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How Was the Solar System Formed? – The Nebular Hypothesis

Since time immemorial, humans have been searching for the answer of how the Universe came to be. However, it has only been within the past few centuries, with the Scientific Revolution, that the predominant theories have been empirical in nature. It was during this time, from the 16th to 18th centuries, that astronomers and physicists began to formulate evidence-based explanations of how our Sun, the planets, and the Universe began.

When it comes to the formation of our Solar System, the most widely accepted view is known as the Nebular Hypothesis . In essence, this theory states that the Sun, the planets, and all other objects in the Solar System formed from nebulous material billions of years ago. Originally proposed to explain the origin of the Solar System, this theory has gone on to become a widely accepted view of how all star systems came to be.

Nebular Hypothesis:

According to this theory, the Sun and all the planets of our Solar System began as a giant cloud of molecular gas and dust. Then, about 4.57 billion years ago, something happened that caused the cloud to collapse. This could have been the result of a passing star, or shock waves from a supernova, but the end result was a gravitational collapse at the center of the cloud.

From this collapse, pockets of dust and gas began to collect into denser regions. As the denser regions pulled in more and more matter, conservation of momentum caused it to begin rotating, while increasing pressure caused it to heat up. Most of the material ended up in a ball at the center while the rest of the matter flattened out into disk that circled around it. While the ball at the center formed the Sun, the rest of the material would form into the protoplanetary disc .

The planets formed by accretion from this disc, in which dust and gas gravitated together and coalesced to form ever larger bodies. Due to their higher boiling points, only metals and silicates could exist in solid form closer to the Sun, and these would eventually form the terrestrial planets of Mercury , Venus , Earth , and Mars . Because metallic elements only comprised a very small fraction of the solar nebula, the terrestrial planets could not grow very large.

In contrast, the giant planets ( Jupiter , Saturn , Uranus , and Neptune ) formed beyond the point between the orbits of Mars and Jupiter where material is cool enough for volatile icy compounds to remain solid (i.e. the Frost Line ). The ices that formed these planets were more plentiful than the metals and silicates that formed the terrestrial inner planets, allowing them to grow massive enough to capture large atmospheres of hydrogen and helium. Leftover debris that never became planets congregated in regions such as the Asteroid Belt , Kuiper Belt , and Oort Cloud .

Artist's impression of the early Solar System, where collision between particles in an accretion disc led to the formation of planetesimals and eventually planets. Credit: NASA/JPL-Caltech

Within 50 million years, the pressure and density of hydrogen in the center of the protostar became great enough for it to begin thermonuclear fusion. The temperature, reaction rate, pressure, and density increased until hydrostatic equilibrium was achieved. At this point, the Sun became a main-sequence star. Solar wind from the Sun created the heliosphere and swept away the remaining gas and dust from the protoplanetary disc into interstellar space, ending the planetary formation process.

History of the Nebular Hypothesis:

The idea that the Solar System originated from a nebula was first proposed in 1734 by Swedish scientist and theologian Emanual Swedenborg. Immanuel Kant, who was familiar with Swedenborg’s work, developed the theory further and published it in his Universal Natural History and Theory of the Heavens  (1755). In this treatise, he argued that gaseous clouds (nebulae) slowly rotate, gradually collapsing and flattening due to gravity and forming stars and planets.

A similar but smaller and more detailed model was proposed by Pierre-Simon Laplace in his treatise Exposition du system du monde (Exposition of the system of the world), which he released in 1796. Laplace theorized that the Sun originally had an extended hot atmosphere throughout the Solar System, and that this “protostar cloud” cooled and contracted. As the cloud spun more rapidly, it threw off material that eventually condensed to form the planets.

This image from the NASA/ESA Hubble Space Telescope shows Sh 2-106, or S106 for short. This is a compact star forming region in the constellation Cygnus (The Swan). A newly-formed star called S106 IR is shrouded in dust at the centre of the image, and is responsible for the surrounding gas cloud’s hourglass-like shape and the turbulence visible within. Light from glowing hydrogen is coloured blue in this image. Credit: NASA/ESA

The Laplacian nebular model was widely accepted during the 19th century, but it had some rather pronounced difficulties. The main issue was angular momentum distribution between the Sun and planets, which the nebular model could not explain. In addition, Scottish scientist James Clerk Maxwell (1831 – 1879) asserted that different rotational velocities between the inner and outer parts of a ring could not allow for condensation of material.

It was also rejected by astronomer Sir David Brewster (1781 – 1868), who stated that:

“those who believe in the Nebular Theory consider it as certain that our Earth derived its solid matter and its atmosphere from a ring thrown from the Solar atmosphere, which afterwards contracted into a solid terraqueous sphere, from which the Moon was thrown off by the same process… [Under such a view] the Moon must necessarily have carried off water and air from the watery and aerial parts of the Earth and must have an atmosphere.”

By the early 20th century, the Laplacian model had fallen out of favor, prompting scientists to seek out new theories. However, it was not until the 1970s that the modern and most widely accepted variant of the nebular hypothesis – the solar nebular disk model (SNDM) – emerged. Credit for this goes to Soviet astronomer Victor Safronov and his book Evolution of the protoplanetary cloud and formation of the Earth and the planets (1972) . In this book, almost all major problems of the planetary formation process were formulated and many were solved.

For example, the SNDM model has been successful in explaining the appearance of accretion discs around young stellar objects. Various simulations have also demonstrated that the accretion of material in these discs leads to the formation of a few Earth-sized bodies. Thus the origin of terrestrial planets is now considered to be an almost solved problem.

While originally applied only to the Solar System, the SNDM was subsequently thought by theorists to be at work throughout the Universe, and has been used to explain the formation of many of the exoplanets that have been discovered throughout our galaxy.

Although the nebular theory is widely accepted, there are still problems with it that astronomers have not been able to resolve. For example, there is the problem of tilted axes. According to the nebular theory, all planets around a star should be tilted the same way relative to the ecliptic. But as we have learned, the inner planets and outer planets have radically different axial tilts.

Whereas the inner planets range from almost 0 degree tilt, others (like Earth and Mars) are tilted significantly (23.4° and 25°, respectively), outer planets have tilts that range from Jupiter’s minor tilt of 3.13°, to Saturn and Neptune’s more pronounced tilts (26.73° and 28.32°), to Uranus’ extreme tilt of 97.77°, in which its poles are consistently facing towards the Sun.

The latest list of potentially habitable exoplanets, courtesy of The Planetary Habitability Laboratory. Credit: phl.upr.edu

Also, the study of extrasolar planets have allowed scientists to notice irregularities that cast doubt on the nebular hypothesis. Some of these irregularities have to do with the existence of “hot Jupiters” that orbit closely to their stars with periods of just a few days. Astronomers have adjusted the nebular hypothesis to account for some of these problems, but have yet to address all outlying questions.

Alas, it seems that it questions that have to do with origins that are the toughest to answer. Just when we think we have a satisfactory explanation, there remain those troublesome issues it just can’t account for. However, between our current models of star and planet formation, and the birth of our Universe, we have come a long way. As we learn more about neighboring star systems and explore more of the cosmos, our models are likely to mature further.

We have written many articles about the Solar System here at Universe Today. Here’s The Solar System , Did our Solar System Start with a Little Bang? , and What was Here Before the Solar System?

For more information, be sure to check out the origin of the Solar System and how the Sun and planets formed .

Astronomy Cast also has an episode on the subject – Episode 12: Where do Baby Stars Come From?

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5 Replies to “How Was the Solar System Formed? – The Nebular Hypothesis”

So… the transition from the geocentric view and eternal state the way things are evolved with appreciation of dinosaurs and plate tectonics too… and then refining the nebular idea… the Nice model… the Grand Tack model… alittle more? Now maybe the Grand Tack with the assumption of mantle breaking impacts in the early days – those first 10 millions years were heady times!

And the whole idea of “solar siblings” has been busy the last few years…

Nice overview, and I learned a lot. However, there are some salient points that I think I have picked up earlier:

“something happened that caused the cloud to collapse. This could have been the result of a passing star, or shock waves from a supernova, but the end result was a gravitational collapse at the center of the cloud.”

The study of star forming molecular clouds shows that same early, large stars form that way. In the most elaborate model which makes Earth isotope measurements easiest to predict, by free coupling the processes, the 1st generation of super massive stars would go supernova in 1-10 million years.

That blows a 1st geeration of large bubbles with massive, compressed shells that are seeded with supernova elements, as we see Earth started out with. The shells would lead to a more frequent 2nd generation of massive stars with a lifetime of 10-100 million years or so. These stars have powerful solar winds.

That blows a 2nd generation of large bubbles with massive, compressed shells, The shells would lead to a 3d generation of ~ 500 – 1000 stars of Sun size or less. In the case of the Sun the resulting mass was not enough to lead to a closed star cluster as we can see circling the Milky Way, but an open star cluster where the stars would mix with other stars over the ~ 20 orbits we have done around the MW.

“The ices that formed these planets were more plentiful”.

The astronomy course I attended looked at the core collapse model of large planets. (ASs well as the direct collapse scenario.) The core grew large rapidly and triggered gas collapse onto the planet from the disk, a large factor being the stickiness of ices at the grain stage. The terrestrial planets grow by slower accretion, and the material may have started to be cleared from the disk. by star infall or radiation pressure flow outwards, before they are finished.

An interesting problem for terrestrial planets is the “meter size problem” (IIRC the name). It was considered hard to grow grains above a cm, and when they grow they rapidly brake and fall onto the star.

Now scientists have come up with grain collapse scenarios, where grains start to follow each other for reasons of gravity and viscous properties of the disk, I think. All sorts of bodies up to protoplanets can be grown quickly and, when over the problematic size, will start to clear the disk rather than being braked by it.

“But as we have learned, the inner planets and outer planets have radically different axial tilts.”

Jupiter can be considered a clue, too massive to tilt by outside forces. The general explanation tend to be the accretion process, where the tilt would be randomized. (Venus may be an exception, since some claim it is becoming tidally locked to the Sun – Mercury is instead locked in a 3:2 resonance – and it is in fact now retrograde with a putative near axis lock.) Possible Mercury bit at least Earth and Mars (and Moon) show late great impacts.

A recent paper show that terrestrial planets would suffer impacts on the great impact scale, between 1 to 8 as norm with an average of 3. These would not be able to clear out an Earth mass atmosphere or ocean, so if Earth suffered one such impact after having volatiles delivered by late accretion/early bombardment, the Moon could result.

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Solar System

Our solar system.




Introduction: (Initial Observation)

Exploration and learning about stars and planets has a long history, but only in the past few years we have been able to see photographic pictures and colorful details of other planets. Making a model is an excellent way for learning about Solar System. Many attempt to make a scale model, but since actual planets are very far from each other, pieces of the model also need to be relatively far; so you will not be able to see the entire solar system in one room. That often creates the idea of using the scale only for the diameter of planets, not for distances.

Dear This project guide contains information that you need in order to start your project. If you have any questions or need more support about this project, click on the “Ask Question” button on the top of this page to send me a message.

If you are new in doing science project, click on “How to Start” in the main page. There you will find helpful links that describe different types of science projects, scientific method, variables, hypothesis, graph, abstract and all other general basics that you need to know.

Project advisor

Information Gathering:

Find out about what you want to investigate. Read books, magazines or ask professionals who might know in order to learn about the effect or area of study. Keep track of where you got your information from.

Following are some good references:

  • Build a solar system model
  • Scale Model Solar System
  • The worlds largest model of solar system
  • A Space Library
  • The Nine Planets and its An Overview of the Solar System
  • Explore the Solar System interactive applet
  • Sun, Planet and Satellite Data

Question/ Purpose:

How many planets revolve around the sun? Which planet is the closest and which is the farthest from the sun? What are the smallest and largest planets in our solar system? We want to make a model that helps us to learn more about the Solar System.

Two additional specific questions that can be the subject of this project are:

What would be your Age on Other Planets?

How much do you weight on other planets?

Identify Variables:

When you think you know what variables may be involved, think about ways to change one at a time. If you change more than one at a time, you will not know what variable is causing your observation. Sometimes variables are linked and work together to cause something. At first, try to choose variables that you think act independently of each other.

Solar system is a display project. In other words you need to make a model that can show the relative size of the planets and the relative distance of planets to the sun. Display projects do not need defining variables.


Based on your gathered information, make an educated guess about what types of things affect the system you are working with. Identifying variables is necessary before you can make a hypothesis. Display projects do not need question, hypothesis.

If you want to have a hypothesis, first you must come up with a question. For example you may ask “Which planet moves faster around the sun? Earth or Mars?” Hypothesis is an educated guess. For example you may say “Mars moves faster than earth.” as your hypothesis. Later, your observations or investigations may support or oppose your hypothesis.

Experiment Design:

Design an experiment to test each hypothesis. We want to build a model of our solar system. It is good if we can find information that can help us building a scaled model. A scaled model is a model that all sizes in that are reduced at a certain ratio. First we need to find the actual diameter of sun and its nine planets. then we reduce these sizes at a ratio that matches our plan. We want to make a model of solar system that is about 2 to 3 feet wide. We also want to use plastic balls to make our model. By searching the net and books we found the diameter of sun and it’s planets as well as their distance to the sun. Following table reflects these information.

   Body Diameter (km)  Orbit radius (km)
 Sun  1,391,900
Mercury  4,866  57950000
Venus  12,106  108110000
Earth  12,742  149570000
 Mars  6,760  227840000
Jupiter  139,516  778140000
Saturn  116,438  1427000000
Uranus  46,940  2870300000
 Neptune  45,432  4499900000
Pluto  2,274  5913000000

Now we want to use a 5 inch ball to represent the sun. 5 inches is about 12 Centimeters. We need to find out how many times do we need to reduce the size of sun to reach to 12 centimeters. To do that we divide the real diameter of the sun (in centimeters) by 12 centimeters. The diameter of the sun in centimeter is 1,391,900,000,000 and by dividing it by 12 we get 115991666666 and we round it up to 116,000,000,000 or 116 billion times. Now we need to reduce all other diameters and distances with the same ratio. So we simply divide all of them by 116 billion. The result is in the following table.

   Body Diameter (in)  Orbit radius (ft)
 Sun  5
Mercury  0.0174  17
Venus  0.0434  32
Earth  0.0457  44
 Mars  0.0242  68
Jupiter  0.5011  232
Saturn  0.4182  427
Uranus  0.1686  859
 Neptune  0.1632  1347
Pluto  0.008  1770

This calculation shows that if the diameter of our sun is only 5 inches, the Pluto orbit must be 1770 feet away from the sun and its size should be as small as a dust particle and will be invisible.

So we decided to make our model with a 5 inch ball to be the sun and for all other bodies we use smaller balls for all other planets.

One way to construct a model is to buy 10 Styrofoam balls in 10 different sizes and let the largest be sun and the smallest be Pluto. Paint the balls, place the sun in the center and connect all planets to the sun using straight wires.

The other way is using the sizes from the above table. You may buy a 5″ ball to be the sun and buy small bids for Jupiter, Saturn, Uranus and Neptune. All others will be the size of a small dot on a wall.

When it comes to presentation, you keep the sun at your desk, Put a small dot somewhere in the wall about 17 feet away from the sun to be the Mercury. Put another small dot about 32 feet away to be the Venus. Two other small dots at 44 feet and 68 feet away will be Earth and Mars. At 232 feet you put a 0.5″ bide to be the Jupiter and another at 427 feet to be the Saturn. The last three are also 3 small dots, Uranus at 859 feet, Neptune at 1347 feet and Pluto at 1770 feet away from our 5″ Sun.

Materials and Equipment:

Material commonly used to make a model of Solar System are:

  • Styrofoam balls
  • Paper and cardboard

Results of Experiment (Observation):

Experiments are often done in series. A series of experiments can be done by changing one variable a different amount each time. A series of experiments is made up of separate experimental “runs.” During each run you make a measurement of how much the variable affected the system under study. For each run, a different amount of change in the variable is used. This produces a different amount of response in the system. You measure this response, or record data, in a table for this purpose. This is considered “raw data” since it has not been processed or interpreted yet. When raw data gets processed mathematically, for example, it becomes results.



Summary of Results:

Summarize what happened. This can be in the form of a table of processed numerical data, or graphs. It could also be a written statement of what occurred during experiments.

It is from calculations using recorded data that tables and graphs are made. Studying tables and graphs, we can see trends that tell us how different variables cause our observations. Based on these trends, we can draw conclusions about the system under study. These conclusions help us confirm or deny our original hypothesis. Often, mathematical equations can be made from graphs. These equations allow us to predict how a change will affect the system without the need to do additional experiments. Advanced levels of experimental science rely heavily on graphical and mathematical analysis of data. At this level, science becomes even more interesting and powerful.

Image on the right shows a sample of solar system model that you may make as a part of your project.

Making a model will help you to memorize with planets are larger or smaller than the earth. It will also help you to memorize which planets are closer or further away from the sun.


Using the trends in your experimental data and your experimental observations, try to answer your original questions. Is your hypothesis correct? Now is the time to pull together what happened, and assess the experiments you did.

Related Questions & Answers:

What you have learned may allow you to answer other questions. Many questions are related. Several new questions may have occurred to you while doing experiments. You may now be able to understand or verify things that you discovered when gathering information for the project. Questions lead to more questions, which lead to additional hypothesis that need to be tested.

Possible Errors:

If you did not observe anything different than what happened with your control, the variable you changed may not affect the system you are investigating. If you did not observe a consistent, reproducible trend in your series of experimental runs there may be experimental errors affecting your results. The first thing to check is how you are making your measurements. Is the measurement method questionable or unreliable? Maybe you are reading a scale incorrectly, or maybe the measuring instrument is working erratically.

If you determine that experimental errors are influencing your results, carefully rethink the design of your experiments. Review each step of the procedure to find sources of potential errors. If possible, have a scientist review the procedure with you. Sometimes the designer of an experiment can miss the obvious.


Visit a local library and review some books related to astronomy and solar system. Also search the Internet with keywords such as “Solar System” and “Size of planets”.

My project sheet is asking me for problem statement, hypothesis, acknowledgments

You have elected a primary project; so, I assume that you are in first grade up to 4th grade. At these ages you must do display projects. Display projects do not require problem statement and hypothesis.

Higher grades, specially 8th graders do experimental projects that require problem and hypothesis.

If you really need to do this project and need to have a hypothesis, this is my recommendation:

Start with a question or problem statement like this:

Which planets are closer to the sun?

Why do we need to know that?

In future, people from the earth may need to travel to other planets. Obviously planets that are closer to the sun are hotter and the planets that are further away from the sun are colder. By knowing the distances of planets to the sun, we can decide which planet is best for us to move in. (Sounds like science fiction!)

Write a hypothesis. In your hypothesis write which planet do you think is the next closest planet to the sun after the earth. Offer a hypothesis like this:

I think Mars is the next closest planet to the sun. (Note that the hypothesis does not have to be correct).

With the help of this project guide, some books and your parents find out which planets are closer to the sun. Then report your results in a table like this:

Mercury Orange/Red
Venus Grey/Blue
Earth Blue and Green
Mars Red
Jupiter Red and Orange
Saturn with ring Mint Green and Peach
Uranus Rust and Green
Neptune Rust and Green
Pluto Purple

solar system project hypothesis

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How Did the Solar System Form?

Click here to download this video (1280x720, 14 MB, video/mp4).

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8.5 x 11 inches, 8.5 x 13 inches, 11 x 17 inches, click here to read a transcript of this story.

The solar system is a pretty busy place. It’s got all kinds of planets, moons, asteroids, and comets zipping around our Sun.

But how did this busy stellar neighborhood come to be?

Our story starts about 4.6 billion years ago, with a wispy cloud of stellar dust.

This cloud was part of a bigger cloud called a nebula.

At some point, the cloud collapsed—possibly because the shockwave of a nearby exploding star caused it to compress.

When it collapsed, it fell in on itself, creating a disk of material surrounding it.

Finally the pressure caused by the material was so great that hydrogen atoms began to fuse into helium, releasing a tremendous amount of energy. Our Sun was born!

Even though the Sun gobbled up more than 99% of all the stuff in this disk, there was still some material left over.

Bits of this material clumped together because of gravity. Big objects collided with bigger objects, forming still bigger objects. Finally some of these objects became big enough to be spheres—these spheres became planets and dwarf planets.

Rocky planets, like Earth, formed near the Sun, because icy and gaseous material couldn’t survive close to all that heat.

Gas and icy stuff collected further away, creating the gas and ice giants.

And like that, the solar system as we know it today was formed.

There are still leftover remains of the early days though.

Asteroids in the asteroid belt are the bits and pieces of the early solar system that could never quite form a planet.

Way off in the outer reaches of the solar system are comets. These icy bits haven’t changed much at all since the solar systems formation.

In fact, it is the study of asteroids and comets that allows scientists to piece together this whole long story.

Quick and fun movies that answer big science questions!

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Origins of the Solar System

  • Teacher Resource
  • Posted 08.23.12
  • NOVA scienceNOW

This video segment adapted from NOVA scienceNOW follows scientists analyzing meteorites—the oldest rocks in the solar system—to determine what triggered the birth of our solar system from a vast cloud of gas and dust. Remnants of an isotope of iron in the meteorites could have come from a distant massive star that died in a violent, luminous explosion, called a supernova. Some scientists think the supernova shock wave triggered the collapse of the gas and dust cloud. Others disagree, suggesting that radiation emitted by a nearby star might have been enough to collapse the cloud, which led to the birth of the Sun, planets, and, eventually, all life on Earth.

NOVA scienceNOW

  • Media Type: Video
  • Running Time: 5m 38s
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  • Level: Grades 6-12
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Source : NOVA scienceNOW : "Origins of the Solar System"

This media asset was adapted from NOVA scienceNOW : "Origins of the Solar System" .

About 5 billion years ago, a vast, cold cloud of interstellar gas and dust, called a nebula, began to collapse. A star formed at the center, surrounded by a spinning disk of gas and dust. Small grains condensed, accreted into smaller rocky bodies, and ultimately grew into planets. Thus was born our solar system—the Sun and all the planets, moons, comets, asteroids, and meteoroids that revolve around it.

This solar nebula theory is well supported by modern astronomical observations of other nebulas, disks, and planets, and by radiometric dating. Samples of the Moon, Earth, Mars, and meteorites (the remnants of meteoroids that entered the atmosphere as meteors and landed on Earth) have a similar age of 4.56 billion years. The evidence implies that planets are a natural byproduct of star formation and that solar systems are common throughout the universe.

However, what initially caused the nebula to begin to collapse remains unknown. Some researchers suggest the death of a distant massive star in a violent, luminous explosion called a supernova sent a shock wave through space, compressing the cloud. Others argue that a supernova shock wave would scatter a gas and dust cloud, rather than collapse it. They propose instead that radiation emitted by a nearby massive star prior to its death could have nudged the nebula to collapse.

The debate centers on evidence from meteorites, which preserve a record of the chemical composition and conditions at the time of solar system formation. Researchers have found nickel-60 in the grains of meteorites. Nickel-60 is a radioactive decay product of iron-60, which forms inside massive stars and is dispersed by supernovas. Iron-60 decays rapidly into nickel-60, allowing scientists to use it like a clock. The presence of nickel-60 means that a supernova happened around the time that solid grains (later found in meteorites) began to condense out of the cloud.

This is tantalizing evidence that a supernova may have triggered the birth of the solar system—with the shock wave injecting iron and triggering the collapse at the same time. In contrast, proponents of the hypothesis that radiation from a nearby star collapsed the cloud believe the iron was injected later—by the supernova of that star—after the Sun and planets had already begun to form.

More research is needed to determine whether the birth of the solar system was triggered by a distant supernova or radiation from a nearby star, or possibly something else. An emerging hypothesis holds that no one star contributed the iron-60 and other short-lived radionuclides, but that they instead came from an ensemble of massive stars that formed in the nebula before the solar system.

In addition to iron-60, scientists are studying other short-lived radionuclides found in meteorites, such as aluminum-26, manganese-53, and iodine-129, which form in different ways, in stars of different masses, and have different half-lives. Thus, the proportion in which they appear can isolate which events occurred—in the right place, at the right time—to trigger the birth of the solar system and, ultimately, all life on Earth.

Teaching Tips

Here is a suggested way to engage students with an activity related to this topic.

  • Do a research project—groups: Have students work together to describe the formation of the solar system, from the early nebula phase to the planets that we see today. Ideas include drawing a comic, making a timeline, drawing a picture, making a presentation or video, etc.

Questions for Discussion

  • What evidence is presented in the video to support the idea that a supernova caused the birth of our solar system?
  • What are meteorites? How do they help us learn about the early solar system?
  • Why is the snowplow a good model for how a supernova shock wave could affect a nebula?
  • Why do you think scientists test meteorites to learn about the early solar system?

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solar system project hypothesis

The shock wave from a supernova may have triggered the formation of our sun and planets five billion years ago.

How the Inner Solar System Fo...

In this video segment adapted from NOVA, learn how the four rocky planets of the inner solar system formed.

The Origin of the Elements

Watch and learn about the origin of the elements and how scientists use element profiles to identify supernova types.

The Origin of the Moon

This video segment follows the Apollo 15 astronauts as they collect samples of ancient rock from the Moon's crust.

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September 29, 1917

17 min read

The Origin of the Solar System

An Outline of the Three Principal Hypotheses

By Harold Jeffreys

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THE question of the origin of the solar system is one that has been a source of speculation for over a hundred years; but, in spite of the attention that has been devoted to it, no really satisfactory answer has yet been obtained. There are at present three principal hypotheses that appear to contain a large element of truth, as measured by the closeness of the approximation of their consequences to the facts of the present state of the system, but none of them is wholly satisfactory. These are the Nebular Hypothesis of Laplace, the Planetesimal Hypothesis of Chamberlin and Moulton, and the Capture Theory of See. Darwings theory of Tidal Friction is scarcely a distinct hypothesis, but is mentioned separately on account of its application to all of the others. The main features of these hypotheses will be outlined in the present paper. The Hypothesis of Laplace.According to Laplace, the solar system formerly consisted of a very much flattened mass of gas, extending beyond the orbit of Neptune, and rotating like a rigid body. In consequence of radiation of energy this slowly contracted, and in so doing gained so much in angular velocity that the centrifugal force at the equator became greater than gravity, and a ring of matter was left behind along the equator. Further contraction would detach a series of rings. These were then expected to break up in such a way that each produced a gaseous planet. This might later evolve in the same way as the original nebula, thus producing satellites. The criticisms of this hypothesis in its original form are very well known, and will only be summarized here. Forest ranger beating out a fire in one of the National Forests in Oregon FIGHTING FOREST FIRES [See page 200] The angular momentum of the system when the gaseous central body extended to the orbit of any planet can be calculated, and is not nearly sufficient to cause detachment of matter. Poincare showed that this objection could be met if the nebula were initially highly heterogeneous, with all but gAtj of its mass in the central body. The matter left behind would not form definite rings; for a gas has no cohesion, and consequently the separation of matter along the equator would be continuous and lead to another gaseous nebula, not rotating like a rigid body. A ring could not condense into a planet. According to the latest work of Jeans, viscosity is inadequate to make a mass of gas as large as a Lapla- cian nebula rotate like a rigid body. No satellite could revolve in a shorter time than it takes its primary to rotate: this condition is violated by Phobos, the inner satellite of Mars, and by the particles constituting the inner edge of Saturn's ring. All satellites should revolve in the same direction as their primaries rotate: this condition is violated by one satellite of Saturn and two of Jupiter. The second, third, and fourth objections seem quite unanswerable at present. The theory of Gravitational Instability, due to Jeans, is an attempt to pass directly from the symmetrical nebula to an unsymmetrical one with a secondary nucleus, without the ring as an intermediate stage. It will be noticed that Laplace's hypothesis implies that all the planets were formerly gaseous, and hence must have been liquid before they became solid. The question of the course of evolution of a gaseous mass initially heterogeneous with several strong secondary condensations has not hitherto been considered; such a mass would be free from at least the first four of the objections offered to the standard forms of Laplace's hypothesis, and its history would serve as a hypothesis intermediate between this and the Planetesimal Hypothesis. The Planetesimal Hypothesis.This hypothesis has been formulated by Chamberlin and Moulton1 to avoid the serious defects of the Nebular Hypothesis. It really consists of two separate assumptions, either of which could be discarded without necessarily invalidating the other. The first of these involves the close approach of some wandering star to the sun. This would raise two tidal projections at opposite sides of the sun, and if the disturbance was sufficiently violent, streams of matter would be expelled from them. On account of the perturbations of their paths by the second body, these would not fall back into the sun, but would go on revolving round it as a system of secondary nuclei, with a large number of very fine particles also revolving round the sun; each particle, however small, would revolve independently, so that the system would in this respect resemble the heterogeneous nebula mentioned at the close of the last paragraph. The mathematical investigation of this hypothesis would be extremely difficult, but there seems to be no obvious objection to it. It will be seen that the nuclei would be initially liquid or gaseous, having been expelled from the sun. Thus this hypothesis implies a formerly molten earth. The smaller particles would soon become solid, but the gaseous part initially expelled and not under the influence of a secondary nucleus would remain gaseous, although its density would be very small. The orbits would be highly eccentric. The second part of the hypothesis deals with the latef- evolution of the secondary nuclei. Its authors believe that these would steadily grow by picking up the smaller particles, which are called planetesimals, and in the process they would have the eccentricities of their orbits reduced. That this is qualitatively correct can easily be proved mathematically. There is, however, a serious objection to its quantitative adequacy. Consider any arbitrary planetesimal. Its chance of colliding with another planetesimal in a definite time is proportional to the sum of the surfaces of the planetesimals, while its chance of colliding with a nucleus is proportional to the sum of the surfaces of the nuclei. Further, if the eccentricities of the planetary orbits are to be considerably affected by accretion, the mass picked up by each planet must be at least as great as the original mass of the planet. Now the more finely divided the matter is, the more surface it exposes, and hence before accretion the mass picked up must have presented a much larger surface than the planet did. Hence collisions between planetesimals must have been far commoner than collisions between planets and planetesimals. Further, as the velocity of impact must have been comparable with an orbital velocity on account of the high eccentricity of the orbits, the colliding planetesimals must in nearly all cases have turned to gas; for it is known that meteors entering the earth's atmosphere at such velocities are volatized. Hence nearly all of the planetesimals must have turned to gas before the nuclei could be much affected by accretion. We are thus back to the heterogeneous gaseous nebula. If the planetesimals moved initially in nearly circular orbits this objection does not arise, but it can then be shown that the product of the mass and the orbital eccentricity of each nucleus would diminish with the time. It can thus be seen that Jupiter could never have been smaller than Uranus is now. There is no obvious objection to this form of the hypothesis, but there is no reason to suppose that solid planetesimals did originally move in nearly circular orbits.2 A further hypothesis that has come to be associated with the present one, although not an essential part of it, is the belief that the earth has always been solid. There are many serious difficulties in the way of this. The mode of formation of the nuclei described in the first part of the Planestesimal Hypothesis implies that they were initially liquid or gaseous. This is not, however, a direct objection; one part of the hypothesis might be true and the other false, as they are not interdependent. Only one satisfactory explanation of the elevation of mountains by the folding of the earth's crust has been offered; this attributes it to a horizontal compression at the surface. Now, if a solid earth grew by the addition of small particles from outside, these would be deposited in a layer on the surface, in a perfectly unstrained condition. Thus, during the whole process of growth the same surface condition would always hold, namely, that there is no horizontal compression at the surface, however much deformation may take place within. Hence any stresses available for mountain- building must have been accumulated after accretion ceased; if the theory that the earth was formerly molten should be proved to give insufficient surface compression to account for known mountains, then a fortiori the theory of a permanently solid earth gives insufficient compression, as the available fall of temperature is less. 3. It is by no means clear that a solid earth growing by accretion would remain solid. A particle falling from an infinite distance to the earth under the earth's attraction alone would develop a velocity almost enough to volatilize it on impact, and the actual velocities must have been considerably greater than this, as the planetesimals would have a velocity relative to the earth before entering its sphere of influence. If, then, the particles required to form the earth were all brought together at once, the resulting body would be gaseous. On the other hand, if the accretion were spread over a long enough time, heat would be radiated away as fast as it was produced, and the body would remain solid. In the absence of a criterion of the rate of growth it is impossible to state whether an earth growing by accretion could remain solid or not. Holmes3 has found that the hypothesis of a cooling earth, initially in a liquid state, leads to temperatures within the crust capable of accounting for igneous activity, whereas the view that the earth is now in a steady state, its temperature gradient being maintained wholly by radio-activity, is by no means certain to lead to adequate internal temperatures. Assuming the former fluidity of the earth, he has developed a wonderfully consistent theory of the earth's thermal state. The present writer, using Holmes's data, finds4 that the available compression of the crust is of the same order of magnitude as that required to produce the existing mountain-ranges. 2Monthly Notices of R.A.S. vol. lxxvn. 1916. It seems, then, that whatever we may assume about the origin of the earth, the hypothesis that it has at some stage of its existence been liquid or gaseous agrees best with its present state. The hypothesis of Laplace, however modified, implies the former fluidity of the earth, and so does the standard form of the Planetesimal Hypothesis. The Capture Theory of See.hLike the Planetesimal Hypothesis, this has been developed during the present century to avoid the objections that have been offered to that of Laplace. The main features of the two theories are very similar. Both involve the idea of a system of secondary nuclei revolving in independent orbits about the primitive sun, with sparsely distributed small particles between them, and the impacts of the small particles on the nuclei are supposed in course of time to act on the orbits of the latter in the same way as a resisting medium; namely, the eccentricities of the orbits tend to diminish, and satellites tend to approach their primaries. The Capture Theory is not, however, stated in so precise a form as the Planetesimal Theory. It is not definitely stated whether all the small particles would revolve in the same direction or not. If they did, then there would be little or no secular effect on the mean distance of a planet. If, however, they moved indifferently in the direct and retrograde senses, then their collective effect would be the same as that of a medium at rest, and the friction encountered by the planets in their motion would cause them to approach the sun. The fact that such a secular effect is stated by See to occur implies that the particles at any point are not on an average supposed to move with the velocity appropriate to a circular orbit at that point, so that the conditions would be such as to ensure that collisions between them would be violent. The small particles are described by the somewhat vague term of “cosmical dust”; if this means that they were solid, the Capture Theory, like the Planetesimal Theory, fails on the ground that the collisions between the small particles would cause the system to degenerate to a gaseous nebula long before any important effect had been produced on the nuclei. If, on the other hand, they were discrete molecules, then the system would be a heterogeneous gaseous nebula at the commencement, and this objection does not apply. It is clear, however, that the planets cannot have entered the system from outer space, for then their orbital planes would be inclined to one another at large angles, which the subsequent action of the medium could scarcely affect, whereas actually all the major planets keep very close to the ecliptic. All must, then, be regarded as having always been members of the solar system, however much their orbits may have changed. They are supposed to be derived from the secondary nuclei of a soiral nebula. The most important difference between the Planetesimal and Capture theories lies in the history attributed to the satellites. In the former, each satellite is supposed to have always been associated with its present primary, having been near it when originally expelled from the sun. In the Capture Theory, primaries and satellites are both supposed to have initially moved independently round the sun in highly eccentric orbits. If, in the course of its movement”, a small body came sufficiently near a large one, and had a sufficiently small relative velocity, then a permanent change would take place in the character of its orbit, and it is possible that, under the influence of the resisting medium, this would ultimately lead to its becoming a satellite. The mechanism of the process has not been worked out in detail, and, in view of the extremely complicated nature of the problem, it would be very dangerous to predict whether it is feasible. All the satellites in the system are supposed to have been captured in this way by their primaries. In both hypotheses the satellites are considered to have approached their primaries after becoming associated with them owing to the secular effect of the resisting medium. 3”Padio-activity and the Earth's Thermal History,” Geol. Mag. FebruaryMarch 1915, June 1916. *Phil. Mag. vol. xxxii. Dec m':er 1916. *>The Capture Theory of Cosmical Evolution, by T. J. J. See The Theory of Tidal Friction.All the theories so far mentioned agree in the fact that each commences with a particular distribution of matter, and tries to predict the course of the changes that would follow if this were left to itself. The success or failure of such hypotheses to lead to a system resembling the present solar system is the measure of their truth or falsehood. The method is thus essentially one of trial and error, and when a theory is found unsatisfactory, the next step is to modify it in such a way as to avoid the defects that have been detected. In this way a succession of different hypotheses may be Obtained, each giving a better representation of the facts than the previous one. Destructive criticism may thus be of positive value. Such a method must necessarily yield the truth very slowly, and must further involve a large number of assumptions concerning the initial conditions; in addition, the set of initial conditions that leads to the correct final state may not be unique. The Theory of Tidal Friction, due to Sir G. H. Darwin,6 is of a totally different character. It? starts with the present conditions, and by means of a single highly plausible hypothesis obtains relations that the properties of the system must have satisfied at any epoch, provided only that this is not too remote for the calculation to be possible, and that no unknown causes have operated that could invalidate the work. The initial conditions thus obtained are then unique, and the only way of disproving the hypothesis would be to discover some new agency of sufficient magnitude to upset the course of the involution. Whatever hypothesis may ultimately be found to account for the present solar system, the Theory of Tidal Friction must therefore form a part of it. The physical basis of the theory is very simple. The attractive force due to the moon is always greatest on the side of the earth nearest to it, and least on that farthest away, while its value at the center of the earth is intermediate. The center of the earth being regarded as fixed, then, the moon tends to cause the parts of the earth nearest to and farthest from it to protrude, thus forming a bodily tide. If the earth were perfectly elastic, the high tide would always occur with the moon in the zenith or nadir; no energy would be dissipated, and there would be no secular effect. If, however, it is viscous the tides would lag somewhat, and their attractions on the moon would, in general, produce a calculable secular effect on the moon's motion and the rotation of the earth. The only case where viscosity would produce no secular effect is when the deformed body rotates in the same time as the deforming one revolves. The tide then does not move round relatively to the body, but becomes a constant fixed deformation, directly under the deforming body, and ceases to produce a secular effect. In the ultimate steady state of a viscous system, then, the viscous body will always keep the same face turned towards the perturbing one. In the solar system system there are certainly two examples of this condition, and no other explanation of it has been advanced. Mercury always keeps the same face towards the sun, and the moon towards the earth; with less certainty it is believed that the same is true of Venus and the satellites of Jupiter. Now if the viscosity of a substance be zero, that substance is a perfect fluid, and there can be no dissipation of energy inside it. If, on the other hand, it be infinite, then we have the case of perfect elasticity, and again there can be dissipation. If the viscosity steadily increase from 0 to infinity, then the rate of dissipation of energy when the same periodic stress is applied increases to a maximum and then diminishes again to zero. The balance of probability seems to imply that the earth was formerly fluid, and, if this can be granted, the fact that most of it is now almost perfectly elastic at once indicates that dissipation of energy by tidal friction must have been important in the past. On this hypothesis Sir G. H. Darwin traced the system of the earth and moon back to a state where the moon was close to the earth, the two always keeping the same face towards each other, and revolving in some time between three and five hours. The lunar orbit was practically in the plane of the equator; the initial eccentricity is uncertain, as it depends altogether on the actual variation of the viscosity with the time. Scientific Papers, vol. ii. The question that next arises is, what was the condition just before this? The natural suggestion is that the two bodies formed one mass. The cause of the separation is, however, open to some doubt. It has been thought that the rapidity of the rotation would be enough to cause instability, in which case the original body might break up into two parts. Moulton, on the other hand, has shown that the actual rotation could not be so rapid as to make the system unstable. It is more likely that Darwin's original suggestion is correct, namely, that at the epoch considered the period of rotation was nearly double the period of one of the free vibrations of the mass; consequently the amplitude of the semidiurnal tide would be enormous, and might easily lead to fission in a system not possessing much strength. The Prevalence of Direct Motion in the Solar System. On all of the theories of the origin of the solar system that have here been described it is necessary that the planets should revolve in the same direction. On the Planetesimal Theory this would be the direction of the motion of the perturbing body relative to the sun at the time of the initial disruption. In addition to this, however, all the planets except probably Uranus and Neptune have a direct rotation, and all the satellites except those of these two planets and the outer ones of Jupiter and Saturn have a direct revolution. The fact that three satellites revolve in the opposite direction to the rotation of their primaries is in flagrant contradiction to the original form of the Nebular Hypothesis. It was, however, suggested by Darwin that all the planets might have originally had a retrograde rotation, and that the friction of the solar tides has since reversed the rotation of all except the two outermost. Jupiter and Saturn would then be supposed to have produced their outer satellites before the reversal took place, and the others afterwards. An objection to this theory has been raised by Moulton, who points out that the secular retardation of the rotation of Saturn due to solar tides is only about tsooo of that of the earth, so that there probably was not time for this to occur. On the other hand, this retardation is proportional to the seventh power of the diameter of the planets: if we can grant then that these planets were formerly much more distended than at present, the viscosity remaining the same, the available time may be adequate. At the same time, solar tidal friction may be adequate to explain the facts that one of the satellites of Mars and the particles at the inner edge of Saturn's ring revolve more rapidly than their primaries rotate, which would not be the case on the unmodified Nebular Hypothesis. Direct rotation and revolution of satellites on the Planetesimal Theory are shown by Moulton to be probable as a result of a very ingenious argument involving the mode of accretion. Whether it is quantitatively adequate is not proved, and the present writer would prefer to regard these motions as having been direct since the initial disruption. Let us suppose, for instance, that disruption would occur when the disruptive force had reached a definite fraction of surface gravity. It can easily be seen that both are proportional to the diameter of the disturbed body, and hence their ratio is independent of it. Other things being equal, then, a nucleus of any size would be equally likely to be broken up and give a set of dependent nuclei, which would then revolve round it in the direct sense. Secondary nuclei expelled at the same time and close together would remain together, and their relative motion might be in either sense. Thus we should expect both direct and retrograde revolution, but the former would predominate. The fact that the retrograde satellites are on the outside of their systems is to be attributed partly to the greater stability of retrograde orbits of larger size and partly to the fact that they would experience less resistance from the medium. Capture may be possible; in the present state of our knowledge we can neither affirm nor deny it. Direct rotation is presumably to be attributed to the attraction of the disturbing body on the tidal protuberance before and during expulsion, and to secondary nuclei with direct motions falling back into the parent body. Subsequent evolution would take place in a similar way to that indicated by Darwin. The Hypothesis of a Heterogeneous Nebula.A system of nuclei revolving in a tenuous gaseous nebula would experience a viscous resistance from it, and hence would probably evolve in much the same way as See has indicated in the Capture Theory; accretion must probably be almost negligible, so that the original nuclei must have had nearly their present masses. The original eccentricities of the orbits of both planets and satellites would be considerably reduced; the inclination to the plane of the ecliptic would be small at the commencement, and would remain so; if the medium revolved the effect on the major axes of the orbit, and hence on the periods, would probably be small. Direct satellites would approach their primaries, and retrograde ones would ultimately be left on the outskirts of their subsystems. Given suitable initial conditions, then, a system might be developed that would bear a strong resemblance to the existing solar system. The resisting medium itself would gradually degenerate and approach the sun on account of its internal friction; the zodiacal light may be the last remnant of it. It may, however, be regarded as certain that there has been no large amount of resisting matter near the earth's orbit for a very long time; there has probably been ample time for the evolution of the earth and moon to take place from the state that Darwin traced them back to. The moon was then probably formed from the earth by the disruptive action of the solar tides; but, as this would be a resonance effect, increasing in amplitude over thousands of vibrations, whereas the formation of a system of nuclei in the way suggested by Moulton would take place at once, there need be no surprise that the former event led to a single satellite of of the mass of the primary, while the latter formed several, the largest having a mass of tTjjfu of its primary. The unsymmetrical nebula here considered might have been produced in the manner described in the last section. A symmetrical nebula becoming gravitationally unstable would lead to an unsymmetrical one, as was proved by Jeans, but it is difficult to see how the phenomenon of retrograde and direct motions occuring to the same subsystem could occur on this hypothesis. On the whole, then, the most plausible hypothesis seems to be that a gaseous neubla with a system of secondary and tertiary nuclei was formed round the sun by tidal disruption owing to the close passage of another star, and that this has been subsequently modified by gaseous viscosity, and at a later stage by tidal friction. The moon was probably formed from the earth by solar tidal disruption, this method being abnormal in the system, and the later evolution of the earth and moon has been dominated by bodily tidal friction.

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Article Contents

Early theories, what is a good theory, the accretion theory, the floccule/protoplanet theory, the solar nebula theory, forming planets from a diffuse medium, comments and residual difficulties, the capture theory, satellite formation, orbital evolution, star formation, planetary collision and terrestrial planets, the moon, mars and mercury, smaller bodies, isotopic anomalies in meteorites, the modern laplacian theory.

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The origin and evolution of the solar system

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Michael Woolfson, The origin and evolution of the solar system, Astronomy & Geophysics , Volume 41, Issue 1, February 2000, Pages 1.12–1.19, https://doi.org/10.1046/j.1468-4004.2000.00012.x

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M Woolfson discusses theories of how the Sun and the planets began.

The heliocentric nature of the solar system with its major components — the Sun, planets and satellites — was firmly established well before the end of the 17th century. After the publication of Newton's Principia in 1687 it became possible to apply scientific principles to the problem of its origin.

Most theories that have been advanced in the last 300 years are obviously untenable, but some contain the germs of what might be part of a viable theory. It would not be practical to attempt to deal with all theories in detail in a short review article. Here we shall mention five theories, recently developed or still in the process of development, that have a reasonable scientific basis. Two of them, the Solar Nebula Theory and the Capture Theory, will be described in more detail, emphasizing what they have and have not explained and what their remaining difficulties are. Two early theories will be described first, chosen because they relate closely to the extant ones and illustrate the major problems for theories.

Based on ideas and observations by Descartes, Kant and Herschel, Pierre Laplace (1796) put forward the first really scientific theory (summarized in figure 1 ). A slowly spinning cloud of gas and dust cooled and collapsed under gravity. As it collapsed, so it spun faster and flattened along the spin axis. It eventually took on a lenticular form with equatorial material in free orbit around the central mass. Thereafter material was left behind as a set of rings within which clumping occurred. Clumps orbiting at slightly different rates combined to give a protoplanet in each ring. A smaller version of the scenario, based on the collapse of protoplanets, produced satellite systems. The central bulk of the original cloud collapsed to form the Sun.

An illustration of Laplace's nebula theory. (a) A slowly rotating and collapsing gas-and-dust sphere. (b) An oblate spheroid forms as the spin rate increases. (c) The critical lenticular form. (d) RIngs left behind in the equatorial plane. (e) One planet condensing in each ring.

This monistic theory, that produced the Sun and the planets in a single process, has an attractive simplicity but a fatal flaw. It suggests that most of the angular momentum of the system is in the Sun — which is not so. The Sun with 99.86% of the mass of the system has only 0.5% of the total angular momentum contained in its spin; the remainder is in the planetary orbits. All 19th century attempts to rescue the theory were unsuccessful. The theory, although based on scientific principles, did not agree with observation and so had to be abandoned.

Some time later James Jeans (1917) suggested a dualistic theory, one for which the Sun and planets were produced by different mechanisms. A massive star passed by the Sun, drawing from it a tidal filament (shown in figure 2 ). The gravitationally unstable filament broke up with each condensation forming a protoplanet. The protoplanets, attracted by the retreating star, were retained in heliocentric orbits. At first perihelion passage a small-scale version of the same mechanism led to a filament being drawn from a protoplanet within which protosatellites formed.

An illustration of Jeans' theory. (a) The escape of material from the tidally distorted Sun. (b) Protoplanetary condensation in the ejected filament. (c) Protoplanets attracted by the retreating massive star.

The theory had a good reception — especially as it was supported by some elegant analysis. Jeans found how a tidally affected star would distort and eventually lose a filament of material from the tidal tip. He showed that the filament would fragment through gravitational instability and he also derived a condition for the minimum mass of a filament clump that could collapse. Despite the initial enthusiastic acceptance of the theory, it soon ran into trouble. Harold Jeffreys (1929) , by a mathematical argument involving the concept of circulation, suggested that Jupiter, which has the same mean density as the Sun, should have a similar spin period. The periods differ by a factor of 70. Other simpler, and hence more readily accepted, objections followed. Henry Norris Russell (1935) showed that material pulled from the Sun could not go into orbit at more than four solar radii — well within Mercury's orbit. This was another type of angular momentum problem. Then Lyman Spitzer (1939) calculated that a Jupiter mass of solar material would have a temperature of about 10 6 K and would explode into space rather than collapse. Later, other objections were raised concerning the presence of lithium, beryllium and boron in the Earth's crust, light elements that are readily consumed by nuclear reactions in the Sun.

Jeans tried to rescue his theory by having a cool extended Sun with the radius of Neptune's orbit, but this created new problems — not least that the newly formed planets in a diffuse form would be ploughing through the Sun. He finally conceded that “the theory is beset with difficulties and in some respects appears to be definitely unsatisfactory”.

The Laplace and Jeans theories were scientifically based but finally succumbed to scientific criticism. They both had angular momentum problems although of different kinds. Nevertheless all the modern theories described here involve ideas that they introduced. They also illustrate important problems that theories must address to be considered as plausible.

Those producing cosmogonic theories usually provide lists of “facts to be explained” but, as the scientific historian Stephen Brush concluded, such lists often emphasize those facts that the individual's theory best deals with. This could well be true. To avoid that possibility, I give below the union of all “facts” suggested by various workers. They are separated into groups according to whether they are gross features or relate to details of the system.

Gross features :

the distribution of angular momentum between Sun and planets

a planet-forming mechanism

planets to form from “cold” material

direct and almost coplanar orbits

the division into terrestrial and giant planets

the existence of regular satellites.

Secondary features:

the existence of irregular satellites

the 7° tilt of the solar spin axis to the normal to the mean plane of the system

the existence of other planetary systems.

Finer details of the solar system:

departures from planarity of the system

the Earth-Moon system variable directions of planetary spin axes

Bode's law or commensurabilities linking planetary and satellite orbits

asteroids: origin, compositions and strutures

comets: origin, compositions and structures

the formation of the Oort cloud

the physical and chemical characteristics of meteorites

isotopic anomalies in meteorites

Pluto and its satellite, Charon

Kuiper-belt objects.

The least that a theory should deliver is convincing explanations of the gross features. A theory without a slowly spinning Sun and a planar system of planets with regular satellite systems for some is, at best, implausible.

If alternative plausible theories are available then one may resort to the principle first enunciated by the English philosopher William of Occam (1285–1349), known generally as Occam's razor. Loosely translated from the Latin this implies that “if alternative theories are available that explain the observations equally well then the simpler is to be preferred”.

The goal then is to find a simple theory based on well-established scientific principles, that explains what is known and that cannot be refuted by scientific arguments. We shall now look at the ideas that have been put forward over the last half century, roughly in their date order of presentation.

In 1944, Soviet planetary scientist Otto Schmidt suggested a new kind of dualistic theory. It was known from telescopic observations that cool dense clouds occur in the galaxy and Schmidt argued that a star passing through one of these clouds would acquire a dusty-gas envelope. Schmidt believed from energy considerations that, for two isolated bodies, material from one body could not be captured by the other and so he introduced a third body nearby, another star, to remove some energy. The need for a third body made the model rather implausible but, as Lyttleton showed in 1961, Schmidt's argument was invalid since the cloud was of large extent and the star-plus-cloud behaved like a manybody system. Lyttleton proposed capture of material by an accretion mechanism first suggested by Bondi and Hoyle (1944) and illustrated in figure 3 . The cloud material moves relative to the star at speed V , greater than the escape speed. Deflected interacting streams, such as at point G, lose their component of velocity perpendicular to the original direction of motion and the residual speed can then be less than the escape speed.

Streams of material arriving at point G cancel their velocity components perpendicular to the axis.

Lyttleton used parameters for the model that gave the mass and angular momentum of captured material compatible with that of the planets, although no process was suggested for producing planets from the diffuse envelope. However, Lyttleton's parameters were implausible. The temperature of the cloud was 3.18 K, in equilibrium with galactic radiation, and the relative velocity of cloud and star was 0.2 kms -1 . A cloud temperature of 10–20 K or even greater is more consistent with observation, and the relative speed is more likely to be of order 20 kms -1 . The proposed mechanism does no more than suggest a source of planetary material. It cannot be regarded as a convincing theory, especially as planet formation from diffuse material presents additional difficulties, as we shall see later.

In 1960, McCrea suggested a theory that linked planetary formation with the production of a stellar cluster and also explained the slow rotation of the Sun. McCrea's starting point was a cloud of gas and dust that was to form a galactic cluster. Due to turbulence, gas streams collided and produced regions of higher-than-average density. The high-density regions, referred to as “floccules”, moved through the cloud and combined whenever they collided. When a large aggregation formed, it attracted other floccules in its region so producing a protostar. Since floccules joined the accreting protostar from random directions, the net angular momentum of the protostar was small; for a particular set of parameters it would be only a few times the present angular momentum of the Sun and the excess can be removed after formation by various physical processes.

It was assumed that star-forming regions were isolated and McCrea showed that the angular momentum contained in a region due to the original floccules was much greater than that residing in the protostar. The missing angular momentum was assumed to be taken by smaller aggregations of floccules that were captured by the protostar to form a set of planets.

In the original form of the theory, each floccule had about three times the mass of the Earth so many of them had to combine to form the giant planets. The resultant planetary aggregations contained much more angular momentum than the present planets. McCrea turned this apparent problem into an asset. As the protoplanet collapsed it would have become rotationally unstable and behaved as described by Lyttleton (1960) and shown in figure 4 . The protoplanet would have broken into two parts with a mass ratio of about 8:1. The smaller part, moving faster relative to the centre of mass, could escape from the solar system, with most of the angular momentum. In a neck between the two separating parts, small condensations would form and be retained by the larger part as a satellite family. To explain the terrestrial planets, McCrea had to assume that the fission process took place in a dense core of the protoplanet. In the inner part of the solar system, with higher escape speeds, both parts were retained and formed the pairs Earth-Mars and Venus-Mercury.

The fission of a rapidly spinning protoplanet with the formation of protosatellite droplets.

With some parameters deduced from the present solar system and others chosen to give the best possible results, the Sun plus planets and satellites system could be explained. Nevertheless the theory has severe problems. First, the floccules were unstable, with lifetimes much less than the time between floccule collisions. In response, McCrea (1988) produced a modified form of the theory where the initial condensations, now called “protoplanets”, were of Saturn's mass and stable. The initial system would not have been coplanar and indeed there could have been retrograde orbits although, with motion in a resisting medium and collisions to remove a minority population of retrograde objects, the system could have evolved to the present state. However, what is highly suspect is the idea that the angular momentum not present in the protostar must necessarily reside in a planetary system. It is much more likely that the “missing” angular momentum would reside in relative motions of protostars than in planetary systems.

Over the past 30 years a paradigm has arisen — a model that has wide acceptance and is the basis of thinking about contingent matters. This is the Solar Nebula Theory (SNT).

In the 1960s it became clear that many features of meteorites were interpretable in terms of condensation from a hot vapour, encouraging the view that early solar system material had been in a hot gaseous form. In addition, in the 1960s Victor Safronov was working on planet formation from diffuse material and in a seminal paper translated into English ( Safronov 1972 ) he summarized this work. Driven by these twin developments a new Solar Nebula Theory (SNT) quickly took off as a major research activity. It was believed that new knowledge and approaches should enable the original problems of Laplace's nebula theory to be solved.

An early worker on the SNT concluded quite quickly: “At no time, anywhere in the solar nebula, anywhere outwards from the orbit of Mercury, is the temperature in the unperturbed solar nebula ever high enough to evaporate completely the solid materials contained in interstellar grains,” ( Cameron 1978 ). Although this undermined an important raison d'être for the revival of nebula ideas, by this time the work was in full flow and proceeded without interruption.

Work on the redistribution of angular momentum has been central in the development of the SNT. Lynden-Bell and Pringle (1974) described a mechanism in which, given turbulence and energy dissipation in a disk, the disk would evolve to conserve angular momentum by inner material moving inwards while outer material moved outwards. This is tantamount to the outward transfer of angular momentum. However, it does not solve the basic angular momentum problem. Material joining the central condensation gradually spirals inwards so that it is always in a near-Keplerian orbit around the central mass. A useful way of thinking about the spin angular momentum of the Sun is to equate it to onequarter of a Jupiter mass orbiting at the Sun's equator. If the Sun could form in its present condensed configuration by material spiralling inwards, which it could not, then it would still have hundreds of times its present angular momentum. Realistically, without having much less angular momentum it could not form at all. Various mechanisms have been suggested for transferring angular momentum ( Larson 1989 ). An example is by gravitational torques due to spiral arms in the disk ( figure 5 ). To be effective this requires a massive nebula, which is undesirable for other reasons, but any mechanism giving a spiralling motion for material does not solve the problem.

The gravitational effect of a massive trailing spiral arm is to add orbital angular momentum at P and subtract it at Q.

An effective mechanism for removing angular momentum from a pre-existing star involves a loss of ionized material from the star plus a strong stellar magnetic field, both likely in a young active star. Ionized material moves outwards locked to a magnetic field line. The field rotates with the star so the ionized matter moves outwards with constant angular speed; the increased angular momentum it acquires is removed from the star. It remains attached to the field line until the kinetic pressure of the ion flow exceeds the magnetic pressure that, in the case of a dipole field, varies as r -6 . Analysis shows that, with plausible stellar winds and fields, some 90% or so of the original angular momentum can be removed in this way.

T-Tauri emission, at the deduced rate of 10 -7 M ⊙ year -1 for a period of 10 6 years, is often cited as a model for mass loss. However, spectroscopic evidence shows that T-Tauri emitted material is only lightly ionized and hence would be feebly coupled to the field. In addition, low-mass stars, for which no T-Tauri emission occurs, also spin slowly so a second mechanism would be needed for these stars.

Forming the Sun requires inward movement of material while the magnetic field mechanism for removing angular momentum requires outward movement. If a way could be found whereby the nebula core would grow and simultaneously lose highly ionized material which coupled to a strong stellar magnetic field (∼10 5 times as strong as the present solar field) then the angular momentum problem would be solved. For example, one could envisage a bipolar inflow of neutral material adding to the mass of the star with an equatorial loss of ionized material to remove angular momentum — although it seems unlikely that such a pattern would arise naturally. To summarize, while it is not possible to say that the angular momentum problem cannot be solved, it has certainly not been convincingly solved as yet although general papers on the evolution of disks appear from time to time (e.g. Pickett and Durisen 1997 ).

There are two possible planet-forming scenarios for the SNT. In the first, the nebula disk had about a solar mass and a density and temperature such that regions of it contained a Jeans critical mass and spontaneously collapsed to produce planets. This gives planets, but so many that there is a challenging disposal problem. SNT theorists no longer seriously consider this possibility.

The other scenario is with a disk of mass between 0.01 M ⊙ and 0.1 M ⊙, similar to that considered by Safronov (1972) whose work has been developed by others. Recent observed infrared excess radiation from young stars is almost certainly due to the presence of dusty disks. These observations, taken as supporting the SNT, also impose a constraint; stars older than a few million years do not show infrared excess radiation. It has been inferred, and generally accepted by the SNT community, that planet production has to be completed within 10 million years of disk formation.

What emerges is a multi-stage process:

(i)Dust within the disk settles into the mean plane. For dust grains as small as normal ISM grains this process would take too long. Weidenschilling et al. (1989) suggested that grains were sticky so that large dust particles formed, thus drastically shortening the settling time. There is controversy about the need for sticky dust but general agreement that the dust disk must form in a reasonably short time.

(ii)The dust disk is gravitationally unstable and fragments to form kilometre-size bodies, called “planetesimals”. The early nebula might have had to be turbulent to allow transfer of angular momentum but a quieter nebula is now required to allow the planetesimals to form.

(iii)Planetesimals accumulate to form planets. This is the awkward part of the process. Planets would form in the terrestrial region within 107 years but, according to Safronov's theory, it would take 1.5 × 10 8 years to produce a Jupiter core and 10 10 years or more to produce Neptune — more than twice the age of the solar system.

There are conflicting requirements here. Short formation times require a turbulent environment to bring planetesimals together quickly while, for planetesimals to amalgamate, approach speeds must be low. Stewart and Wetherill (1988) suggested conditions that would lead to runaway growth. These include local density enhancements in the disk, viscous forces to slow down planetesimals and the application of an energy equipartition principle so that larger bodies would move more slowly and hence be able to combine more readily. These are ad hoc assumptions but reduce formation times to within the allowed period — except for Uranus and Neptune. In the first programme of a recent BBC television series The Planets, an SNT theorist said, “according to our theories, Uranus and Neptune do not exist”! (iv) Planetary cores accrete gaseous envelopes. This would take about 10 5 years for Jupiter.

Satellite formation is taken as a miniature version of planet formation although angular momentum transfer is not such a serious problem in this case. The ratio (intrinsic orbital angularmomentum of the secondary body)/(intrinsic equatorial spin angular momentum of the primary body) is 7800 for Jupiter-Sun and 17 for Callisto-Jupiter so that only a modest outwards transfer of angular momentum is required.

The difficulties of angular momentum transfer and planet formation have not been convincingly resolved after 30 years of concentrated effort so the SNT per se has not progressed beyond these basic problems.

Papers are produced from time to time on planet formation, usually involving special assumptions that are not justified other than that they lead to a desired outcome. For example Pollack et al. (1996) , by numerical simulations involving the simultaneous accretion of solid planetesimals and gas, gave the formation times of Jupiter, Saturn and Uranus as a few million years. The major assumption they made was that the growing planet was in a disk of gas and planetesimals with uniform surface density and that planetesimals had to remain within the feeding zone of the planet. More recently Chambers and Wetherill (1998) have simulated the formation of terrestrial planets on the assumption of a pre-existing Jupiter and Saturn but, even then, the period covered by the simulation is an unacceptable 3 × 10 8 years. There is no model for planet formation that has commanded general support from the SNT community which describes a progression from a believable initial condition through a series of well-founded physical processes to planetary formation.

The division of planets into terrestrial and giant categories is related to the temperature of their formation. Mercury is formed where only iron and silicate grains can survive and the Mercury region would have been iron-rich. However, there is no simple explanation for the seemingly erratic pattern of densities of the terrestrial planets. Beyond the orbit of Mars, ice grains would have been stable, so allowing massive planetary cores to form that attracted extensive atmospheres.

On the question of angular momentum transfer the situation is perhaps less favourable than for planet formation. Again papers appear giving rather general results which are not, and cannot be, directly related to the problem of a slowly spinning Sun.

The SNT should yield the solar spin axis strictly perpendicular to the mean plane. An explanation for the 7° tilt could be perturbation by a passing star that disturbed the orbital planes of the planets subsequent to their formation. There are some tricky problems with this explanation. Neptune's orbit is almost perfectly circular and any perturbation that significantly changed its inclination would also have greatly changed its eccentricity. There is, however, a ready explanation for the tilts of the planetary spin axes. Planetesimals, or larger aggregations, will build up planets by collisions from random directions and spin axes could be in almost any direction, although the preponderance of direct planetary spins may require explanation.

The Capture Theory (CT) ( Woolfson 1964 ) actually predated the advent of the SNT by several years but its arrival was largely unnoticed. The basis of the CT, as first presented, is illustrated in figure 6 which shows a point-mass model, an early one of its kind, in which interpoint forces simulated the effects of gravity, gas pressure and viscosity. It depicts a tidal interaction between the Sun and a diffuse cool protostar, of mass 0.15 M ⊙ and radius 15 AU. As Jeans had deduced, the protostar distorts and eventually a filament of material escapes from the tidal tip. The model was too coarse to show filament fragmentation, but individual mass points were captured by the Sun. This model, which involved mechanisms analysed by Jeans, was free of all the criticism that had been raised against the original tidal model. The angular momentum of the planetary orbits comes from the protostar-Sun orbit and the range of perihelia given by the model, up to 38 AU, matches that of planetary orbital radii. Since the material is cold it satisfies the chemical constraints. The orbital planes are close to the Sun-protostar orbital plane although, due to protostar spin throwing material slightly out of the plane, there would be some variation of inclinations.

The disruption of a model protostar. Captured points are marked with their orbital perihelion distances (10 12 m) and eccentricities (in brackets). Escaping points are marked H (hyperbolic orbits).

It was seven years before the next CT paper was published. This paper ( Dormand and Woolfson 1971 ) improved the original model by exploiting the dramatic increase in available computer power. The paper confirmed the validity of the capture process and showed, from several simulations, that the calculated radial distributions of planetary material agreed reasonably well with that in the solar system ( figure 7 ). From the properties of the filament it seemed that six or so protoplanet condensations would be expected. Much later, by the use of a smoothed particle hydrodynamics (SPH) approach, Dormand and Woolfson (1988) modelled filament fragmentation that was found to take place much as Jeans had described.

The mass distribution from four Sun-protostar encounters together with the smoothed-out distribution for the solar system

The modelling showed that the protoplanets began moving towards the aphelia of very eccentric orbits. If the collapse time of a protoplanet was substantially less than its orbital periods (>100 years) then this would enable it to condense before being subjected to disruptive tidal forces at perihelion. The collapse of a Jupiter-like protoplanet, under the conditions of CT formation, was modelled in detail by Schofield and Woolfson (1982) . This indicated planetary collapse time as short as 20 years with reasonable model parameters.

While the planets could survive, they were subjected to considerable tidal forces during their first orbit. Consequently they would go into their final collapse stage in a distorted form that included a tidal protuberance. The characteristic of a nearly free-fall collapse is to amplify any distortion so that what began as a tidal bulge turned into a tongue or filament. Condensations within this filament would give a family of regular satellites. Williams and Woolfson (1983) found good quantitative agreement between predictions based on this model and the properties of the regular satellite families of Jupiter, Saturn and Uranus. Actually, this mechanism is similar to that suggested by Jeans for satellite formation — a small-scale version of his planet-forming process. The Jeans tidal theory had insuperable angular momentum problems for planets but not for satellite formation.

Dormand and Woolfson (1974) , investigating the effect of a resisting medium around the Sun, found that protoplanet orbits quickly round off. In one simulation, with a medium with five times Jupiter's mass, it was found that Jupiter rounded-off in 10 5 years, Saturn in 3 × 10 5 years and Uranus and Neptune in 2 × 10 6 years. The times depend on the density of the medium and were also approximately proportional to the inverse of the planet's mass. They are comfortably less than the inferred lifetimes of disks around young stars if, indeed, the resisting medium acts as a disk.

The periods of Jupiter and Saturn and those of Uranus, Neptune and Pluto are close to being commensurate. Melita and Woolfson (1996) showed that orbital evolution in a resisting medium leads to resonance locking between pairs of planets. During the evolution of the orbits with energy loss, the periods reach some commensurability. Thereafter an automatic feedback mechanism ensures a difference between the energy lost by the outer planet and its gain of energy from the inner planet such that the resonance is maintained. This does not give Bode's law — but it does explain commensurabilities that have a firmer physical foundation.

The original solar spin axis could have been in any direction. However, during the dispersal of the resisting medium — mostly by being pushed outwards by radiation pressure and the solar wind - larger solid grains would have spiralled inwards due to the Poynting-Robertson effect. As they joined the Sun, their angular momentum contribution pulled the solar spin axis towards the normal to the mean plane. Absorption of a fraction of a Jupiter mass in this way would give the spin axis nearly, but not quite, normal to the mean plane — not a problem for, but a natural consequence of, this model.

The basic CT provides an explanation of the tilts of the planetary spin axes as due to strong tidal interactions between planets that approached closely while their orbits were still highly eccentric. Woolfson (2000) describes a point-mass model of a proto-Uranus with a radius of 0.25 AU in an orbit of semi-major axis 35.6 AU and eccentricity 0.69 interacting with a model Jupiter on an orbit with semimajor axis 14.8 AU and eccentricity 0.826. Jupiter passes over Uranus with nearest approach 1.15 AU and the spin axis of Uranus changes from being normal to the original orbit to being at an angle of 98.7° to the almostunchanged new orbit. Other planetary spin-axis inclinations are readily explained in this way.

The CT is a dualistic one and offers no explanation for the slow solar spin, something that must always be of concern to the cosmogonist. To address this concern, Woolfson (1979) described a model for star formation within a galactic cluster and similar ideas have been investigated by Pongracic et al . (1991) . The model followed the evolution of a collapsing dark cool cloud within which turbulent energy steadily increased. The collision of turbulent gas elements gave compressed hot regions that cooled much faster than they re-expanded. If the free-fall time of the cool dense region was less than the coherence time for the whole cloud, during which matter was completely redistributed within it, then a star could form. Producing stars this way, with subsequent accretion to form more massive stars, gave spin rates for different classes of stars similar to those observed. Additionally, the rate of star formation and the variation of the masses of formed stars with time agreed with observations from young clusters. The predicted mass index of stars, that gives the stellar mass distribution, also agreed with observation. Given at least one star-forming model that explains solar spin in the context of the spin characteristics of all stars, it is reasonable for a dualistic theory to confine itself to the problem of planetary orbital angular momenta.

The basic CT gives planets formed from cold material, in direct almost coplanar orbits of the right dimensions and accompanied by natural satellites. However, there were problems with the original model. Dormand and Woolfson (1971) reported that, according to their model, terrestrial planets would have gone too close to the Sun and so have been disrupted.

The first orbital round-off calculations by Dormand and Woolfson (1974) were two-dimensional but later they explored a threedimensional scenario. They found, as expected, reducing orbital inclinations but they also found other, unexpected, orbital behaviour. Due to the medium's gravitational influence the eccentric orbits precessed in a complex way. The original inclined orbits did not intersect in space but, because of differential precession, pairs of orbits did occasionally intersect. Strong interactions could occur if planets arrived together near a point of intersection. A tidal interaction between a proto-Uranus and proto-Jupiter was previously described, but Dormand and Woolfson (1977) considered much stronger interactions where either one or other of the planets was ejected from the solar system or where there was a direct collision. Straightforward calculations showed that characteristic times for strong interactions were similar to those for orbital round-off.

Dormand and Woolfson took an initial system with six major planets, the present four plus two others denoted by A and B in table 1 . The characteristics of A and B are speculative but the conclusions that follow are insensitive to the parameters chosen. From table 2 , it appears that at least one major event was more likely than not in the early solar system.

Planets in the early solar system according to the Capture Theory

Planets in the early solar system according to the Capture Theory

Characteristic times for (a) planet 1 to be expelled from the system, (b) planet 2 to be expelled and (c) a collision

Characteristic times for (a) planet 1 to be expelled from the system, (b) planet 2 to be expelled and (c) a collision

Dormand and Woolfson (1977) modelled a collision between protoplanets A and B and showed that A could be expelled from the solar system while B was sheared into two parts that would have rounded off to the present orbits of the Earth and Venus. The largest terrestrial planets were interpreted as two non-volatile residues of a disrupted major planet.

The possible outcomes for the planetary satellites were that they could leave the solar system, go into independent heliocentric orbits, or be retained or captured by one or other of the B fragments. Thus, in one computational model the Earth fragment captured a satellite of A into a very stable orbit with an eccentricity of 0.4. The capture readily occurred in the presence of other bodies that removed energy from the Earth-satellite (Moon) system.

This scenario explains a curious feature of the Moon. The Moon's far side lacks large mare features, so characteristic of the near side. Since altimetry from lunar orbiters shows the presence of large basins on the far side, the usually accepted and sensible conclusion is that the solid crust was thicker on the far side so that magma was unable to reach the surface. Complicated explanations for this have been advanced yet simple tidal effects should lead to a thicker crust on the near side. Planetary collision is a straightforward explanation. Collision debris, travelling at more than 100 kms-1, would have bombarded the satellites and abraded their surfaces. A thickness of a few tens of kilometres of the Moon's original surface could have been removed in this way - but only from the planet-facing hemisphere.

Protoplanets A and B would have had small perihelia and, because of large solar tidal forces, families of large satellites. A satellite origin for Mars explains its hemispherical asymmetry. The surface features of Mars, and their relationship to its spin axis, were explained by Connell and Woolfson (1983) who also considered the early water-rich evolution of that planet. Mercury too could be an escaped satellite, originally of similar mass to Mars but so heavily abraded that its surface completely reformed and it was left with a high density (Woolfson 2000).

The CT model does not predict large satellites for the outer planets. Neptune's large satellite, Triton, is also anomalous in its retrograde orbit. Woolfson (1999) described a computational model in which Triton was an escaped satellite from the collision. This collided with an existing regular satellite of Neptune, Pluto, which was expelled into a heliocentric orbit like its present one while Triton was captured by Neptune. The collision sheared off a portion of Pluto to give its satellite, Charon.

Debris from the planetary collision would have had the greatest concentration in the inner part of the system. Near-surface volatilerich material from the colliding planets would have moved out furthest and, interacting with protoplanets near the aphelia of their original elliptical orbits, have provided a comet reservoir beyond the present planetary region. Inner larger members of this reservoir form Kuiper belt objects. Others, perturbed outwards by occasional close passages of stars or giant molecular clouds, formed the Oort cloud. Perturbations now remove Oort cloud comets and replenish them from the inner reservoir.

Debris closer in provided the early heavy bombardment within the solar system for which there is so much evidence. Those bodies that were in “safe” orbits remain today as asteroids or as captured irregular satellites.

Models of a planetary collision (Woolfson 2000) show a collision-interface temperature in excess of 3 × 10 6 K. With a wide range of temperatures available there would have been an abundance of molten and vaporized material to explain chondrule formation and rapid cooling to give unequilibrated mineral assemblages within chondrules. There are interesting isotopic anomalies in meteorites including important ones for oxygen, magnesium, neon, silicon, carbon and nitrogen. An intriguing anomaly in some meteorites is neon-E, almost pure 22 Ne, assumed to be the daughter product of 22 Na with a half-life of 2.6 years. This sodium isotope was produced by nucleosynthesis and trapped in a cold rock within a few years.

Most explanations of isotopic anomalies deal with them individually on an ad hoc basis. The excess 16O in some meteorites is ascribed to formation from 12 C in some far region of the galaxy, then transport in grains to the solar system and then exchange with normal oxygen.

One widespread anomaly within the solar system is the D/H ratio — 2 × 10 -5 for Jupiter, 1.6 × 10 -4 for the Earth, a few times the Earth value for some meteorites and 100 times the Earth value on Venus. Michael (1990) showed that the early evolution of intermediate-mass protoplanets could lead to differential loss of D and H and a D/H ratio as high as that of Venus. The consequence of a colliding planet having such a high D/H ratio was quantitatively examined by Holden and Woolfson (1995) . A triggering temperature of 3 × 10 6 K sets off a nuclear reaction chain, at first involving D but later other nuclei as the temperature rises. All the isotopic anomalies referred to above can be well explained as mixtures of processed and unprocessed material; there is no need for ad hoc explanations. For example, figure 8 shows the variation of the concentration of oxygen isotopes (and 17 F and 18 F that decay quickly to 17 O and 18 O) with temperature during the nuclear reaction. At ∼5 × 10 8 K the system explodes, the collision region expands and cools and reactions virtually cease. The oxygen content of processed material is almost pure 16 O; mixing it with unprocessed material explains the anomaly.

The variation of the concentration of stable oxygen isotopes and radioactive fluorine isotopes with temperature. ( Holden and Woolfson 1995 .)

The Solar Nebula Theory is clearly related to the original Laplace model but the Modern Laplacian Theory ( Prentice 1974 ) follows the Laplace scenario much more closely.

To solve the problem of a slowly spinning Sun, Prentice followed a suggestion of Reddish and Wickramasinghe (1969) and assumed that the Sun formed from grains of solid molecular hydrogen settling within a dense cool cloud to which they were strongly coupled. The gravitational energy of the collapse vaporized the grains to give a cloud of hydrogen of radius 10 4 R⊙ with a dense core formed by fasterfalling CNO grains. By the time the radius of the cloud equalled that of Neptune's orbit, the boundary material was in free orbit. At this stage Prentice introduced turbulent stress. Supersonic turbulence within the cloud gave density variations and less dense regions were propelled outwards from the surface by buoyancy effects in the form of needle-like elements. Motion outwards would have been fast but inward motion slower, giving a higher density in the surface region ( figure 9 ). Prentice showed that an instability would occur from time to time at the cloud equator so that material would be lost in the equatorial plane in the form of rings, much as Laplace postulated. All the rings had a similar mass, about 10 3 M ⊙, with temperatures falling off with increasing ring radius. Prentice postulated that the several rings within the orbit of Mercury were vaporized, for a terrestrial ring there would have been silicate and metal grains with total mass 4 M ⊙ and in major planet regions there would have been additional ice grains giving a total ring mass of 11–13 M ⊙.

Needle-like elements due to supersonic turbulence. Material in the shaded region slowly falls back to the surface of the proto-Sun.

Prentice presented an analysis in which solid material fell towards the axis of each ring and then came together to form a single planet or planetary core. In the major planet region the cores were sufficiently massive to accrete gas. While this gas contracted, a smaller scale version of the process, including supersonic turbulence, was taken to produce planetary systems.

This theory is by far the most complex of the current theories but despite its attention to the fine details of the system it does have severe drawbacks. The several rings within Mercury would have had an angular momentum several hundred times that of the Sun so they would not fall into the Sun. It can be shown that the rings would not have been stable and have had lifetimes much shorter than the time required for material within them to aggregate. The process by which material falls towards a ring axis is based on rather dubious mechanics requiring quite large solid bodies to be strongly coupled to a very diffuse gas. Finally, the system produced by this model would be highly coplanar and could not explain the tilt of the solar spin axis.

The current paradigm, the SNT, has not yet been successful in explaining the structure of the solar system at a very basic level. The observation that young stars are accompanied by dusty disks does not necessarily confirm the validity of the SNT because it predicts and depends upon a disk. Indeed, it is difficult to envisage a star-forming process that would not provide extraneous material that would form a disk. The important thing is not the disk but whether or not it gives planets. Nevertheless all observations are interpreted in terms of the SNT. For example, the nebula concept naturally suggests that radioactive isotopes were uniformly distributed in the early solar system. Hence, by looking for daughter products of particular decays in various types of object one can get relative times for when they became closed systems. The timings thus deduced are confusing and inconsistent — although the measurements are of good quality. Conformity reigns supreme and there is reluctance to consider that the SNT may not be valid. A more fruitful approach would be to find out what the experiments and observations are indicating rather than trying to force them into a theoretical strait-jacket. To quote Richard Feynman: “The test of all knowledge is experiment. Experiment is the sole judge of scientific ‘truth’.” This is applicable to cosmogony where “experiment” is usually observation.

By contrast the CT provides a coherent selfconsistent model where single events explain many observations and events occur in causally related sequences. Figure 10 shows a schematic flow diagram for the CT including a planetary collision. Explanations have been given for all but one of the 20 features referred to previously in this article — the existence of other planetary systems. It turns out that CT interactions would probably be common in an evolving stellar cluster. Recently there has been much discussion of the embedded phase in the evolution of a galactic cluster (Gaidos 1995) where stellar density can be of order 10 5 pc- 3 . Recent work, as yet unpublished, has not only realistically modelled planetary formation in great detail, showing the formation of single-planet or multiple- planet systems, but also indicated that the predicted frequency of planetary systems is consistent with recent observations.

A schematic representation of the Capture Theory and related events.

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Solar System

While astronomers have discovered thousands of other worlds orbiting distant stars, our best knowledge about planets, moons, and life comes from one place. The Solar System provides the only known example of a habitable planet, the only star we can observe close-up, and the only worlds we can visit with space probes. Solar System research is essential for understanding the origin and evolution of planets, along with the conditions necessary for life.

Center for Astrophysics | Harvard & Smithsonian scientists study the Solar System in many ways:

Participating in current and next-generation astronomical surveys mapping a large part of the sky. The multi-year Pan-STARRS survey has revealed many comets, asteroids, and other small Solar System bodies. Pan-STARRS Releases Largest Digital Sky Survey to the World

Studying the Sun using probes and instruments on rockets. Much of the Sun’s radiation is blocked by Earth’s atmosphere, requiring the use of stratospheric rockets such as the Hi-C mission or solar probes like the Solar Dynamics Observatory (SDO). These missions allow researchers to observe the high-energy radiation driven by the Sun’s magnetic field. Hi-C Launches to Study Sun's Corona

Observing comets in X-ray light to understand the effect the Sun has on them. Using NASA’s Chandra X-ray Observatory, astronomers study the reactions of molecules on a comet’s surface and in its tails respond when bombarded by sunlight and high-energy particles from the solar wind. Comets ISON & PanSTARRS: Comets in the ‘X’-Treme

Using the Chandra X-ray Observatory to study the icy worlds of the outer Solar System. Pluto in particular reacts relatively strongly to particles from the solar wind, to the point where its atmosphere shows up in X-ray telescopes. That behavior is similar to what we see from Mars’ atmosphere, even though Pluto is much farther from the Sun. X-ray Detection Sheds New Light on Pluto

Studying the chemical composition of Earth’s Moon and other satellites. Using data from robotic space probes, researchers have discovered water on bodies throughout the Solar System, including the Moon. While many of these places were once thought to be dry, astronomers now know there is far more water around the Solar System than just on Earth. Water on the Moon?

Looking for new and possibly unexpected worlds at the edges of the Solar System. Some astronomers think there might be a large “Planet Nine” far beyond the orbit of Pluto, based on unexplained observed motion of icy worlds in the Kuiper Belt. Planet Nine: A World That Shouldn't Exist

The Mysterious Worlds We Know Best

The heart of the Solar System is the Sun, a yellow star of moderate mass somewhere in the middle of its life cycle. That star is what drives most of the physical processes in the system, from heating Earth’s atmosphere to allow life, to gently pushing asteroids around and giving comets their tails . The rest of the Solar System is its eight major planets, five dwarf planets, hundreds of moons, and a large number of comets, asteroids, and other small bodies of rock and ice.

The extent of the Solar System is defined by the solar wind — particles driven by the Sun’s magnetic field — and gravitational influence. The heliopause is the boundary created when solar wind particles collide with interstellar gas as the Solar System moves through the galaxy. The gravitational edge is much farther and is defined by the Oort Cloud, a halo of icy debris left over from the formation of the Solar System. The Oort Cloud is the origin of many comets, and reaches nearly halfway to the nearest star.

As we’ve learned through centuries of study, the various planets, moons, and other objects in the Solar System are the products both of their common origin and of their unique history. Missions to Jupiter and Saturn have revealed that some moons might have habitable oceans beneath the ice. Comets and asteroids are the remaining planetesimals from the nebula that made the Solar System, which provide us with a look at the chemistry and physical processes that produced the planets.

Researchers use all that information to understand where we came from, and how the Solar System fits in with the thousands of known exoplanet systems. We can study the worlds of our Solar System in more detail than these alien planets, but no other star system so far resembles ours. The contrast between these systems and ours helps us understand the general rules governing planet formation and evolution.

Pluto in visible light and X-rays

Pluto's atmosphere is barely visible as seen in the optical light image captured by NASA's New Horizons spacecraft. However, in the inset X-ray picture from NASA's Chandra X-ray Observatory, the atmosphere shines out, revealing details about the environment of the outer Solar System.

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solar system

What is the solar system?

The solar system comprises 8 planets , approximately 170 natural planetary satellites (moons), and countless asteroids , meteorites , and comets .

There are eight planets in the solar system. The four inner terrestrial planets are Mercury , Venus , Earth , and Mars , all of which consist mainly of rock. The four outer planets are Jupiter , Saturn , Neptune , and Uranus , giant planets that consist mainly of either gases or ice. Pluto was considered the ninth planet until 2006, when the International Astronomical Union voted to classify Pluto as a dwarf planet instead.

Where is the solar system?

The solar system is situated within the Orion-Cygnus Arm of the Milky Way Galaxy . Alpha Centauri , made up of the stars Proxima Centauri, Alpha Centauri A, and Alpha Centauri B, is the closest star system to the solar system.

Scientists have multiple theories that explain how the solar system formed. The favoured theory proposes that the solar system formed from a solar nebula , where the Sun was born out of a concentration of kinetic energy and heat at the centre, while debris rotating the nebula collided to create the planets .

Is there life in the solar system aside from on Earth?

Europa and Enceladus , moons of Jupiter and Saturn respectively, are ice-covered rocky objects that scientists think may harbour life in the water beneath the surface. Some geological evidence points to the possibility of microorganisms on Mars .

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solar system , assemblage consisting of the Sun —an average star in the Milky Way Galaxy —and those bodies orbiting around it: 8 (formerly 9) planets with more than 210 known planetary satellites (moons); many asteroids , some with their own satellites; comets and other icy bodies; and vast reaches of highly tenuous gas and dust known as the interplanetary medium . The solar system is part of the " observable universe ," the region of space that humans can actually or theoretically observe with the aid of technology. Unlike the observable universe, the universe is possibly infinite .

The Sun, Moon , and brightest planets were visible to the naked eyes of ancient astronomers, and their observations and calculations of the movements of these bodies gave rise to the science of astronomy . Today the amount of information on the motions, properties, and compositions of the planets and smaller bodies has grown to immense proportions, and the range of observational instruments has extended far beyond the solar system to other galaxies and the edge of the known universe. Yet the solar system and its immediate outer boundary still represent the limit of our physical reach, and they remain the core of our theoretical understanding of the cosmos as well. Earth -launched space probes and landers have gathered data on planets, moons, asteroids, and other bodies, and this data has been added to the measurements collected with telescopes and other instruments from below and above Earth’s atmosphere and to the information extracted from meteorites and from Moon rocks returned by astronauts. All this information is scrutinized in attempts to understand in detail the origin and evolution of the solar system—a goal toward which astronomers continue to make great strides.

Composition of the solar system

solar system project hypothesis

Located at the centre of the solar system and influencing the motion of all the other bodies through its gravitational force is the Sun , which in itself contains more than 99 percent of the mass of the system. The planets, in order of their distance outward from the Sun, are Mercury , Venus , Earth , Mars , Jupiter , Saturn , Uranus , and Neptune . Four planets—Jupiter through Neptune—have ring systems, and all but Mercury and Venus have one or more moons. Pluto had been officially listed among the planets since it was discovered in 1930 orbiting beyond Neptune, but in 1992 an icy object was discovered still farther from the Sun than Pluto. Many other such discoveries followed, including an object named Eris that appears to be at least as large as Pluto. It became apparent that Pluto was simply one of the larger members of this new group of objects, collectively known as the Kuiper belt . Accordingly, in August 2006 the International Astronomical Union (IAU), the organization charged by the scientific community with classifying astronomical objects, voted to revoke Pluto’s planetary status and place it under a new classification called dwarf planet . For a discussion of that action and of the definition of planet approved by the IAU, see planet .

Planet Neptune

Any natural solar system object other than the Sun, a planet, a dwarf planet, or a moon is called a small body ; these include asteroids , meteoroids , and comets . Most of the more than one million asteroids, or minor planets, orbit between Mars and Jupiter in a nearly flat ring called the asteroid belt. The myriad fragments of asteroids and other small pieces of solid matter (smaller than a few tens of metres across) that populate interplanetary space are often termed meteoroids to distinguish them from the larger asteroidal bodies.

The solar system’s several billion comets are found mainly in two distinct reservoirs. The more-distant one, called the Oort cloud , is a spherical shell surrounding the solar system at a distance of approximately 50,000 astronomical units (AU)—more than 1,000 times the distance of Pluto’s orbit. The other reservoir, the Kuiper belt , is a thick disk-shaped zone whose main concentration extends 30–50 AU from the Sun, beyond the orbit of Neptune but including a portion of the orbit of Pluto. (One astronomical unit is the average distance from Earth to the Sun—about 150 million km [93 million miles].) Just as asteroids can be regarded as rocky debris left over from the formation of the inner planets, Pluto, its moon Charon , Eris, and the myriad other Kuiper belt objects can be seen as surviving representatives of the icy bodies that accreted to form the cores of Neptune and Uranus. As such, Pluto and Charon may also be considered to be very large comet nuclei. The Centaur objects , a population of comet nuclei having diameters as large as 200 km (125 miles), orbit the Sun between Jupiter and Neptune, probably having been gravitationally perturbed inward from the Kuiper belt. The interplanetary medium —an exceedingly tenuous plasma (ionized gas) laced with concentrations of dust particles —extends outward from the Sun to about 123 AU.

solar system project hypothesis

The solar system even contains objects from interstellar space that are just passing through. Two such interstellar objects have been observed. ‘Oumuamua had an unusual cigarlike or pancakelike shape and was possibly composed of nitrogen ice. Comet Borisov was much like the comets of the solar system but with a much higher abundance of carbon monoxide .

solar system project hypothesis

All the planets and dwarf planets, the rocky asteroids, and the icy bodies in the Kuiper belt move around the Sun in elliptical orbits in the same direction that the Sun rotates. This motion is termed prograde, or direct, motion. Looking down on the system from a vantage point above Earth’s North Pole , an observer would find that all these orbital motions are in a counterclockwise direction. In striking contrast, the comet nuclei in the Oort cloud are in orbits having random directions, corresponding to their spherical distribution around the plane of the planets.

The shape of an object’s orbit is defined in terms of its eccentricity . For a perfectly circular orbit, the eccentricity is 0; with increasing elongation of the orbit’s shape, the eccentricity increases toward a value of 1, the eccentricity of a parabola. Of the eight major planets, Venus and Neptune have the most circular orbits around the Sun, with eccentricities of 0.007 and 0.009, respectively. Mercury, the closest planet, has the highest eccentricity, with 0.21; the dwarf planet Pluto, with 0.25, is even more eccentric . Another defining attribute of an object’s orbit around the Sun is its inclination , which is the angle that it makes with the plane of Earth’s orbit—the ecliptic plane. Again, of the planets, Mercury’s has the greatest inclination, its orbit lying at 7° to the ecliptic; Pluto’s orbit, by comparison, is much more steeply inclined, at 17.1°. The orbits of the small bodies generally have both higher eccentricities and higher inclinations than those of the planets. Some comets from the Oort cloud have inclinations greater than 90°; their motion around the Sun is thus opposite that of the Sun’s rotation, or retrograde.


11 min read

Life in Our Solar System? Meet the Neighbors

solar system illustration

A tour of our solar system reveals a stunning diversity of worlds, from charbroiled Mercury and Venus to the frozen outer reaches of the Oort Cloud.

In between are a few tantalizing prospects for life beyond Earth ­– subterranean Mars, maybe, or the moons of giant planets with their hidden oceans – but so far, it’s just us.

“There’s nothing else in the solar system with lots of life on it,” said Mary Voytek, senior scientist for astrobiology at NASA Headquarters in Washington, D.C. “Otherwise, we would have likely detected it.”

Still, NASA continues searching the solar system for signs of life, past or present, and decades of investigation have begun to narrow down the possibilities. The broiling inner solar system seems unlikely (though the high-altitude clouds of Venus remain a possibility).

The same goes for the cloud-covered gas giants, with their crushing atmospheric pressures and seemingly bottomless depths – perhaps no solid surface at all, or if there is one, it’s no place for any living being.

The farthest provinces, with their dwarf planets and would-be comets locked in deep freeze, also seem a poor bet, though they can’t be ruled out. Same for dwarf planet Ceres in the asteroid belt, considered a possible “water world” either now or earlier in its history.

That brings us back to those tantalizing prospects. There’s Mars, now a cold, nearly airless desert, but once temperate and flowing with water.

And much hope remains out among the gas giants – not the big planets themselves, but their long list of moons. Jupiter’s Europa and Saturn’s Enceladus, despite their frozen, forbidding surfaces, are hiding vast oceans beneath the ice – among several moons with subsurface oceans.

Let’s begin the tour with our hottest planet.

Venus, a tantalizing target

Often called our “sister planet,” Venus, of similar size and structure to Earth, has critical differences: a surface hot enough to melt lead, a crushingly heavy atmosphere and an extremely volcanic geology. Venus began its existence much as Earth did, perhaps even with globe-spanning oceans. But the two planets took very different paths. A runaway greenhouse effect likely boiled off Venus’s oceans and turned the planet into a perpetual inferno – the hottest world in the solar system.

Yet Venus also exerts an irresistible pull for astrobiologists – scientists who study how life begins, its necessary ingredients and the planetary environments that it might require. Venus is a kind of negative to Earth’s positive; by studying what went so very wrong, we might learn what it takes to get life right.

“Venus gives us an example of an alternative evolution for planets,” said Vikki Meadows, an astrobiologist who heads the Virtual Planetary Laboratory in NASA’s Nexus for Exoplanet System Science.

The planet’s divergent path includes “loss of habitability, loss of water on the surface, sulfuric acid clouds, and a dense carbon-dioxide atmosphere,” Meadows said. “It’s also a warning – how terrestrial planets die.”

Venus has deep implications as well for the study of exoplanets – planets that orbit other stars. Many close to their stars are probably Venus-like worlds; Venus is a nearby laboratory showing how such planets might evolve.

Persistent, dark streaks in Venus’s clouds, where temperatures and pressure are more congenial, also prompt intriguing speculation: Could they be wind-whipped bands of microbial lifeforms? A recent study even suggested the presence of one potential life sign, a gas called phosphine, in the Venusian atmosphere. Bacteria on Earth produce it. For now, this possibility remains in the “unlikely but possible” column, scientists say; only further investigation will offer a definite answer.

Earth as an analog in search for life

As we cruise past our sole example of a life-bearing world, we might take a page from an earlier era of planetary exploration, courtesy of Carl Sagan. The astronomer and prize-winning author also was a key member of science teams for a variety of NASA’s solar system exploration missions, including Galileo.

In 1990, as the space probe zipped past Earth for a gravitational kick that would hurtle it toward the outer solar system, it turned its instruments on the home planet. Sagan’s question: Could Galileo detect signs of life on Earth?

And it did. Oxygen. Methane. A spike in the infrared part of the light spectrum, called a “red edge,” the telltale sign of reflective vegetation on the surface. Galileo even detected what today might be called a “technosignature” – a sign of intelligent life. In this case, powerful radio waves that were unlikely to come from natural sources.

“It’s vital to think about what our own planet would look like to an alien,” said Giada Arney, an astronomer and astrobiologist at NASA’s Goddard Space Flight Center in Greenbelt, Maryland. “It’s important to think about what signs of life they could actually see from space.”

Arney, who says much of her work involves “thinking about Earth as an exoplanet,” focuses on haze-shrouded worlds. As we search for signs of life around other stars, she reminds us that our own planet would have looked very different at various epochs in the deep past.

The Earth of billions of years ago, in the Archean era, might not even have been Sagan’s “pale blue dot.” Before the atmosphere became oxygen rich, Earth might occasionally have been a “pale orange dot,” Arney says, its orange haze created by complex atmospheric chemistry involving methane generated by microbes. A similar haze is found today in the atmosphere of Saturn’s moon, Titan, though in this case, not generated by life.

To find an analog of our own planet out among the stars, we must consider “not just modern Earth, but Earth through time,” she said. “The kinds of planets that could be (considered) Earth-like may be very different from modern Earth.”

Mars: Potentially habitable at some point

In a sense, the Red Planet tells a tale echoing that of Venus, but from the other side of the temperature scale. Investigations by orbiters, and rovers on the surface, confirm that Mars was once wet, with rivers, lakes and perhaps even oceans, and like Earth potentially habitable.

“The most exciting thing about Mars is that, at some point in time, 3.5 billion years ago, it’s clear the climate on Mars was more similar to Earth’s and had liquid water on its surface,” Voytek said.

Then solar wind and radiation stripped most of its atmosphere away. Its minimally active core ceased to generate a protective magnetic field. Its surface became forbiddingly cold and dry even as it was bombarded with radiation.

Habitable Worlds? 2

Is anything alive on Mars, perhaps beneath the surface, or in the frozen polar caps? Or might Earth’s future robotic explorers – one day maybe human explorers – stumble upon evidence of extinct forms from early Mars?

Two strikes against Mars, Voytek said, are its lack of available water and the absence of plate tectonics – the process on Earth that moves continents over eons and recycles buried nutrients back up to the surface.

“A lot of people think the planet may be dead – no life now because it doesn’t have that recycling going on,” she said.

Strikes in its favor might include detection of methane in the Martian atmosphere. On Earth, methane, otherwise short-lived in the atmosphere, is replenished by the metabolic action of life forms. Methane also can be produced through reactions of water and rock, but microbial life beneath the surface is another possibility.

“While surface conditions are not suitable, we may find evidence of past life, or perhaps some life that’s still hanging on,” said Morgan Cable, a researcher with the Astrobiology and Ocean Worlds Group at NASA’s Jet Propulsion Laboratory.

A newly launched Mars rover, Perseverance, is designed to collect samples of Martian soil – called regolith ­– that would be returned to Earth later for analysis. And the European Space Agency’s Rosalind Franklin lander, expected to launch in 2022, will drill beneath the Mars surface to search for signs of life.

Ocean worlds: The moons of gas giants

Our solar system’s majestic giants – Jupiter, Saturn, Uranus, Neptune – and their trains of moons might almost be considered solar systems in their own right. Some of these moons could well be habitable worlds; one of them, Titan, has a thick atmosphere, rain, rivers and lakes, though composed of methane and ethane instead of water.

We first glide toward Europa, a moon of Jupiter with an icy shell. Beneath the frozen surface, however, space probes have detected evidence of a vast ocean of liquid water. Two other Jovian moons, Ganymede and Callisto, also are likely to host subsurface oceans, though these might be sandwiched between layers of ice. That makes life less likely, Cable says.

“Europa, we think, has a nice contact between the liquid water ocean and the rocky interior,” she said. “That’s important because the energy you can generate through chemistry can be utilized by life.”

A potentially more accessible example can be found among the moons of Saturn, the next planet out. Enceladus, though tiny, also hides a liquid water ocean beneath an icy shell. But in this case, scientists know the little moon is doing something extraordinary.

“Luckily, it happens to be sending free samples from its ocean into space,” Cable says. “Enceladus is the only place in the solar system with guaranteed access to a subsurface ocean without the need to dig or drill.”

NASA’s Cassini spacecraft detected convincing evidence of hydrothermal vents on its sea floor, and jets of ocean water shoot through cracks in the moon’s surface, known as tiger stripes (Europa might have similar plumes). The material from Enceladus’s jets, in fact, forms one of Saturn’s rings.

Cassini flew through the plume, and although its instruments were not designed to analyze ocean-water samples – when it was built, the nature of these distant ocean worlds was unknown – it did pick up important clues.

These include complex organic molecules, salts similar to those in Earth’s oceans, and silicate “nanograins” and other evidence indicating the presence of hydrothermal activity.

Gases detected in the plume , hydrogen and methane, suggest enough energy is present to provide fuel for life.

“If there’s that much energy, why isn’t there life eating it?” Cable asks. So far, no one knows the answer.

“Hopefully a future mission will journey back to Enceladus and bring today’s modern sensitive instruments to this test,” she said.

Then there’s Titan.

Though smaller and with lighter gravity than Earth, Titan reminds us of our own world, if perhaps reflected through a fun-house mirror. Nitrogen dominates this moon’s atmosphere, as it does Earth’s. And Titan is the only other body in the solar system with rain, lakes and rivers – a whole hydrologic cycle in fact. Its flowing lakes and rivers are made of the hydrocarbons, methane and ethane.

Flowing water is not an option; Titan is nightmarishly cold, and water is essentially rock on its surface.

Titan also possesses a subsurface ocean of water, though deep down, and it’s unknown whether the ocean makes contact with anything from the surface. If it does, mixing with complex chemistry on the surface could provide fuel for life.

If it doesn’t, there’s another possibility. The chemical brew on the surface could power life as we don’t know it: exotic forms based on completely different components and chemical reactions.

“Titan allows us to test a completely separate hypothesis of life,” Cable said. “It has a completely different liquid on its surface.”

The extreme cold on Titan’s surface, of course, means chemistry happens very slowly if at all. That could make “weird life” far less likely.

NASA is planning a mission called “Dragonfly,” a rotary flier that will hop from place to place on the surface – and maybe solve some of Titan’s mysteries.

"The more we study our own cosmic backyard, the more surprises we find," Cable said. "And I'm excited. We'll be surprised more and more as we continue to extend our senses to the outer solar system and beyond."

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NASA’s Webb Reveals Long-Studied Star Is Actually Twins

Managed by NASA’s Jet Propulsion Laboratory through launch, Webb’s Mid-Infrared Instrument also revealed jets of gas flowing into space from the twin stars. Scientists recently got a big surprise from NASA’s James Webb Space Telescope when they turned the observatory toward a group of young stars called WL 20. The region has been studied since […]

solar system project hypothesis

Coming in Hot — NASA’s Chandra Checks Habitability of Exoplanets

This graphic shows a three-dimensional map of stars near the Sun. These stars are close enough that they could be prime targets for direct imaging searches for planets using future telescopes. The blue haloes represent stars that have been observed with NASA’s Chandra X-ray Observatory and ESA’s XMM-Newton. The yellow star at the center of […]

A yellowish star is at the center of the image. It is surrounded by a mottled disk of gas and dust that transitions from bright yellow to darker orange as you move outward. The disk stretches from about 8 o'clock to 2 o'clock and is tilted so that the nearer side is toward the viewer.

Webb Finds Plethora of Carbon Molecules Around Young Star

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Table of Contents

Grades K-2 or Adult Naive Learner

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Do you know what a planet is? A planet is a big, round world, floating in space. It can be made mostly of rock or even mostly of gas, just like the air all around us.

You, me, and everyone we know lives on a planet called Earth. Our planet is in space and goes around the Sun. Now, did you know that the Sun is a star? Well, there are also seven other planets going around our star, the Sun. The Sun and the planets are part of what we call the Solar System.

The Solar System is really old. The Sun and all of the planets came from a big cloud of stuff in space. Do you know that raindrops come from clouds in the sky? Well, it turns out that stars and even planets can come from clouds in space. Our Sun came from the middle of a big cloud in space, and the planets of our solar system also formed from that same cloud, moving around the Sun in the same kind of pattern that they follow today.

Disciplinary Core Ideas

ESS1.C: The History of Planet Earth: Some events happen very quickly; others occur very slowly, over a time period much longer than one can observe. (2-ESS1-1)

PS3.B: Conservation of Energy and Energy Transfer: Sunlight warms Earth’s surface. (K-PS3-1, K-PS3-2)

Crosscutting Concepts

Patterns in the natural world can be observed, used to describe phenomena, and used as evidence. (1-ESS1-1, 1-ESS1-2)

Big Ideas: The solar system consists of Earth and seven other planets all spinning around the Sun. Planets are big, round worlds floating in space. The Earth is a planet that goes around a much larger star called the Sun. The Sun and planets formed from a big cloud of gas and dust. The Earth, moon, Sun and planets all move in a pattern called an orbit.

Boundaries: By the end of 2nd grade, seasonal patterns of Sunrise and Sunset can be observed, described and predicted. Temperature (i.e. the Sun warms Earth) is limited to relative measurements such as warmer/cooler. (K-PS3-1)

K-5 The Science of the Sun. In this unit, students focus on the Sun as the center of our solar system and as the source for all energy on Earth. By beginning with what the Sun is and how Earth relates to it in size and distance, students gain a perspective of how powerful the Sun is compared to things we have here on Earth, and the small fraction of its energy we receive. Students also gain an understanding of how Earth relates to the other planets in the solar system. The Sun as a Star (page 17) Students identify the sun as a star. The Scale of Things (page 27). Students explore the scale of the solar system. The Size of Things (page 33) Students describe the relative sizes of the planets in the solar system by making a play-doh model. What is a year (page 37) Students act out the motion of Earth as it travels (revolves) around the Sun. Goddard Space Flight Center/NASA. https://sdo.gsfc.nasa.gov/assets/docs/UnitPlanElementary.pdf

2-12 Toilet Paper Solar System. Even in our own “cosmic neighborhood,” distances in space are so vast they are difficult to imagine. In this activity, participants build a scale model of the distances in the solar system using a roll of toilet paper. https://astrosociety.org/file_download/inline/cfdf9b2c-5947-4c19-9a23-a790ac3c7ae0

Grades 3-5 or Adult Emerging Learner

For us to learn about where we came from, we need to understand how our solar system formed.

The Sun and the planets and all of the asteroids and comets and other stuff in our solar system all formed from a really big cloud of gas and dust in space. There are clouds of gas and dust all around our galaxy. Sometimes these clouds can slowly turn into stars and planets when enough material is available and clumps together forming massive collections of ice and rock.

Do you know what kind of pattern the planets make when they go around the Sun? It kind of looks like a big circle, right? Well, when the planets were first forming from that cloud in space, the cloud itself was spinning in the same way, with the Sun forming in the middle. That’s why we see the planets moving around the Sun the way that they do today! We call that pattern of how a planet moves around the Sun an “orbit.” Have you heard of anything else that has an “orbit”? Our Moon orbits around our Earth, just like our Earth orbits around our Sun, and our entire solar system is also orbiting around the galaxy. Orbits are really important for us to learn about if we want to know where we came from.

ESS1.C: The History of Planet Earth: Local, regional, and global patterns of rock formations reveal changes over time due to earth forces, such as earthquakes. The presence and location of certain fossil types indicate the order in which rock layers were formed. (4-ESS1-1)

PS1.A: Structure and Properties of Matter: Matter of any type can be subdivided into particles that are too small to see, but even then the matter still exists and can be detected by other means. (5-PS1-1)

PS2.B: Types of Interactions: Objects in contact exert forces on each other. (3-PS2-1) The gravitational force of Earth acting on an object near Earth’s surface pulls that object toward the planet’s center. (5-PS2-1)

Patterns can be used as evidence to support an explanation. (4-ESS1-1, 4-ESS2-2) *Science assumes consistent patterns in natural systems. (4-ESS1-1)

Big Ideas: The Solar system formed through condensation from a big cloud of gas and dust. The solar system consists of Earth and seven other planets all orbiting around the Sun. The Sun, moon, and planets all move in predictable patterns called orbits. Many of these orbits are observable from Earth. The entire solar system orbits around the Milky Way galaxy.

Boundaries: In this grade band, students are learning about the different positions of the Sun, moon, and stars as observable from Earth at different times of the day, month, and year. Students are not yet defining the unseen particles or explaining the atomic-scale mechanism of condensation.

3-5 SpaceMath Problem 543: Timeline for Planet Formation. Students calculate time intervals in millions and billions of years from a timeline of events [Topics: time calculations; integers] https://spacemath.gsfc.nasa.gov/Grade35/10Page6.pdf

3-5 SpaceMath Problem 541: How to Build a Planet. Students study planet growth by using a clay model of planetessimals combining to form a planet by investigating volume addition with spheres. [Topics: graphing; counting] https://spacemath.gsfc.nasa.gov/Grade35/10Page4.pdf

3-5, 6-8, 9-12 Marsbound! In this NGSS aligned activity (three 45-minute sessions), students in grades become NASA project managers and design their own NASA mission to Mars. Mars is significant in astrobiology and more needs to be learned about this planet and its potential for life. Students create a mission that must balance the return of science data with mission limitations such as power, mass and budget. Risk factors play a role and add to the excitement in this interactive mission planning activity. Arizona State University/NASA. http://marsed.asu.edu/lesson_plans/marsbound

3-5 or 6-8 Strange New Planet. This 5E hands-on lesson (2-3 hours) engages students in how scientists gain information from looking at things from different perspectives. Students gain knowledge about simulated planetary surfaces through a variety of missions such as Earth-based telescopes to landed missions. They learn the importance of remote sensing techniques for exploration and observation. NASA /Arizona State University. http://marsed.asu.edu/strange-new-planet

4-8 SpaceMath Problem 300: Does Anybody Really Know What Time It Is? Students use tabulated data for the number of days in a year from 900 million years ago to the present, to estimate the rate at which an Earth day has changed using a linear model. [Topics: graphing; finding slopes; forecasting] https://spacemath.gsfc.nasa.gov/earth/6Page58.pdf

4-12 Meet the Planets. In this activity, kids identify the planets in the solar system, observe and describe their characteristics and features, and build a scale model out of everyday materials. They are also introduced to moons, comets, and asteroids. (Finding life Beyond Earth, page 13) NOVA . https://d43fweuh3sg51.cloudfront.net/media/assets/wgbh/nvfl/nvfl_doc_collection/nvfl_doc_collection.pdf

5-12 Exploring Meteorite Mysteries: The Meteorite Asteroid Connection (4.1). In this lesson, students build an exact-scale model of the inner solar system; the scale allows the model to fit within a normal classroom and also allows the representation of Earth to be visible without magnification. Students chart where most asteroids are, compared to the Earth, and see that a few asteroids come close to the Earth. Students see that the solar system is mostly empty space unlike the way it appears on most charts and maps. NASA . https://er.jsc.nasa.gov/seh/Exploring_Meteorite_Mysteries.pdf

5-12 Exploring Meteorite Mysteries: Building Blocks of Planets (10.1). Chondrites are the most primitive type of rock available for study. The chondrules that make up chondrites are considered the building blocks of planets. In this lesson, students experiment with balloons and static electricity to illustrate the theories about how dust particles collected into larger clusters. Students also manipulate magnetic marbles and steel balls to illustrate the accretion of chondritic material into larger bodies like planets and asteroids. NASA . https://er.jsc.nasa.gov/seh/Exploring_Meteorite_Mysteries.pdf

5-12 Exploring Meteorite Mysteries: Exploration Proposal (17.1). Exploration of the outer Solar System provides clues to the beginnings of the solar system. This is a group-participation simulation based on the premise that water and other resources from the asteroid belt are required for deep space exploration. Students brainstorm or investigate to identify useful resources, including water, that might be found on an asteroid. NASA . https://er.jsc.nasa.gov/seh/Exploring_Meteorite_Mysteries.pdf

5-12 Big Explosions and Strong Gravity. In this one-two day activity, students work in groups to examine the crushing ability of gravity, equilibrium, and a model for the creation of heavy elements through a supernova. This active lesson helps students visualize the variation and life cycle of stars. NASA http://imagine.gsfc.nasa.gov/educators/programs/bigexplosions/activities/supernova_demos.html

Grades 6-8 or Adult Building Learner

Earth is the only world that we know of that has life. All of the plants and animals and microbes and other living things on Earth have evolved here. So, for us to understand where life as we know it came from, we need to understand where our planet came from.

The Sun and the planets and all of the other stuff in our solar system all formed from a really big cloud of gas and dust in space. We call such a cloud a “nebula” and more than one of them we refer to as “nebulae.” There are nebulae all around our galaxy, and it’s from these nebulae that stars and planets form. Nebulae are massive clouds of dust and debris in space and have all the ingredients to form stars and planets. When enough material is available, it begins to stick together forming a large mass. In time, the mass can grow large enough to form a planet or even a new star.

We currently think that our solar system formed from a large nebula, perhaps after the explosion of a nearby star. Some big stars can explode, something called a supernova, and that explosion has enough energy to make the gas and dust in nearby nebulae start swirling and spinning about. As this happened, it caused a lot of the material in the nebula to fall into its center, and that’s where the Sun started forming. Meanwhile, the rest of the gas and dust in the nebula began colliding and sticking together, making little pieces of metal and rock. Those small pieces then collided with each other, forming larger pieces, which then collided with each other to form even larger ones. These were young planets, and eventually, over a long time and through many, many collisions, our eight planets were formed – Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune.

We call the pattern that the planets make when they go around the Sun an “orbit.” Well, when the planets were first forming from that cloud in space, the cloud itself was spinning in the same direction as the orbits of the planets today, with the Sun forming in the middle and also spinning in the same direction. That’s why we see the planets moving around the Sun the way that they do today!

You might also know that the Moon orbits around Earth. For something to be a moon, it needs to be in orbit around a planet. One thing that makes a planet is that a planet has to be orbiting a star. But star systems also have orbits. They orbit around their entire galaxy. So, orbits are really important for us to learn about if we want to know where we came from.

ESS1.A: The Universe and Its Stars: - Patterns of the apparent motion of the Sun, the Moon, and stars in the sky can be observed, described, predicted, and explained with models. (MS-ESS1-1) - Earth and its solar system are part of the Milky Way galaxy, which is one of many galaxies in the universe. (MS-ESS1-2)

ESS1.B: Earth and the Solar System: - The solar system consists of the Sun and a collection of objects, including planets, their moons, and asteroids that are held in orbit around the Sun by its gravitational pull on them. (MS-ESS1-2, MS-ESS1-3) - This model of the solar system can explain eclipses of the Sun and the Moon. Earth’s spin axis is fixed in direction over the short-term but tilted relative to its orbit around the Sun. The seasons are a result of that tilt and are caused by the differential intensity of sunlight on different areas of Earth across the year. (MS-ESS1-1) - The solar system appears to have formed from a disk of dust and gas, drawn together by gravity. (MS-ESS1-2)

PS1.A: Structure and Properties of Matter: All substances are made from some 100 different types of atoms, which combine with one another in various ways. Atoms form molecules that range in size from two to thousands of atoms. Pure substances are made from a single type of atom or molecule; each pure substance has characteristic physical and chemical properties that can be used to identify it. (MS-PS1-1)

Cause and effect relationships may be used to predict phenomena in natural or designed systems. (MS-PS1-4)

Big Ideas: Condensation causes rain drops to form inside of clouds, and sometimes can cause entire star systems to form inside of clouds. The Solar system formed through condensation from big clouds of gas and dust called nebulae after a supernova, or the explosion of a large star. Planets move around the Sun in an orbit, and the Solar system orbits around the entire galaxy.

Boundaries: Emphasis is on gravity as the force that holds together the solar system and Milky Way galaxy and controls orbital motions within them. (MS-ESS1-2) Does not include Kepler’s Laws of orbital motion or the apparent retrograde motion of the planets as viewed from Earth. (MS-ESS1-2)

6-8 SpaceMath Problem 542: The Late Heavy Bombardment Era. Students estimate the average arrival time of large asteroids that impacted the moon. They work with the formula for the volume of a sphere to estimate how much additional mass was added to the moon and Earth during this era. [Topics: volume of spheres; proportions] https://spacemath.gsfc.nasa.gov/earth/10Page5.pdf

6-8 SpaceMath Problem 60: When is a planet not a planet? In 2003, Dr. Michael Brown and his colleagues at CalTech discovered an object nearly 30% larger than Pluto, which is designated as 2003UB313. Is 2003UB313 really a planet? In this activity, students examine this topic by surveying various internet resources that attempt to define the astronomical term ‘planet’. [Topics: non-mathematical essay; reading to be informed] https://spacemath.gsfc.nasa.gov/astrob/2page17.pdf

6-8 SpaceMath Problem 59: Getting A Round in the Solar System! How big does a body have to be before it becomes round? In this activity, students examine images of asteroids and planetary moons to determine the critical size for an object to become round under the action of its own gravitational field. [Topics: data analysis; decimals; ratios; graphing] https://spacemath.gsfc.nasa.gov/astrob/2page20.pdf

6-8 Explore! Jupiter’s Family Secrets. This one-hour lesson for formal or informal education settings has students connecting their own life story to a cultural creation story and then to the “life” story of Jupiter, including the Big Bang as the beginning of the universe, the creation of elements through stars and the creation of the solar system. JPL /NASA. http://www.lpi.usra.edu/education/explore/solar_system/activities/birthday/

6-9 Rising Stargirls Teaching and Activity Handbook. 1.2. Art & the Cosmic Connection: (page 19). This activity engages students in space and science education by becoming explorers. Using the elements of art: line, color, texture, shape, and value: students learn to analyze the mysterious surfaces of our rocky celestial neighbors; planets, moons, comets and asteroids, as well as the Earth. Name That Planet (page 25) Students communicate their knowledge about the solar system using different modes of communication—visual, verbal, and kinesthetic. Distance Calculation (page 27) Students calculate the distances between planets using a unit of measurement that is personal to them—themselves! Rising Stargirls activities fuse science and the arts to create enlightened future scientists and imaginative thinkers. Rising Stargirls. https://static1.squarespace.com/static/54d01d6be4b07f8719d7f29e/t/5748c58ec2ea517f705c7cc6/1464386959806/Rising_Stargirls_Teaching_Handbook.compressed.pdf

6-12 Science Fiction Stories with Good Astronomy & Physics: A Topical List: Cosmology. 1.2. The Astronomical Society of the Pacific created this list of short stories and novels that use more or less accurate science and can be used for teaching or reinforcing astronomy or physics concepts including the origin of the universe. https://astrosociety.org/file_download/inline/621a63fc-04d5-4794-8d2b-38e7195056e9

6-12 Where are the Small Worlds? Through an immersive digital experience (1-2 hours), students use a simulation/model of the solar system in order to investigate small worlds in order to learn more about the solar system and its origin. The experience can be standalone or has options to track student tasks or modify the simulation as needed by the teacher. Arizona State University. https://infiniscope.org/lesson/where-are-the-small-worlds/

6-12 Astrobiology Math. This collection of math problems provides an authentic glimpse of modern astrobiology science and engineering issues, often involving actual research data. Students explore concepts in astrobiology through calculations. Relevant topics include Habitable Zones and Stellar Luminosity (page 57) and Ice or Water? (page 49). NASA . https://www.nasa.gov/pdf/637832main_Astrobiology_Math.pdf

6-12 Pocket Solar System. This activity involves making a simple model to give students an overview of the distances between the orbits of the planets and other objects in our solar system. It is also a good tool for reviewing fractions. https://astrosociety.org/file_download/inline/5c27818a-e947-46ad-a9dc-f4af157af7d8

6-12 Origins: The Universe. In this web interactive, scientists use a giant eye in the southern sky to unravel how galaxies are born. Video, pictures, and print weave information for the learner as they more deeply understand the scientific pursuit of astrobiology. UW-Madison. https://origins.wisc.edu/

7-9 SpaceMath Problem 8: Making a Model Planet. Students use the formula for a sphere, and the concept of density, to make a mathematical model of a planet based on its mass, radius and the density of several possible materials (ice, silicate rock, iron, basalt). [Topics: volume of sphere; mass = density x volume; decimal math; scientific notation] https://spacemath.gsfc.nasa.gov/astrob/Week14.pdf

Grades 9-12 or Adult Sophisticated Learner

As the physical context for life as we know it, it is important to learn about Earth’s origins so we can understand life’s origins. Although life may exist in situations other than that of a planet orbiting a star, it makes sense to explore the phenomenon of planetary system formation as a context for the emergence and evolution of life.

The story of the formation of our solar system begins in a region of space of called a “giant molecular cloud”. You might have heard before that a cloud of gas and dust in space is also called a “nebula,” so the scientific theory for how stars and planets form from molecular clouds is also sometimes called the Nebular Theory. Nebular Theory tells us that a process known as “gravitational contraction” occurred, causing parts of the cloud to clump together, which would allow for the Sun and planets to form from it.

Before gravitational contraction, the majority of the material within the giant molecular cloud that formed our solar system consisted of hydrogen and helium produced at the time of the big bang, with small amounts of heavier elements such as carbon and oxygen which were made via nucleosynthesis in prior generations of stars (see 1.1 above). The material in this giant cloud was not uniformly distributed – there were regions of higher density (more dust and gas within a specific volume of space) and regions of lower density (less gas and dust within that same volume).

Evidence from meteorites suggests that the energy produced by a nearby exploding star (a supernova) passed through a higher density region in the cloud and caused it to begin to swirl and twist about. This area of the cloud is sometimes called the pre-solar nebula (“pre” = before; “solar” = star or Sun). As molecules in the pre-solar nebula were swirling about, some of them started bumping into each other and sometimes would even stick together. As more and more of these clumps formed, gravity caused them to start sticking together and to fall into the center of the pre-solar nebula, which only caused gravity to pull even more of the material into the center of the cloud, and this is the process that’s referred to as gravitational contraction.

While all of this was happening, the action of molecules bumping into each other over and over slowly caused the pre-solar nebula to flatten into a spinning disk of dust and gas. This is sometimes called a circumstellar disk (“circum” = around; “stellar” = star) or protoplanetary disk (“proto” = first or before). Almost all of the material in the disk collected in the center, giving rise to the young Sun. However, some of the particles in the spinning disk began colliding with each other and sticking together, forming larger and larger fragments. The larger a fragment became, the more mass it had and therefore the more gravitational pull it exerted. Which in turn drew more and more material to it, and the larger it became, and so on. This process is called “accretion,” and resulted in the production of many planetesimals (small objects that build up into planets), and eventually, the planets themselves.

While the young Sun was starting to heat up in the middle of the protoplanetary disk, it warmed up the disk so much that nothing could stay solid really close to the Sun (it all melted). A little further out from the Sun, stuff like metal and rock was able to cool enough to make solid materials for forming the planets. But it was still so hot there that molecules that are often liquids or gases here on Earth (like water, ammonia, carbon dioxide and methane) couldn’t easily stick to the solid planet-forming materials. Those molecules could only really be added to planets that were a lot further from the Sun, where it was cold enough for them to clump together with the other solid stuff. This is why we have gas giant planets like Jupiter and Saturn which are very different from the rocky planets like Earth and Venus.

ESS1.A: The universe and its Stars: Nearly all observable matter in the universe is hydrogen or helium, which formed in the first minutes after the big bang. Elements other than these remnants of the big bang continue to form within the cores of stars. (HS-ESS1-2) *Nuclear fusion within stars produces all atomic nuclei lighter than and including iron, and the process releases the energy seen as starlight. Heavier elements are produced when certain massive stars achieve a supernova stage and explode. (HS-ESS1-2, HS-ESS1-3) *Stars go through a sequence of developmental stages — they are formed; evolve in size, mass, and brightness; and eventually burn out. Material from earlier stars that exploded as supernovas is recycled to form younger stars and their planetary systems.

ESS1.B: Earth and the Solar System: Kepler’s laws describe common features of the motions of orbiting objects, including their elliptical paths around the Sun. (HS-ESS1-4) *The solar system consists of the Sun and a collection of objects of varying sizes and conditions — including planets and their moons — that are held in orbit around the Sun by its gravitational pull on them. This system appears to have formed from a disk of dust and gas, drawn together by gravity.

PS1.C: Nuclear Processes: Nuclear processes, including fusion, fission, and radioactive decays of unstable nuclei, involve release or absorption of energy. The total number of neutrons plus protons does not change in any nuclear process. (HS-PS1-8)

Scientific knowledge is based on the assumption that natural laws operate today as they did in the past and they will continue to doe so in the future (HS-ESS1-2). Science assumes the universe is a vast single system in which basic laws are consistent. (HS-ESS1-2)

Big Ideas: The phenomenon of planetary system formation serves as a context for the emergence and evolution of life. A cloud of gas and dust in space is called a “nebula”. The Nebular Theory is the scientific theory for how stars and planets form from molecular clouds and their own gravity. The majority of the material within the giant molecular cloud that formed our solar system consisted of hydrogen and helium produced at the time of the big bang. Nuclear fusion within stars forms heavier elements under extreme pressure and temperature. The larger the star, the heavier the elements that can be produced through fusion and Supernova. Heavier elements were also made via nucleosynthesis. The circumstellar disk gave rise to the young Sun.

Boundaries: Emphasis is on the way nucleosynthesis, and therefore the different elements created, varies as a function of the mass of a star and the stage of its lifetime.(HS-ESS1-3) Does not include details of the atomic and subatomic processes involved with the Sun’s nuclear fusion. (HS-ESS1-1)

9-10 Voyages through Time: Cosmic Evolution. This comprehensive integrated curriculum includes the universe, the totality of all things that exist, origins (beginning with an explosion of space and time and the expansion of a hot, dense mass of elementary particles and photons), and how it has evolved over billions of years into the stars and galaxies we observe today. Sample lesson on the website and the curriculum is available for purchase. SETI . http://www.voyagesthroughtime.org/cosmic/index.html

9-11 SpaceMath Problem 302: How to Build a Planet from the Inside Out. Students model a planet using a spherical core and shell with different densities. The goal is to create a planet of the right size, and with the correct mass using common planet building materials. [Topics: geometry; volume; scientific notation; mass=density x volume] https://spacemath.gsfc.nasa.gov/astrob/6Page72.pdf

9-12 Genesis Science Modules: Cosmic Chemistry: Planetary Diversity. The goal of this module is to acquaint students with the planets of the solar system and some current models for their origin and evolution. The lessons in the Genesis Science Modules challenge students to look for patterns in data, to generate observations, and critically analyze where the data does not fit with the current nebular model. This mini-unit reveals the essence of scientific research and argument within the context of the formation of solar systems. JPL /NASA http://genesismission.jpl.nasa.gov/educate/scimodule/PlanetaryDiversity/index.html

9-12 A101 Slide Set: From Supernovae to Planets. This slide set explains the discoveries of the SOFIA mission and the implications of the new data explaining how supernovae and dust push planet formation and how this is the physical context for life. SOFIA /NASA https://slideplayer.com/slide/8679314/ Teacher’s Guide:


11-12 SpaceMath Problem 305: From Asteroids to Planets. Students explore how long it takes to form a small planet from a collection of asteroids in a planet-forming disk of matter orbiting a star based on a very simple physical model. [Topics: integral calculus] https://spacemath.gsfc.nasa.gov/astrob/6Page82.pdf

11-12 SpaceMath Problem 304: From Dust Balls to Asteroids. Students calculate how long it takes to form an asteroid-sized body using a simple differential equation based on a very simple physical model. [Topics: integral calculus] https://spacemath.gsfc.nasa.gov/astrob/6Page81.pdf

11-12 SpaceMath Problem 303: From Dust Grains to Dust Balls. Students create a model of how dust grains grow to centimeter-sized dust balls as part of forming a planet based on a very simple physical model. [Topics: integral calculus] https://spacemath.gsfc.nasa.gov/astrob/6Page80.pdf

Storyline Extensions

The planets are named after stories from long ago:.

Our planets are named Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Seven of the planets are named after gods from Roman mythology. These are Mercury, Venus, Mars, Jupiter, Saturn, and Neptune. However, Uranus is a name from Greek mythology (Uranus was the god of the sky). Also, the name for our planet, Earth, comes from Old English, and appears to have come from people who lived in Northern Europe long ago.

Our location in the galaxy:

Our Milky Way galaxy is really big! If we could travel outside of the galaxy and look back at it, it would look like a big disk of dust and gas and stars, with a big bulging sphere of stars near the middle. The disk of the galaxy is about 100,000 lightyears in diameter. That means that it takes light about 100,000 years to travel from one side to the other. Our little solar system (little in comparison to the galaxy, that is) lies about 30,000 lightyears from the center of the galaxy. Just as moons orbit around planets, and planets orbit around stars, star systems also orbit around the center of the galaxy. Our own solar system is traveling through the galaxy at over 500,000 miles per hour! And our very long orbit around the galaxy takes almost 250 million years! But we’re not alone out here. There are lots of other stars and other worlds in the galaxy. Our best estimates right now are that there are about 100-400 billion stars in the Milky Way. And, even though we’ve only just begun finding exoplanets, some astronomers believe there is evidence for more planets than stars in the milky way and other galaxies. That’s an awful lot of worlds!

problem statement and hypothesis

solar system project hypothesis

NASA’s Juno Gets a Close-Up Look at Lava Lakes on Jupiter’s Moon Io

Juno spacecraft captured two volcanic plumes rising above the horizon of Jupiter’s moon Io

The JunoCam instrument aboard NASA’s Juno spacecraft captured two volcanic plumes rising above the horizon of Jupiter’s moon Io. The image was taken Feb. 3 from a distance of about 2,400 miles (3,800 kilometers).

Infrared imagery from the solar-powered spacecraft heats up the discussion on the inner workings of Jupiter’s hottest moon.

New findings from NASA’s Juno probe provide a fuller picture of how widespread the lava lakes are on Jupiter’s moon Io and include first-time insights into the volcanic processes at work there. These results come courtesy of Juno’s Jovian Infrared Auroral Mapper (JIRAM) instrument, contributed by the Italian Space Agency, which “sees” in infrared light. Researchers published a paper on Juno’s most recent volcanic discoveries on June 20 in the journal Nature Communications Earth and Environment.

Io has intrigued the astronomers since 1610, when Galileo Galilei first discovered the Jovian moon, which is slightly larger than Earth's Moon. Some 369 years later, NASA’s Voyager 1 spacecraft captured a volcanic eruption on the moon. Subsequent missions to Jupiter , with more Io flybys, discovered additional plumes — along with lava lakes . Scientists now believe Io, which is stretched and squeezed like an accordion by neighboring moons and massive Jupiter itself, is the most volcanically active world in the solar system. But while there are many theories on the types of volcanic eruptions across the surface of the moon, little supporting data exists.

Infrared data

Infrared data collected Oct. 15, 2023, by the JIRAM instrument aboard NASA’s Juno shows Chors Patera, a lava lake on Jupiter’s moon Io. The team believes the lake is largely covered by a thick, molten crust, with a hot ring around the edges where lava from Io’s interior is directly exposed to space.

In both May and October 2023, Juno flew by Io, coming within about 21,700 miles (35,000 kilometers) and 8,100 miles (13,000 kilometers), respectively. Among Juno’s instruments getting a good look at the beguiling moon was JIRAM.

Designed to capture the infrared light (which is not visible to the human eye) emerging from deep inside Jupiter, JIRAM probes the weather layer down to 30 to 45 miles (50 to 70 kilometers) below the gas giant’s cloud tops. But during Juno’s extended mission, the mission team has also used the instrument to study the moons Io , Europa , Ganymede , and Callisto. The JIRAM Io imagery showed the presence of bright rings surrounding the floors of numerous hot spots.

“The high spatial resolution of JIRAM’s infrared images, combined with the favorable position of Juno during the flybys, revealed that the whole surface of Io is covered by lava lakes contained in caldera-like features,” said Alessandro Mura, a Juno co-investigator from the National Institute for Astrophysics in Rome. “In the region of Io’s surface in which we have the most complete data, we estimate about 3% of it is covered by one of these molten lava lakes.” (A caldera is a large depression formed when a volcano erupts and collapses.)

This animation is an artist’s concept of Loki Patera, a lava lake on Jupiter’s moon Io, made using data from the JunoCam imager aboard NASA’s Juno spacecraft. With multiple islands in its interior, Loki is a depression filled with magma and rimmed with molten lava.

Fire-Breathing Lakes

JIRAM’s Io flyby data not only highlights the moon’s abundant lava reserves, but also provides a glimpse of what may be going on below the surface. Infrared images of several Io lava lakes show a thin circle of lava at the border, between the central crust that covers most of the lava lake and the lake’s walls. Recycling of melt is implied by the lack of lava flows on and beyond the rim of the lake, indicating that there is a balance between melt that has erupted into the lava lakes and melt that is circulated back into the subsurface system.

“We now have an idea of what is the most frequent type of volcanism on Io: enormous lakes of lava where magma goes up and down,” said Mura. “The lava crust is forced to break against the walls of the lake, forming the typical lava ring seen in Hawaiian lava lakes. The walls are likely hundreds of meters high, which explains why magma is generally not observed spilling out of the paterae” — bowl-shaped features created by volcanism — “and moving across the moon’s surface.”

JIRAM data suggests that most of the surface of these Io hot spots is composed of a rocky crust that moves up and down cyclically as one contiguous surface due to the central upwelling of magma. In this hypothesis, because the crust touches the lake’s walls, friction keeps it from sliding, causing it to deform and eventually break, exposing lava just below the surface.

Never Miss a Discovery

An alternative hypothesis remains in play: Magma is welling up in the middle of the lake, spreading out and forming a crust that sinks along the rim of the lake, exposing lava.

“We are just starting to wade into the JIRAM results from the close flybys of Io in December 2023 and February 2024,” said Scott Bolton, principal investigator for Juno at the Southwest Research Institute in San Antonio. “The observations show fascinating new information on Io’s volcanic processes. Combining these new results with Juno’s longer-term campaign to monitor and map the volcanoes on Io’s never-before-seen north and south poles, JIRAM is turning out to be one of the most valuable tools to learn how this tortured world works.”

Juno executed its 62nd flyby of Jupiter — which included an Io flyby at an altitude of about 18,175 miles (29,250 kilometers) — on June 13. The 63rd flyby of the gas giant is scheduled for July 16.

More About the Mission

NASA’s Jet Propulsion Laboratory, a division of Caltech in Pasadena, California, manages the Juno mission for the principal investigator, Scott Bolton, of the Southwest Research Institute in San Antonio. Juno is part of NASA’s New Frontiers Program, which is managed at NASA’s Marshall Space Flight Center in Huntsville, Alabama, for the agency’s Science Mission Directorate in Washington. The Italian Space Agency (ASI) funded the Jovian InfraRed Auroral Mapper. Lockheed Martin Space in Denver built and operates the spacecraft.

More information about Juno is available at:


News Media Contact

Jet Propulsion Laboratory, Pasadena, Calif.


[email protected]

Karen Fox / Charles Blue

NASA Headquarters

202-385-1600 / 202-802-5345

[email protected] / [email protected]

Southwest Research Institute, San Antonio


[email protected]

Caltech Magazine

Solar Power at All Hours: Inside the Space Solar Power Project

solar system project hypothesis

Caltech researchers hope to harness the sun’s energy and power the planet from 300 miles above.

By ker than.

On a cool, clear evening in May 2023, Caltech electrical engineer Ali Hajimiri and four members of his lab gathered on the roof of the Gordon and Betty Moore Laboratory of Engineering to await a signal from the heavens.

In preparation, the researchers had strewn portable floodlights across the floor and erected a collapsible canopy in a corner of the roof to shelter instruments and monitors stacked atop a small folding table. Two antennae perched nearby on heavy-duty tripods, their electronic gazes steadily tracking an invisible target drifting more than 300 miles overhead. The signal—if it came—would arrive in the form of a weak microwave beam transmitted from the Space Solar Power Demonstrator (SSPD-1), a 110-pound set of Caltech payloads that had launched into space five months earlier aboard a SpaceX rocket on the Momentus Vigoride-5 spacecraft. SSPD-1 is the first spaceborne prototype from Caltech’s Space Solar Power Project (SSPP).

The source of the evening’s anticipated signal was the Microwave Array for Power-transfer Low-orbit Experiment (MAPLE), a series of flexible lightweight microwave power transmitters built in Hajimiri’s lab that make up one of SSPD-1’s three main experiments. Hajimiri, an SSPP co-director and principal investigator, and others have estimated that space-based harvesters founded on the technology demonstrated by MAPLE could one day provide access to eight times as much solar energy on average as their terrestrial counterparts.

“This is a system able to provide stable power over time,” adds aerospace engineer Sergio Pellegrino, an SSPP co-director and principal investigator, whose lab worked on SSPD-1’s ultralight deployable structure. “There is potential for a breakthrough in the provision of clean renewable energy.”

That harvested energy could then be dispatched to any place on Earth, including areas devastated by war or natural disaster, or regions with poor energy infrastructure, explains nanophotonic and solar-energy expert Harry Atwater, who is also one of SSPP’s principal investigators. “You could imagine in places like that, where you want to bring power to a large city, you could immediately do that without building a large power grid,” Atwater says. “The thing that’s really transformative about space solar power is that, unlike solar power on Earth, it has potential to eliminate the need for storage. You get power continuously, 24 hours a day, and you don’t have to come up with day-to-night storage, like in the form of batteries, or season-to-season storage.”

The May rooftop experiment was long planned but had to be rapidly executed on short notice. The Caltech engineers had just three hours to haul their equipment to the roof, having received the green light from Momentus to conduct their experiment only that evening. “It wasn’t like we had everything set up, and we were just sitting around waiting,” recalls Raha Riazati, an undergraduate researcher in Hajimiri’s lab who designed the receiving antennae for the test. “It was definitely a mad rush, and I remember being very stressed and thinking, ‘I really hope we can get everything ready in time, because if we don’t, this will be a wasted opportunity.’ It was pretty nerve-racking.”

Around 10 p.m., the team paused its various activities to huddle around a single monitor. From where they were on the rooftop, there was only a seven-minute window in which the signal could be detected, and the countdown had begun. “In the beginning, we weren’t seeing the signal,” Hajimiri recalls.

A minute passed. Then a few more. Before the team’s apprehension could turn into alarm, a digital peak heralding the signal appeared on-screen. The growing spike carried the precise power level and frequency shift the team had predicted based on the beam’s travel from orbit. “It took a few seconds for it to sink in that, yes, this is happening,” Hajimiri notes. MAPLE’s demonstration marked the first time that power was transmitted and received in space, directed toward Earth, and then detected, according to Hajimiri. “The level of energy, of course, is very small at this point,” he says. “This was mostly about detection, but it is a first step.”

Despite lasting only 90 seconds, the microwave signal detection at Caltech on May 22 marked a major milestone toward realizing a century-long dream to harvest solar energy in space and beam it wirelessly down to Earth.

The experiments on SSPD-1 are designed to test key technologies that could enable Caltech’s unique take on this vision, which involves deploying a fleet of nimble, modular spacecraft, each equipped with a flexible ultralight membrane that can function as both solar panel and energy transmitter. Like a starling murmuration, the spacecraft will come together as a “flock” to create enormous floating power stations above Earth, but each spacecraft will also be able to operate independently.

In addition to MAPLE, SSPD-1 has two other main experiments: Deployable on-Orbit ultraLight Composite Experiment (DOLCE), a structure designed and built in Pellegrino’s lab that measures 6 feet by 6 feet and that will test the architecture, packaging scheme, and deployment mechanisms of the spacecraft; and ALBA (Italian for “dawn”), a collection of 32 different types of photovoltaic (PV) cells, some built in Atwater’s lab and at other Caltech labs, and some sourced from other researchers around the world. Atwater’s team is conducting tests to determine which of these cells operate best in the punishing environment of space. In total, about 35 faculty members, postdocs, graduate students, and undergrads at Caltech worked on the SSPD-1 project.

With the May test successfully concluded, Hajimiri’s team erupted in cheers and high-fived one another before beginning the laborious task of dismantling the equipment and clearing off the roof. It was only later when she was back in her room that Riazati could reflect on the night’s achievement. “That was when it clicked in my head that this project I’d been working on for over a year and a half had finally worked, and that we’d gotten this groundbreaking result,” she says. “I was like, ‘Wow, that was pretty awesome.’”

solar system project hypothesis

Left to right: Sergio Pellegrino, Harry Atwater, and Ali Hajimiri, the principal investigators of the Space Solar Power Project.

A Long Journey

The idea of space-based solar power dates back to as early as 1923 when Russian theorist Konstantin Tsiolkovsky proposed using mirrors in space to concentrate a strong beam of sunlight down to Earth. Years later, the science fiction writer Isaac Asimov, in his 1941 short story “Reason,” imagined solar-powered satellites beaming energy in the form of invisible microwaves to Earth and human settlements across the solar system. Learning of this for the first time, Asimov’s robot character asks, “Do you expect me to believe any such complicated, implausible hypothesis as you have just outlined? What do you take me for?”

The first patent for a microwave-based method of transmitting power from orbit was granted in 1973 to NASA engineer Peter Glaser, who also outlined the engineering concepts for his proposal in an influential 1968 Science article titled “Power from the Sun: Its Future.” Glaser’s ambitious plan called for massive satellites equipped with solar-panel arrays capable of harvesting sunlight in space, converting the sunlight into energy, and then beaming that energy wirelessly toward 5-mile-wide receiving antennae on Earth. “It is an incredibly complex piece of infrastructure. It needed to be giant to make sense,” Pellegrino says.

Glaser also articulated the rationale for harvesting solar energy in space: high above the atmosphere where the sun never sets, sunlight can be collected around the clock, irrespective of clouds, weather, or nightfall. This prospect so intrigued the U.S. government that it spent $20 million to investigate the technology (inspired by the shift toward reducing dependence on fossil fuel due to the oil crisis of the 1970s), only to deem it too complex and expensive a few years later. But thanks to recent advances in photovoltaics, materials engineering, and electronics, combined with decreasing launch costs and urgent calls for more clean energy sources, space-based solar power is enjoying a new moment in the sun.

The European Space Agency recently approved two concept studies of a European space-solar network as part of its SOLARIS initiative, which aims to establish the technical, political, and programmatic viability of space-based solar power. The China Academy of Space Technology plans to launch its own power-beaming satellite prototype by 2028, and the U.S. Naval Research Laboratory recently tested technology to convert sunlight into microwaves in space, although it did not actually transmit that energy anywhere. India, Japan, and the United Kingdom have also expressed interest in developing their own technologies.

Of these global efforts, Caltech’s is arguably the furthest along: SSPD-1 is the first space-based solar power demonstrator to reach orbit and demonstrate wireless energy transfer in space. “Demonstration of wireless power transfer in space using lightweight structures is an important step toward space solar power and broad access to it globally,” Atwater says.

The Caltech Concept

The Caltech effort began after philanthropist Donald Bren, chairman of Irvine Company and a life member of the Caltech community, first learned about the potential for space-based solar energy manufacturing as a young man after reading an article in Popular Science magazine. Intrigued by the potential for space solar power, Bren approached Caltech’s then-president Jean-Lou Chameau in 2011 to discuss the creation of a space-based solar power research project. In the years to follow, Bren and his wife, Brigitte Bren, a Caltech trustee, agreed to make a series of donations (which ultimately amounted to a total commitment of $100 million) through the Donald Bren Foundation to fund the project and endowed professorships. “The hard work and dedication of the brilliant scientists at Caltech have advanced our dream of providing the world with abundant, reliable, and affordable power for the benefit of all humankind,” Donald Bren says.

In addition to the support received from the Brens, Northrop Grumman Corporation also provided Caltech $12.5 million between 2014 and 2017 through a sponsored research agreement that aided the development of technology and advancement of science for the project.

Bren charged Caltech with making solar power feasible and—equally as important—economically viable. The Institute responded by asking Hajimiri, Pellegrino, and Atwater’s teams to invent the necessary new technologies, materials, and manufacturing processes. “You could characterize our work at Caltech as a component-led revolution,” Atwater says. “In the solar-energy-technology part of SSPP, we need to achieve a kind of photovoltaic technology that does not exist today that is ultralight, efficient, low cost, and resistant to radiation.”

solar system project hypothesis

Space Solar Power Project transmitters are designed to direct power toward Earth using the physical phenomenon of interference.

The Brens approached Hajimiri due to his work in electronics and photonics that laid the groundwork for 5G communications and radar sensors in cars. But at first Hajimiri had reservations. “The way that space solar power had been envisioned previously, it was not practical at all,” Hajimiri remembers. Atwater had a similar initial reaction.

“It took me a long time to overcome my own skepticism,” he recalls. “But it’s one of those things: you start thinking about how you might do it, and it really gnaws at you, and you can’t let go of it.”

The more Atwater, Hajimiri, and Pellegrino began chewing on the problem together, the more realistic it began to seem. “It became clear that we needed to replace the basic components that other people had imagined being part of the system,” Atwater says. “If you change the components, suddenly you can have a much higher power-to-weight ratio, and that reduces the mass to orbit and, therefore, the launch cost.”

The trio eventually came up with a design plan now known as the Caltech Concept, which is radically different from the one Glaser outlined decades earlier. “The Caltech Concept is not a giant monolithic object. It is a collection of spacecraft—many, many, many spacecraft—that are all identical,” Pellegrino explains. “They’re synchronized, and they provide energy to Earth.”

As initially envisioned, each spacecraft will carry a square-shaped membrane measuring roughly 200 feet on each side. The membrane is made up of hundreds or thousands of smaller units, called tiles, which have PV cells embedded on one side and a microwave transmitter on the other. The tiles are arranged into long strips suspended between four telescoping booms that provide structure and tension for the membrane. The strips are folded and coiled for launch and unfurl once the spacecraft reaches orbit. Each spacecraft would operate and maneuver in space on its own but also possess the ability to hover in formation and configure an orbiting power station spanning several kilometers with the potential to produce about 1.5 gigawatts of continuous power.

Since each power station would consist of individual spacecraft operating collectively, there would be no need for complex wiring and a heavy central antenna. “This is a paradigm shift,” Hajimiri says. “The analogy I use is going from one big elephant to an army of ants.”

Ants Versus Elephant

The ability of each tile to transmit energy wirelessly through space is based on a physical phenomenon called interference, which arises due to the wave-like nature of light. To understand interference, Hajimiri says, imagine sitting at the edge of a pond and putting both of your hands in the water and moving them up and down. Each hand makes a wave, but because of how the waves and their energy interact, some waves will be bigger and others will be smaller. Like big waves in water, synchronized light waves overlap, their peaks meet and create a greater peak; this is called constructive interference.

“If you have multiple sources that are operating in concert, in the same phase, you can actually direct energy in one direction so all of them will only add in one direction and will cancel each other out in all other directions,” Hajimiri explained in a recent video that accompanied an announcement of SSPD-1’s launch. “The same way that a magnifying glass can focus light into a small point, you can actually control the timing of this in such a way that you can focus all of that energy in a smaller area than the area that you started with.”

The upshot of being able to control direction by manipulating timing is that no moving mechanical parts are required, and energy can be redirected in mere nanoseconds to a receiver in space or on the ground. “It’s as if you have an army of ants that are working in perfect synchronization, and each one of them contributes a little bit of energy, but as a whole they send it to the right place,” Hajimiri said in the video.

Engineering “Backward”

If the Caltech Concept is ever to be realized, everything about current PV-cell technology will need to be rethought and vastly improved, Atwater says. The PV cells used in space to power satellites and the International Space Station are about 32 percent efficient at converting sunlight to energy. They weigh about 2.1 kilograms per square meter and have a power-to-weight ratio, or specific power, of 200 watts per kilogram. They cost about $10,000 per square meter to manufacture.

SSPP aims to develop a PV cell with an efficiency level of 25 percent that is 100 times less expensive ($100 per square meter), 40 times lighter (0.05 kilograms per square meter), and with a specific power 33 times greater (6.6 kilowatts per kilogram) than current space PV cells. Another way to think about it: An SSPP spacecraft with a 60-meter-by-60-meter surface area made using today’s space PV-cell technology would cost $36 million and weigh nearly 9,000 pounds, or almost as much as a Ford F-450 truck. With the ultra-lightweight PV-cell technology Atwater envisions, it would cost just $450,000 and weigh about 300 pounds, or about as much as an IKEA three-seat sofa.

To achieve these lofty goals, Atwater’s team is investigating novel manufacturing techniques and exotic materials for the creation of its PV cells. “We’re inverting the normal methodology that you use to make solar panels,” Atwater says. “What we said was, ‘We have to make this very cheap, so we’re going to start with the economic analysis that says it has to cost $100 a square meter. Then we’re going to design the cell-manufacturing process, and out of that we’re going to make the cell.’ It’s completely backward.”

Atwater’s team is using a simple method called spalling to create highly efficient PV cells made from gallium arsenide and indium phosphide. Spalling involves peeling a layer of ceramic material from a bulk crystal to create a film layer that is thinner than the thinnest piece of plastic found in your home. Crucially, spalling doesn’t require a vacuum environment, and the PV cells can be baked in a furnace comparable to a consumer oven. “We’re using the same kind of processing that we teach Caltech firstyear students,” Atwater says. “It’s very inexpensive.”

The ALBA tests, which began in June, have so far shown that gallium arsenide cells perform well in space even though they do not have a protective coating. “It’s a point of validation that these low-cost cells we made with processing that you can do in your kitchen will work in space, and they work nicely,” Atwater says.

The two other solar-cell technologies being tested by ALBA include PV cells made from thin-film perovskite, and semiconductors known as quantum dots that utilize nanotechnology and quantum mechanics to convert sunlight to energy. In total, 32 PV-cell samples, each made using a variation of one of these three technologies, make up ALBA’s science payload. “We’re testing their current voltage characteristics and how they perform as a function of temperature,” Atwater explains. “This is the first time these kinds of PV cells have ever been tested in space.”

Delivery and Deployment

The third SSPD-1 experiment, DOLCE, demonstrates the packaging and deployment mechanism for the flexible membranes populated with PV and radio-frequency components that, although not included in DOLCE, will be required in a complete space-based solar-power spacecraft. The first stage in the deployment occurred in May 2023, and the process was completed in September. Pellegrino’s team has developed a novel method called slip wrapping that packages large membranes tightly and efficiently by first dividing them into precise strips that are compactly folded into a star shape and then carefully wrapped around a central axis to form a tight cylindrical package. DOLCE is the first engineering-scale demonstration of the slip-wrapping technique in space.

The edges of the membranes aboard SSPD-1 are reinforced with deployable “longerons” that are the result of extensive research in Pellegrino’s lab. They consist of two tape-measure-like sections of ultrathin composite material made of glass and carbon fiber that are bonded together on one side. The longerons’ curved cross section is thin walled and provides high bending stiffness, which allows the longerons to store energy during packaging; this “strain energy” is then used to self-deploy the structure in space.

When combined, the longerons and the slip-wrapping technique allow each membrane strip to be tightly stowed in a cylindrical mechanism. Deployment occurs in two steps: first, the folded strips with the membrane uncoil from a central spool into a star shape, and then the star unfolds into a flat structure. The uncoiling step is controlled by a motor, whereas the unfolding process is driven by the stored strain energy of the longerons. “We discovered that this is a highly repeatable process, very robust,” Pellegrino says. “In fact, we’ve never broken any of the structures we built by folding and unfolding them, but it took much study to develop the technique for packaging, and even more study to acquire the courage to let the structure deploy by itself.”

Looking back, Hajimiri acknowledges it has been a long journey to get to this point, and there is still much ground—and space—left to cover. “Movies often portray a direct path to success. The real path to success has a lot of meandering and a lot of dead ends. But people don’t talk about that. There are a lot of things that can go wrong. The key is to learn from each of them and take the next step.”

Harry Atwater is the Otis Booth Leadership Chair of the Division of Engineering and Applied Science; Howard Hughes Professor of Applied Physics and Materials Science; director of the Liquid Sunlight Alliance; and a principal investigator of the SSPP.

Ali Hajimiri is the Bren Professor of Electrical Engineering and Medical Engineering, and a co-director and a principal investigator of the SSPP. Sergio Pellegrino is the Joyce and Kent Kresa Professor of Aerospace and Civil Engineering and a senior research scientist at JPL, which Caltech manages for NASA. He is also a co-director and a principal investigator of the SSPP.

Sergio Pellegrino is the Joyce and Kent Kresa Professor of Aerospace and Civil Engineering and a senior research scientist at JPL, which Caltech manages for NASA. He is also a co-director and a principal investigator of the SSPP.


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The fifth Planetary Defense Interagency Tabletop Exercise focused on an asteroid impact scenario designed by NASA JPL’s Center for Near Earth Object Studies.

A large asteroid impacting Earth is highly unlikely for the foreseeable future. But because the damage from such an event could be great, NASA leads hypothetical asteroid impact “tabletop” exercises every two years with experts and decision-makers from federal and international agencies to address the many uncertainties of an impact scenario. The most recent exercise took place this past April, with a preliminary report being issued on June 20.

Making such a scenario realistic and useful for all involved is no small task. Scientists from the Center for Near Earth Object Studies ( CNEOS ) at NASA’s Jet Propulsion Laboratory in Southern California, which specializes in the tracking and orbital determination of asteroids and comets and finding out if any are hazards to Earth, have played a major role in designing these exercises since the first 11 years ago.

“These hypothetical scenarios are complex and take significant effort to design, so our purpose is to make them useful and challenging for exercise participants and decision-makers to hone their processes and procedures to quickly come to a plan of action while addressing gaps in the planetary defense community’s knowledge,” said JPL’s Paul Chodas, the director of CNEOS.

This year’s scenario: A hypothetical asteroid, possibly several hundred yards across, has been discovered, with an estimated 72% chance of impacting Earth in 14 years. Potential impact locations include heavily populated areas in North America, Southern Europe, and North Africa, but there is still a 28% chance the asteroid will miss Earth. After several months of being tracked, the asteroid moves too close to the Sun, making further observations impossible for another seven months. Decision-makers must figure out what to do.

Leading the exercise was NASA’s Planetary Defense Coordination Office ( PDCO ), the Federal Emergency Management Agency Response Directorate, and the Department of State Office of Space Affairs. Over the course of two days in April, participants gathered at the Johns Hopkins Applied Physics Laboratory in Laurel, Maryland, which hosted the event, to consider the potential national and global responses to the scenario.

“This was a very successful tabletop exercise, with nearly 100 participants from U.S. government agencies and, for the first time, international planetary defense experts,” said Terik Daly from APL, who coordinated the exercise. “An asteroid impact would have severe national and international ramifications, so should this scenario play out for real, we’d need international collaboration.”

In real life, CNEOS calculates the orbit of every known near-Earth object to provide assessments of future potential impact hazards in support of NASA’s planetary defense program. To make this scenario realistic, the CNEOS team simulated all the observations in the months leading up to the exercise and used orbital determination calculations to simulate the probability of impact.

“At this point in time, the impact was likely but not yet certain, and there were significant uncertainties in the object’s size and the impact location,” said Davide Farnocchia, a navigation engineer at JPL and CNEOS, who led the design of the asteroid’s orbit. “It was interesting to see how this affected the decision-makers’ choices and how the international community might respond to a real-world threat 14 years out.”

Preparation, planning, and decision-making have been key focal points of all five exercises that have taken place over the past 11 years. For instance, could a reconnaissance spacecraft be sent to the asteroid to gather additional data on its orbit and better determine its size and mass? Would it also be feasible to attempt deflecting the asteroid so that it would miss Earth? The viability of this method was recently demonstrated by NASA’s Double Asteroid Redirection Test ( DART ), which impacted the asteroid moonlet Dimorphos on Sept. 26, 2022, slightly changing its trajectory. Other methods of deflection have also been considered during the exercises.

But any deflection or reconnaissance mission would need many years of preparation, requiring the use of advanced observatories capable of finding hazardous asteroids as early as possible. NASA’s Near-Earth Object Surveyor, or NEO Surveyor , is one such observatory. Managed by JPL and planned for launch in late 2027, the infrared space telescope will detect light and dark asteroids, including those that orbit near the Sun. In doing so, NEO Surveyor will support PDCO’s objectives to discover any hazardous asteroids as early as possible so that there would be more time to launch a deflection mission to potential threats.  

To find out the outcome of the exercise, read NASA’s preliminary summary .

For more information about CNEOS, visit:


Ian J. O’Neill Jet Propulsion Laboratory, Pasadena, Calif. 818-354-2649 [email protected]

Karen Fox / Charles Blue NASA Headquarters 202-358-1600 / 202-802-5345 [email protected] / [email protected]

Related Terms

  • NEA Scout (Near Earth Asteroid Scout)
  • NEO Surveyor (Near-Earth Object Surveyor Space Telescope)
  • Planetary Defense
  • Planetary Defense Coordination Office
  • Potentially Hazardous Asteroid (PHA)

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15 R Projects for Beginners (with Source Code)

R programming projects are essential for gaining practical data science experience. They provide the hands-on practice that bridges the gap between learning the required skills and deomonstrating you meet real-world job requirements. This process is particularly valuable when applying for jobs, as it addresses the common challenge of not having any experience when you're applying for your first data job .

A properly diversified portfolio of R projects will demonstrate your proficiency in:

  • Data manipulation
  • Data visualization
  • Advanced statistical analysis

These skills are fundamental to making informed business decisions―so being able to demonstrate that you have them makes you a valuable asset to potential employers.

In this post, we'll explore 15 practical R project ideas. Each project is designed to highlight critical data science capabilities that will enhance your job prospects. Whether you're a student aiming to launch your career or a professional seeking advancement, these projects on R will show your ability to handle real-world data challenges effectively.

Two individuals collaborating over an R project, highlighting the importance of practical experience

But first, to ensure you're developing in-demand R skills , we'll explain how to build your portfolio of projects on R by selecting the right ones and go over some of the common challenges you might face along the way. After we look at the 15 R project ideas in detail, we'll discuss how you can prepare for an R programming job.

Choosing the right R projects for your portfolio

Looking to improve your chances of landing a data science job? The R project ideas you select for your portfolio can make a big difference. A well-chosen set of projects on R shows off your skills and proves you can tackle real-world problems. Here's how to select R projects that help you grow, match your interests, and impress potential employers.

Find the sweet spot: Your skills, interests, and market demand

The best projects combine what you enjoy, what you're good at, and what employers want. This balance keeps you motivated and makes you more appealing to hiring managers. For example, if you love sports, you might create a project that uses R to predict game outcomes. This type of project lets you practice working with data and creating visualizations—skills that are valuable in many industries.

How to pick your R projects: A step-by-step approach

  • Know your strengths (and weaknesses): Assess your R programming skills. What are you comfortable with? Where do you need practice? Knowing the answers these questions will help you choose projects that challenge you appropriately.
  • Explore different tools and techniques: Pick projects that use various R packages and data types. This shows your versatility as a data scientist.
  • Focus on solving problems: ChR project ideasoose projects with clear goals, like predicting customer behavior or analyzing social media trends. These projects are engaging and show employers you can deliver results.
  • Seek feedback: Ask others to review your code and approach. Their input can help you improve your skills and projects.

Common challenges (and how to overcome them)

Many learners struggle with choosing projects on R that are too complex or aren't able to manage their time effectively. To avoid these issues:

  • Start small : Begin with manageable projects that match your current skill level.
  • Use available resources : When you get stuck, look for help in online tutorials or community forums .

Keep improving: The power of iteration

Don't stop after your first attempt. Reworking and refining your R projects based on feedback is key. This process of continuous improvement enhances the quality of your work and shows potential employers your commitment to excellence. It also helps prepare you for the workplace where iterating on your work is common.

Wrapping up

Carefully selecting your R project ideas can significantly improve your skills and how you present them to potential employers. As you review the list of 15 R project ideas below, use these tips to choose projects that will strengthen your portfolio and align with your career goals.

Getting started with R programming projects

Hands-on projects are key to developing practical R programming skills. They'll boost your understanding of the language and prepare you for real-world data tasks. Here's how to get started:

Common tools and packages

First, familiarize yourself with these R tools and packages:

  • RStudio: An IDE that simplifies code writing, debugging, and visualization .
  • dplyr : Streamlines data manipulation tasks .
  • ggplot2 : Creates complex visualizations easily .
  • data.table : Processes large datasets efficiently .

These tools will streamline your project workflow. For more insights, explore this guide on impactful R packages .

Setting up your project on R

Follow these steps to start your R programming project:

  • Install R and RStudio: These are your foundational tools .
  • Create a new project in RStudio: This keeps your files organized.
  • Learn the RStudio environment: Understand each part of the IDE to get the most out of it .
  • Import necessary packages: Load libraries like tidyverse or shiny as needed.

Overcoming common challenges

As a beginner, you might face some hurdles. Here are some strategies to help:

  • Keep your code organized and use Git for version control.
  • Start small to build confidence before tackling complex projects.
  • Use community forums and official documentation when you need help.

15 R Project Ideas with Source Code

The beauty of the following R projects lies in their diverse range of scenarios. You'll start by investigating COVID-19 virus trends and soon find yourself analyzing forest fire data. This variety ensures that you can apply your R programming skills to uncover valuable insights in different contexts. Although most of these R projects are suitable for beginners, the more advanced ones towards the end of the list may require additional effort and expertise to complete.

Here's what we'll cover:

Beginner R Projects

  • Investigating COVID-19 Virus Trends
  • Creating An Efficient Data Analysis Workflow
  • Creating An Efficient Data Analysis Workflow, Part 2
  • Analyzing Forest Fire Data
  • NYC Schools Perceptions
  • Analyzing Movie Ratings

Intermediate R Projects

  • New York Solar Resource Data
  • Investigating Fandango Movie Ratings
  • Finding the Best Markets to Advertise In
  • Mobile App for Lottery Addiction
  • Building a Spam Filter with Naive Bayes
  • Winning Jeopardy

Advanced R Projects

  • Predicting Condominium Sale Prices
  • Predicting Car Prices
  • Creating a Project Portfolio

In the sections that follow, we'll provide detailed walkthroughs for each project. You'll find step-by-step instructions and expected outcomes to guide you through the process. Let's get started with building your portfolio of projects on R!

1. Investigating COVID-19 Virus Trends

Difficulty Level: Beginner

In this beginner-level R project, you'll step into the role of a data analyst exploring the global COVID-19 pandemic using real-world data. Leveraging R and the powerful dplyr library, you'll manipulate, filter, and aggregate a comprehensive dataset containing information on COVID-19 cases, tests, and hospitalizations across different countries. By applying data wrangling techniques such as grouping and summarizing, you'll uncover which countries have the highest rates of positive COVID-19 tests relative to their testing numbers. This hands-on project will not only strengthen your R programming skills and analytical thinking but also provide valuable experience in deriving actionable insights from real-world health data – a crucial skill in today's data-driven healthcare landscape.

Tools and Technologies


To successfully complete this project, you should be comfortable with data structures in R such as:

  • Creating and working with vectors, matrices, and lists in R
  • Indexing data structures to extract elements for analysis
  • Applying functions to data structures to perform calculations
  • Manipulating and analyzing data using dataframes

Step-by-Step Instructions

  • Load and explore the COVID-19 dataset using readr and tibble
  • Filter and select relevant data using dplyr functions
  • Aggregate data by country and calculate summary statistics
  • Identify top countries by testing numbers and positive case ratios
  • Create vectors and matrices to store key findings
  • Compile results into a comprehensive list structure

Expected Outcomes

Upon completing this project, you'll have gained valuable skills and experience, including:

  • Analyzing a real-world COVID-19 dataset using R and dplyr
  • Applying data manipulation techniques to filter and aggregate data
  • Identifying trends and insights from data using grouping and summarizing
  • Creating and manipulating different R data structures (vectors, matrices, lists)
  • Interpreting results to answer specific questions about COVID-19 testing and positive rates

Relevant Links and Resources

  • R Project Example Solution
  • Original COVID19 Worldwide Testing dataset on Kaggle

Additional Resources

  • WHO Coronavirus (COVID-19) Dashboard

2. Creating An Efficient Data Analysis Workflow

In this hands-on, beginner-level project with R, you'll step into the role of a data analyst for a company selling programming books. Using R and RStudio, you'll analyze their sales data to determine which titles are most profitable. By applying key R programming concepts like control flow, loops, and functions, you'll develop an efficient data analysis workflow. This project provides valuable practice in data cleaning, transformation, and analysis, culminating in a structured report of your findings and recommendations.

To successfully complete this project, you should be comfortable with control flow, iteration, and functions in R including:

  • Implementing control flow using if-else statements
  • Employing for loops and while loops for iteration
  • Writing custom functions to modularize code
  • Combining control flow, loops, and functions in R
  • Load and explore the book sales dataset using tidyverse
  • Clean the data by handling missing values and inconsistent labels
  • Transform the review data into numerical format
  • Analyze the cleaned data to identify top-performing titles
  • Summarize findings and provide data-driven recommendations
  • Applying R programming concepts to real-world data analysis
  • Developing an efficient, reproducible data analysis workflow
  • Cleaning and preparing messy data for analysis using tidyverse
  • Analyzing sales data to derive actionable business insights
  • Communicating findings and recommendations to stakeholders
  • Getting Started with R and RStudio - Dataquest Blog

In this beginner-level R project, you'll step into the role of a data analyst at a book company tasked with evaluating the impact of a new program launched on July 1, 2019 to encourage customers to buy more books. Using R and powerful packages like dplyr, stringr, and lubridate, you'll clean and analyze the company's 2019 sales data to determine if the program successfully boosted book purchases and improved review quality. You'll handle missing data, process text reviews, and compare key metrics before and after the program launch. This project offers hands-on experience in applying data manipulation techniques to real-world business data, strengthening your skills in efficient data analysis and deriving actionable insights.

  • tidyverse (including dplyr)

To successfully complete this project, you should be comfortable with specialized data processing techniques in R , including:

  • Manipulating strings using stringr functions
  • Working with dates and times using lubridate
  • Applying the map function to vectorize custom functions
  • Understanding and employing regular expressions for pattern matching
  • Load and explore the book company's 2019 sales data
  • Clean the data by handling missing values and inconsistencies
  • Process text reviews to determine positive/negative sentiment
  • Compare key sales metrics before and after the program launch date
  • Analyze differences in sales between customer segments
  • Evaluate changes in review sentiment and summarize findings
  • Cleaning and preparing a real-world business dataset for analysis using R
  • Applying powerful R packages to manipulate and process data efficiently
  • Analyzing sales data to quantify the impact of a new business initiative
  • Translating data analysis findings into meaningful business insights
  • Project Dataset

4. Analyzing Forest Fire Data

In this beginner-level data analysis project in R, you'll analyze a dataset on forest fires in Portugal to uncover patterns in fire occurrence and severity. Using R and powerful data visualization techniques, you'll explore factors such as temperature, humidity, and wind speed to understand their relationship with fire spread. You'll create engaging visualizations, including bar charts, box plots, and scatter plots, to reveal trends over time and across different variables. By completing this project, you'll gain valuable insights into the ecological impact of forest fires while strengthening your skills in data manipulation, exploratory data analysis, and creating meaningful visualizations using R and ggplot2.

  • tidyverse (including ggplot2)

To successfully complete this project, you should be comfortable with data visualization techniques in R and have experience with:

  • Working with variables, data types, and data structures in R
  • Importing and manipulating data using R data frames
  • Creating basic plots using ggplot2 (e.g., bar charts, scatter plots)
  • Transforming and preparing data for visualization
  • Load and explore the forest fires dataset using R and tidyverse
  • Process the data, converting relevant columns to appropriate data types (e.g., factors for month and day)
  • Create bar charts to analyze fire occurrence patterns by month and day of the week
  • Use box plots to explore relationships between environmental factors and fire severity
  • Implement scatter plots to investigate potential outliers and their impact on the analysis
  • Summarize findings and discuss implications for forest fire prevention strategies
  • Cleaning and preparing real-world ecological data for analysis using R
  • Creating various types of plots (bar charts, box plots, scatter plots) using ggplot2
  • Interpreting visualizations to identify trends in forest fire occurrence and severity
  • Handling outliers and understanding their impact on data analysis and visualization
  • Communicating data-driven insights for environmental decision-making
  • UCI Machine Learning Repository: Forest Fires Dataset

5. NYC Schools Perceptions

In this beginner-level R project, you'll explore real-world survey data on school quality perceptions in New York City. Using R and various data manipulation packages, you'll clean, reshape, and visualize responses from students, parents, and teachers to uncover insights about school performance. You'll work with a large, complex dataset to build valuable data wrangling and exploration skills while creating an impactful analysis of NYC school quality perceptions across different stakeholder groups.

  • R Notebooks
  • tidyverse (dplyr, tidyr, ggplot2)

To successfully complete this project, you should be comfortable with data cleaning techniques in R including:

  • Manipulating DataFrames using dplyr
  • Joining and combining relational data
  • Handling missing data through various techniques
  • Reshaping data between wide and long formats using tidyr
  • Creating visualizations with ggplot2
  • Load and clean the NYC school survey datasets
  • Join survey data with school performance data
  • Create a correlation matrix to identify relationships between variables
  • Visualize strong correlations using scatter plots
  • Reshape the data to compare perceptions across stakeholder groups
  • Analyze and visualize differences in perceptions using box plots
  • Cleaning and wrangling complex, real-world datasets using tidyverse tools
  • Joining multiple datasets to create a comprehensive analysis
  • Identifying correlations and visualizing relationships in data
  • Reshaping data to facilitate comparisons across different groups
  • Creating informative visualizations to communicate insights about school quality perceptions
  • Interpreting results to draw meaningful conclusions about NYC schools
  • NYC School Survey Data on NYC Open Data

6. Analyzing Movie Ratings

In this beginner-level project with R, you'll analyze movie ratings data from IMDb using web scraping techniques in R. You'll extract information such as titles, release years, runtimes, genres, ratings, and vote counts for the top 30 movies released between March and July 2020. Using packages like rvest and dplyr, you'll practice loading web pages, identifying CSS selectors, and extracting specific data elements. You'll also gain experience in data cleaning by handling missing values. Finally, you'll use ggplot2 to visualize the relationship between user ratings and number of votes, uncovering trends in movie popularity and reception. This project offers hands-on experience in web scraping, data manipulation, and visualization using R, skills that are highly valuable in real-world data analysis scenarios.

To successfully complete this project, you should be familiar with web scraping techniques in R and have experience with:

  • Understanding HTML structure and using CSS selectors to locate specific elements
  • Using the rvest package to extract data from web pages
  • Basic data manipulation and cleaning using dplyr and stringr
  • Working with vectors and data frames in R
  • Load the IMDb web page and extract movie titles and release years
  • Extract additional movie features such as runtimes and genres
  • Scrape user ratings, metascores, and vote counts for each movie
  • Clean the extracted data and handle missing values
  • Create a data frame combi ning all extracted information
  • Visualize the relationship between user ratings and vote counts using ggplot2
  • Implementing web scraping techniques to extract structured data from IMDb
  • Cleaning and preprocessing scraped data for analysis
  • Creating a comprehensive dataset of movie information from multiple web elements
  • Visualizing relationships between movie ratings and popularity
  • Applying R programming skills to solve real-world data extraction and analysis problems
  • IMDb Top 30 Movies (March-July 2020)

7. New York Solar Resource Data

Difficulty Level: Intermediate

In this beginner-friendly R project, you'll step into the role of a data analyst tasked with extracting solar resource data for New York City using the Data Gov API. Using R, you'll apply your skills in API querying, JSON parsing, and data structure manipulation to retrieve the data and convert it into a format suitable for analysis. This project provides hands-on experience in working with real-world data from web APIs, a crucial skill for data scientists working with diverse data sources.

To successfully complete this project, you should be comfortable with working with APIs in R and have experience with:

  • Making API requests using the httr package
  • Parsing JSON responses with jsonlite
  • Manipulating data frames using dplyr
  • Creating basic visualizations with ggplot2
  • Working with complex list structures in R
  • Set up the API request parameters and make a GET request to the NREL API
  • Parse the JSON response and extract relevant data into R objects
  • Convert the extracted data into a structured dataframe
  • Create a custom function to streamline the data extraction process
  • Visualize the solar resource data using ggplot2
  • Extracting data from web APIs using R and the httr package
  • Parsing and manipulating complex JSON data structures
  • Creating custom functions to automate data retrieval and processing
  • Visualizing time-series data related to solar resources
  • Applying data wrangling techniques to prepare API data for analysis
  • NREL Solar Resource Data API Documentation
  • Data.gov - Open Data Source

8. Investigating Fandango Movie Ratings

In this beginner-friendly project with R, you'll investigate potential bias in Fandango's movie rating system. A 2015 analysis revealed that Fandango's ratings were inflated. Your task is to compare movie ratings data from 2015 and 2016 to determine if Fandango's system changed after the bias was exposed. Using R and statistical analysis techniques, you'll explore rating distributions, calculate summary statistics, and visualize changes in rating patterns. This project provides hands-on experience with a real-world data integrity investigation, strengthening your skills in data manipulation, statistical analysis, and data visualization.

To successfully complete this project, you should be familiar with fundamental statistics concepts in R and have experience with:

  • Data manipulation using dplyr (filtering, selecting, mutating, summarizing)
  • Working with string data using stringr functions
  • Reshaping data with tidyr (gather, spread)
  • Calculating summary statistics (mean, median, mode)
  • Creating and customizing plots with ggplot2 (density plots, bar plots)
  • Interpreting frequency distributions and probability density functions
  • Basic hypothesis testing and statistical inference
  • Load and explore the 2015 and 2016 Fandango movie ratings datasets
  • Clean and preprocess the data, isolating relevant samples for analysis
  • Compare distribution shapes of 2015 and 2016 ratings using kernel density plots
  • Calculate and compare summary statistics for both years
  • Visualize changes in rating patterns using bar plots
  • Interpret results and draw conclusions about changes in Fandango's rating system
  • Conducting a comparative analysis of rating distributions using R
  • Applying statistical techniques to investigate potential bias in ratings
  • Creating informative visualizations to illustrate changes in rating patterns
  • Drawing and communicating data-driven conclusions about rating system integrity
  • Implementing end-to-end data analysis workflow in R, from data loading to insight generation
  • Original Fandango Ratings Dataset
  • Original FiveThirtyEight Article on Fandango Ratings

9. Finding the Best Markets to Advertise In

In this beginner-friendly R project, you'll step into the role of an analyst for an e-learning company offering programming courses. Your task is to analyze survey data from freeCodeCamp to determine the two best markets for advertising your company's products. Using R, you'll explore factors such as new coder locations, market densities, and willingness to pay for learning. By applying statistical concepts and data analysis techniques, you'll provide actionable insights to optimize your company's advertising strategy and drive growth.

To successfully complete this project, you should be comfortable with intermediate statistics concepts in R such as:

  • Summarizing distributions using measures of central tendency
  • Calculating variance and standard deviation
  • Standardizing values using z-scores
  • Locating specific values in distributions using z-scores
  • Load and explore the freeCodeCamp survey data
  • Analyze the locations and densities of new coders in different markets
  • Calculate and compare average monthly spending on learning across countries
  • Identify and handle outliers in the spending data
  • Determine the two best markets based on audience size and willingness to pay
  • Summarize findings and make recommendations for the advertising strategy
  • Applying statistical concepts to inform strategic business decisions
  • Using R to analyze real-world survey data and derive actionable insights
  • Handling outliers and cleaning data for more accurate analysis
  • Translating data analysis results into clear recommendations for stakeholders
  • Developing a data-driven approach to optimizing marketing strategies
  • The 2017 freeCodeCamp New Coder Survey Data
  • freeCodeCamp's New Coder Survey Results

10. Mobile App for Lottery Addiction

In this beginner-friendly data science project in R, you'll develop the logical core of a mobile app designed to help lottery addicts understand their chances of winning. As a data analyst at a medical institute, you'll use R programming, probability theory, and combinatorics to analyze historical data from the Canadian 6/49 lottery. You'll create functions to calculate various winning probabilities, check for previous winning combinations, and provide users with a realistic view of their odds. This project offers hands-on experience in applying statistical concepts to a real-world problem while building your R programming portfolio.

  • tidyverse package
  • sets package

To successfully complete this project, you should be comfortable with fundamental probability concepts in R such as:

  • Calculating theoretical and empirical probabilities
  • Applying basic probability rules
  • Working with permutations and combinations
  • Using R functions for complex probability calculations
  • Manipulating data with tidyverse packages
  • Implement core probability functions for lottery calculations
  • Calculate the probability of winning the jackpot with a single ticket
  • Analyze historical lottery data to check for previous winning combinations
  • Develop functions to calculate probabilities for multiple tickets and partial matches
  • Create user-friendly outputs to communicate lottery odds effectively
  • Applying probability and combinatorics concepts to a real-world scenario
  • Implementing complex probability calculations using R functions
  • Working with historical data to inform statistical analysis
  • Developing logical components for a mobile application
  • Communicating statistical concepts to a non-technical audience
  • 6/49 Lottery Dataset on Kaggle

11. Building a Spam Filter with Naive Bayes

In this beginner-friendly project with R, you'll build an SMS spam filter using the Naive Bayes algorithm. Working with a dataset of labeled SMS messages, you'll apply text preprocessing techniques, implement the Naive Bayes classifier from scratch, and evaluate its performance. This project offers hands-on experience in applying probability theory to a real-world text classification problem, providing valuable skills for aspiring data scientists in natural language processing and spam detection. You'll gain practical experience in data preparation, probability calculations, and implementing machine learning algorithms in R.

  • Naive Bayes algorithm

To successfully complete this project, you should be familiar with conditional probability concepts in R and have experience with:

  • Basic R programming and data manipulation using tidyverse
  • Understanding and applying conditional probability rules
  • Calculating probabilities based on prior knowledge using Bayes' theorem
  • Text preprocessing techniques in R
  • Load and preprocess the SMS dataset, creating training, cross-validation, and test sets
  • Clean the text data and build a vocabulary from the training set
  • Calculate probability parameters for the Naive Bayes classifier
  • Implement the Naive Bayes algorithm to classify new messages
  • Evaluate the model's performance and tune hyperparameters using cross-validation
  • Test the final model on the test set and interpret results
  • Implementing text preprocessing techniques for machine learning tasks
  • Building a Naive Bayes classifier from scratch in R
  • Applying probability calculations in a real-world text classification problem
  • Evaluating and optimizing machine learning model performance
  • Interpreting classification results in the context of spam detection
  • UCI Machine Learning Repository: SMS Spam Collection Dataset

12. Winning Jeopardy

In this beginner-friendly R project, you'll analyze a dataset of over 20,000 Jeopardy questions to uncover patterns that could give you an edge in the game. Using R and statistical techniques, you'll explore question categories, identify terms associated with high-value clues, and develop data-driven strategies to improve your odds of winning. You'll apply chi-squared tests and text analysis methods to determine which categories appear most frequently and which topics are associated with higher-value questions. This project will strengthen your skills in hypothesis testing, string manipulation, and deriving actionable insights from text data.

  • Chi-squared test

To successfully complete this project, you should be familiar with hypothesis testing in R and have experience with:

  • Performing chi-squared tests on categorical data
  • Manipulating strings and text data in R
  • Data cleaning and preprocessing techniques
  • Basic data visualization in R
  • Load and preprocess the Jeopardy dataset, cleaning text and converting data types
  • Normalize dates to make them more accessible for analysis
  • Analyze the frequency of question categories using chi-squared tests
  • Identify unique terms in questions and associate them with question values
  • Perform statistical tests to determine which terms are associated with high-value questions
  • Visualize and interpret the results to develop game strategies
  • Applying chi-squared tests to analyze categorical data in a real-world context
  • Implementing text preprocessing and analysis techniques in R
  • Interpreting statistical results to derive actionable insights
  • Developing data-driven strategies for game show success
  • Original Jeopardy Dataset

13. Predicting Condominium Sale Prices

Difficulty Level: Advanced

In this challenging project with R, you'll analyze New York City condominium sales data to predict prices based on property size. Using R and linear regression modeling techniques, you'll clean and explore the dataset, visualize relationships between variables, and build predictive models. You'll compare model performance across NYC's five boroughs (Manhattan, Brooklyn, Queens, The Bronx, and Staten Island), gaining valuable experience in real estate data analysis and statistical modeling. This project will strengthen your skills in data cleaning, exploratory analysis, and interpreting regression results in a practical business context.

  • Linear regression

To successfully complete this project, you should be familiar with linear regression modeling in R and have experience with:

  • Data manipulation and cleaning using tidyverse functions
  • Creating scatterplots and other visualizations with ggplot2
  • Fitting and interpreting linear regression models in R
  • Evaluating model performance using metrics like R-squared and RMSE
  • Basic understanding of real estate market dynamics
  • Load and clean the NYC condominium sales dataset
  • Perform exploratory data analysis, visualizing relationships between property size and sale price
  • Identify and handle outliers that may impact model performance
  • Build a linear regression model for all NYC boroughs combined
  • Create separate models for each borough and compare their performance
  • Interpret results and draw conclusions about price prediction across different areas of NYC
  • Cleaning and preparing real estate data for analysis in R
  • Visualizing and interpreting relationships between property features and prices
  • Building and comparing linear regression models across different market segments
  • Evaluating model performance and understanding limitations in real estate price prediction
  • Translating statistical results into actionable insights for real estate analysis
  • R-bloggers: A great resource for R programming tips and tutorials

14. Predicting Car Prices

In this challenging R project, you'll step into the role of a data scientist tasked with developing a model to predict car prices for a leading automotive company. Using a dataset of various car attributes such as make, fuel type, body style, and engine specifications, you'll apply the k-nearest neighbors algorithm in R to build an optimized prediction model. You'll go through the complete machine learning workflow - from data exploration and preprocessing to model evaluation and interpretation. This project will strengthen your skills in examining relationships between predictors, implementing cross-validation, performing hyperparameter optimization, and comparing different models to create an effective price prediction tool that could be used in real-world automotive market analysis.

  • caret package
  • k-nearest neighbors algorithm

To successfully complete this project, you should be comfortable with fundamental machine learning concepts in R such as:

  • Understanding the key steps in a typical machine learning workflow
  • Implementing k-nearest neighbors for regression tasks
  • Using the caret library for machine learning model training and evaluation in R
  • Evaluating model performance using error metrics (e.g., RMSE) and k-fold cross validation
  • Basic data manipulation and visualization using dplyr and ggplot2
  • Load and preprocess the car features and prices dataset, handling missing values and non-numerical columns
  • Explore relationships between variables using feature plots and identify potential predictors
  • Prepare training and test sets by splitting the data using createDataPartition
  • Implement k-nearest neighbors models using caret, experimenting with different values of k
  • Conduct 5-fold cross-validation and hyperparameter tuning to optimize model performance
  • Evaluate the final model on the test set, interpret results, and discuss potential improvements
  • Applying the end-to-end machine learning workflow in R to a real-world prediction problem
  • Implementing and optimizing k-nearest neighbors models for regression tasks using caret
  • Using resampling techniques like k-fold cross validation for robust model evaluation
  • Interpreting model performance metrics (e.g., RMSE) in the context of car price prediction
  • Gaining practical experience in feature selection, preprocessing, and hyperparameter tuning
  • Developing intuition for model selection and performance optimization in regression tasks
  • Original Automobile Dataset on UCI Machine Learning Repository

15. Creating a Project Portfolio

In this challenging project with R, you'll be tasked with creating an impressive interactive portfolio to showcase your R programming and data analysis skills to potential employers. Using Shiny, you'll compile your guided projects from Dataquest R courses into one cohesive portfolio app. You'll apply your Shiny skills to incorporate R Markdown files, customize your app's appearance, and deploy it for easy sharing. This project will strengthen your ability to create interactive web applications, integrate multiple data projects, and effectively present your work to enhance your job prospects in the data analysis field.

To successfully complete this project, you should be comfortable with building interactive web applications in Shiny and have experience with:

  • Understanding the structure and components of a Shiny app
  • Creating inputs and outputs in the Shiny user interface
  • Programming the server logic to connect inputs and outputs
  • Extending Shiny apps with additional features
  • Basic R Markdown usage for creating dynamic reports
  • Plan the structure and content of your portfolio app
  • Build the user interface with a navigation bar and project pages
  • Incorporate R Markdown files for individual project showcases
  • Develop server logic to handle user interactions and display content
  • Create a utility function to efficiently generate project pages
  • Design an engaging splash page and interactive resume section
  • Deploy your portfolio app to shinyapps.io for easy sharing
  • Building a comprehensive, interactive portfolio app using Shiny
  • Integrating multiple R projects and analyses into a cohesive presentation
  • Creating utility functions to streamline app development
  • Customizing Shiny app appearance and functionality for professional presentation
  • Deploying a Shiny app to a public hosting platform for easy access
  • Effectively showcasing your R programming and data analysis skills to potential employers
  • Resolved R Shiny app issue regarding images in the Dataquest Community
  • Non-Guided Project: Making an R Shiny App to track moths | Dataquest Community

How to Prepare for an R Programming Job

Looking to land your first R programming job? Let's walk through the key steps to prepare yourself for success in this field.

Understand Market Demands

Start by researching what employers want. Browse R programming job listings on popular job listing sites like the ones below. They'll give you a clear picture of the skills and qualifications currently in demand.

Once you have a good idea of the skills employers are looking for, take on projects that help you develop and demonstrate those in-demand skills.

Develop Essential Skills

For entry-level positions, focus on being able to demonstrate these skills:

  • Data manipulation (using packages like dplyr )
  • Data analysis and visualization (with tools like ggplot2 )
  • Basic statistical analysis
  • Fundamental machine learning concepts
  • Core programming principles

To build these skills:

  • Enroll in structured learning paths or bootcamps
  • Work on hands-on coding projects
  • Participate in coding competitions to enhance problem-solving skills

As you learn, you might find some concepts challenging. Don't get discouraged. Instead:

  • Practice coding regularly to improve your speed and accuracy
  • Seek feedback from peers or mentors to refine your code quality and problem-solving approach

Showcase Your Work

Create a portfolio that highlights your R projects. Include examples demonstrating your data analysis, visualization, and statistical computing skills. Consider using GitHub to host your work , ensuring each project is well-documented.

Prepare for the Job Hunt

Tailor your resume to emphasize relevant technical skills and project experiences. For interviews, be ready to discuss your projects in detail . Practice explaining how you've applied specific R functions and packages to solve real-world problems.

Remember, becoming job-ready in R programming is a journey that combines technical skill development, practical experience, and effective self-presentation. By following these steps and persistently honing your skills, you'll be well-equipped to pursue opportunities in the data science field using R.

Bottom line: R programming projects are essential for building real-world skills and advancing your data science career. Here's why they matter and how to get started:

  • Practical application : Projects help you apply theory to actual problems.
  • Career advancement : They showcase your abilities to potential employers.
  • Skill development : Start simple and gradually tackle more complex challenges.

If you're new to R, begin with basic projects focusing on data cleaning and visualization. This approach builds your confidence and expertise gradually. As you progress, adopt good coding practices. Clear, well-organized code is easier to read and maintain, especially when collaborating with others.

Consider exploring Dataquest's Data Analyst in R path . This program covers everything from basic concepts to advanced data techniques.

R projects do more than beef up your portfolio. They sharpen your problem-solving skills and prepare you for real data science challenges. Start with a project that interests you and matches your current skills. Then, step by step, move to more complex problems. Let your interest in data guide your learning journey.

Remember, every R project you complete brings you closer to your data science goals. So, pick a project and start coding!

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  • Chris Johnson, Matthew D. Affolter, Paul Inkenbrandt, & Cam Mosher
  • Salt Lake Community College via OpenGeology

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Our solar system formed at the same time as our Sun as described in the nebular hypothesis. The nebular hypothesis is the idea that a spinning cloud of dust made of mostly light elements, called a nebula, flattened into a protoplanetary disk, and became a solar system consisting of a star with orbiting planets [ 12 ]. The spinning nebula collected the vast majority of material in its center, which is why the sun Accounts for over 99% of the mass in our solar system.


Planet Arrangement and Segregation


As our solar system formed, the nebular cloud of dispersed particles developed distinct temperature zones. Temperatures were very high close to the center, only allowing condensation of metals and silicate minerals with high melting points. Farther from the Sun, the temperatures were lower, allowing the condensation of lighter gaseous molecules such as methane, ammonia, carbon dioxide, and water [ 13 ]. This temperature differentiation resulted in the inner four planets of the solar system becoming rocky, and the outer four planets becoming gas giants.

Both rocky and gaseous planets have a similar growth model. Particles of dust, floating in the disc were attracted to each other by static charges and eventually, gravity. As the clumps of dust became bigger, they interacted with each other—colliding, sticking, and forming proto-planets. The planets continued to grow over the course of many thousands or millions of years, as material from the protoplanetary disc was added. Both rocky and gaseous planets started with a solid core. Rocky planets built more rock on that core, while gas planets added gas and ice. Ice giants formed later and on the furthest edges of the disc, accumulating less gas and more ice. That is why the gas-giant planets Jupiter and Saturn are composed of mostly hydrogen and helium gas, more than 90%. The ice giants Uranus and Neptune are composed of mostly methane ices and only about 20% hydrogen and helium gases.

The planetary composition of the gas giants is clearly different from the rocky planets. Their size is also dramatically different for two reasons: First, the original planetary nebula contained more gases and ices than metals and rocks. There was abundant hydrogen, carbon, oxygen, nitrogen, and less silicon and iron, giving the outer planets more building material. Second, the stronger gravitational pull of these giant planets allowed them to collect large quantities of hydrogen and helium, which could not be collected by the weaker gravity of the smaller planets.

Jupiter’s massive gravity further shaped the solar system and growth of the inner rocky planets. As the nebula started to coalesce into planets, Jupiter’s gravity accelerated the movement of nearby materials, generating destructive collisions rather than constructively gluing material together [ 14 ]. These collisions created the asteroid belt, an unfinished planet, located between Mars and Jupiter. This asteroid belt is the source of most meteorites that currently impact the Earth. Study of asteroids and meteorites help geologist to determine the age of Earth and the composition of its core, mantle, and crust. Jupiter’s gravity may also explain Mars’ smaller mass, with the larger planet consuming material as it migrated from the inner to the outer edge of the solar system [ 15 ].

Pluto and Planet Definition


The outermost part of the solar system is known as the Kuiper belt, which is a scattering of rocky and icy bodies. Beyond that is the Oort cloud, a zone filled with small and dispersed ice traces. These two locations are where most comets form and continue to orbit, and objects found here have relatively irregular orbits compared to the rest of the solar system. Pluto, formerly the ninth planet, is located in this region of space. The XXVIth General Assembly of the International Astronomical Union (IAU) stripped Pluto of planetary status in 2006 because scientists discovered an object more massive than Pluto, which they named Eris. The IAU decided against including Eris as a planet, and therefore, excluded Pluto as well. The IAU narrowed the definition of a planet to three criteria:

  • Enough mass to have gravitational forces that force it to be rounded
  • Not massive enough to create a fusion
  • Large enough to be in a cleared orbit, free of other planetesimals that should have been incorporated at the time the planet formed. Pluto passed the first two parts of the definition, but not the third. Pluto and Eris are currently classified as dwarf planets

12. Montmerle T, Augereau J-C, Chaussidon M, et al (2006) Solar System Formation and Early Evolution: the First 100 Million Years. In: From Suns to Life: A Chronological Approach to the History of Life on Earth. Springer New York, pp 39–95

13. Martin RG, Livio M (2012) On the evolution of the snow line in protoplanetary discs. Mon Not R Aston Soc Lett 425:L6–L9

14. Petit J-M, Morbidelli A, Chambers J (2001) The Primordial Excitation and Clearing of the Asteroid Belt. Icarus 153:338–347. https://doi.org/10.1006/icar.2001.6702

15. Walsh KJ, Morbidelli A, Raymond SN, et al (2011) A low mass for Mars from Jupiter’s early gas-driven migration. Nature 475:206–209


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