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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 .

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.

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.

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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Nebular theory and the formation of the solar system

In the beginning….

How and when does the story of Earth begin? A logical place to start is with the formation of the planet, but as you’ll soon see, the formation of the planet is part of a larger story, and that story implies some backstory before the story, too. The purpose of this case study is to present our best scientific understanding of the formation of our solar system from a presolar nebula, and to put that nebula in context too.

Nebular theory

The prevailing scientific explanation for the origin of the Earth does a good job of not only explaining the Earth’s formation, but the Sun and all the other planets too. Really, it’s not “the Earth’s origin story” alone so much as it is the origin story of the whole solar system . Not only that, but our Sun is but one star among a hundred million in our galaxy, and our galaxy is one of perhaps a hundred million in the universe. So the lessons we learn by studying our own solar system can likely be applied more generally to the formation of other solar systems elsewhere, including those long ago, in galaxies far, far away. The vice versa is also true: Our understanding of our own solar system’s origin story is being refined as we learn more about exoplanets, some of which defy what we see in our own system; “ hot Jupiters ” and “ super-Earths ,” for instance, are features we see in other star systems but not our own.

When we use powerful telescopes to stare out into the galaxy, we observe plenty of other stars, but we observe other things too, including fuzzy looking features called nebulae. A nebula is a big cloud of gas and dust in space. It’s not as bright as a star because it’s not undergoing thermonuclear fusion, with the tremendous release of energy that accompanies that process. An example of a nebula that you are likely to be able to see is in the constellation Orion. Orion’s “belt,” three stars in a row, is a readily identifiable feature in the northern hemisphere’s night sky in winter. A smaller trio of light spots “dangle” from the belt; this is Orion’s sword scabbard. A cheap pair of binoculars will let you examine these objects for yourself; you will discover that the middle point of light in this smaller trio is not a star. It is a nebula called Messier 42.

Nebulae like Messier 42 are common features of the galaxy, but not as common as stars. Nebulae appear to be short-lived features, as matter is often attracted to other matter. All that stuff distributed in that tremendous volume of space is not as stable as it would be if it were all to be drawn together into a few big clumps. Particles pull together with their neighboring particles under the influence of various forces, including “static cling” or electrostatic attraction. This is the same force that makes tiny dust motes clump up into dust bunnies under your couch!

Now, electrostatic force is quite strong for pulling together small particles over small distances, but if you want to make big things like planets and stars out from a nebula, you’re going to need gravity to take over at some point. Gravity is a rather weak force. After all: every time you take a step, you’ve overcoming the gravitational pull of the entire Earth. But gravity can work very efficiently over distance, if the masses involved are large enough. So static cling was the initial organizer, until the “space dust bunnies” got large enough, then gravity was able to take over, attracting mass to mass. The net result is that the gajillions of tiny pieces of the nebula were drawn together, swirling into a denser and denser amalgamation. The nebula began to spin, flattening out from top to bottom, and flattening out into a spinning disk, something between a Frisbee and a fried egg in shape:

Once a star forms in the center, astronomers call the ring of debris around it a protoplanetary disk. Two important processes that helped organize the protoplanetary disk further were condensation and accretion.

Condensation is the process where gaseous matter sticks together to make liquid or solid matter. We have evidence of condensation in the form of small spherical objects with internal layering, kind of like “space hailstones.” These are chondrules, and they represent the earliest objects formed in our solar system. (Occasionally, we are lucky enough to find chondrules that have survived until the present day, entombed inside certain meteorites of the variety called chondrites.)

Chondrules glommed onto other chondrules, and stuck themselves together into primordial “rocks,” building up larger and larger objects. Eventually, these objects got to be big enough to pull their mass into an round shape, and we would be justified to dub them “planetesimals.” Planetesimals gobbled up nearby asteroids, and smashed into other planetesimals, merging and growing through time through the process of accretion. The kinetic force of these collisions heated the rocky and metallic material of the planetesimals, and their temperature also went up as radioactive decay heated them from within. Once warm, denser material could sink to their middles, and lighter-weight elements and compounds rose up to their surface. So not only were they maturing into spheroidal shapes, but they were also differentiating internally, separating into layers organized by density.

Meteorites that show metallic compositions represent “core” material from these planetesimals; core material that we would never get to glimpse had not their surrounding rocky material been blasted off. Iron meteorites such as the Canyon Diablo meteorite below (responsible for Arizona’s celebrated Meteor Crater) therefore are evidence of differentiation of planetesimals into layered bodies, followed by disaggregation: a polite way of saying they were later violently ripped apart by energetic collisions.

If you were to somehow weigh the nebula before condensation and accretion, and again 4.6 billion years later, we’d find the mass to be the same. Rather than being dispersed in a diffuse cloud of uncountable atoms, the condensation and accretion of the nebula resulted in exactly the same amount of stuff, but organized into a smaller and smaller number of bigger and bigger objects. The biggest of these was the Sun, comprising about 99.86% of all the mass in the solar system. Four-fifths of the remaining 0.14% makes up the planet Jupiter.  Saturn, Neptune, and Uranus are huge gas giants as well. The inner rocky planets (including Earth) make up a tiny, tiny fraction of the total mass of the whole solar system – but of course, just because they are relatively small, that doesn’t mean they are unimportant!

The process of accretion continues into the present day, though at a slower pace than the earliest days of the solar system. One place you can observe this is in the asteroid belt, where there are certain asteroids that are basically nothing more than a big 3D pile of space rocks, held together under their own gravity. Consider the asteroid called Itokawa 25143, for instance:

Only about half a kilometer long, and only a few hundred meters wide, Itokawa doesn’t even have enough gravity to pull itself into a sphere. If you were to land on the surface of Itokawa and kick a soccer-ball-sized boulder, it would readily fly off into space, as the force of your kick would be much higher than the force of gravity causing it to stay put.

Another example of accretion continuing to this day is meteorite impacts. Every time a chunk of rock in space intersects the Earth, its mass is added to that of the planet. In that instant, the solar system gets a little bit cleaner (fewer leftover bits rattling around) and the planet gets a little more massive. A spectacular example of this occurred in 1994 with Comet Shoemaker-Levy 9, a  comet which had only been discovered the previous year. Jupiter’s immense gravity broke the comet into chunks, and then swallowed them up one after another. Astronomers on Earth watched with fascination as the comet chunks, some more than a kilometer across, slammed into Jupiter’s atmosphere at 60 km/second (~134,000 mph), creating a 23,700°C fireball and enormous impact scars that were as large as the entire Earth. These scars lasted for months.

This incredibly dramatic event perhaps raises the hair on our necks, seeing the violence and power of cosmic collisions. It’s a reminder that Earthlings are not safe from accretionary impacts even today – as the dinosaurs found out. For the purposes of our current discussion, though, bear in mind that the collision was really a merger between the masses of Comet Shoemaker-Levy 9 and the planet Jupiter, and after the dust settled, the solar system had one fewer object left off by itself, and Jupiter gained a bit more mass. This is the overall trend of the accretion of our solar system from the presolar nebula: under gravity’s influence, the available mass becomes more and more concentrated through time.

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A star is born

Because the Sun is so massive, it is able to achieve tremendous pressures in its interior. These pressures are so high, they can actually force two atoms into the same space , overcoming their immense repulsion for one another, and causing their two nuclei to merge. As two atoms combine to make one more massive atom, energy is released. This process is thermonuclear fusion. Once it begins, stars begin to give off light.

The ability of stars to make big atoms from small ones is key to understanding the history of our solar system and our planet. Planet Earth is made of a wide variety of chemical elements, both lightweight and heavy. All of these elements must have been present in the nebula, in order for them to be included in Earth’s “starting mixture.” Elements formed in the Sun today stay in the Sun, fusing low-weight atoms into heavier atoms. So all the elements on Earth today came from a pre-Sun star. We can go outside on a spring day and enjoy the Sun’s warmth, but the carbon that makes up the skin that basks in that warmth was forged in the heart of another star, a star that’s gone now, a star that blew up.

This exploding star was the source of the nebula where we began this case study: it’s the backstory that occurred before the opening scene. Our solar system is like a “haunted house,” where billions of years ago, there was a vibrant, healthy main-sequence star right here, in this part of the galaxy. Perhaps it had planets orbiting it. Perhaps some of those planets harbored life. We’ll never know: the explosion wiped the slate clean, and “reset” the solar system for the iteration in which we live. The ghostly remnants of this time before our own still linger, in the very stuff we’re made from. This long-dead star fused hydrogen to build the carbon in our bodies, the iron in our blood, the oxygen we breathe, and the silicon in the rocks of our planet.

This is an incredible realization to embrace: everything you know, everything you trust, everything you are , is stardust.

Age of the solar system

So just when did all this happen? An estimate for the age of the solar system can be made using isotopes of the element lead (Pb). There are several isotopes of lead, but for the purposes of figuring out the age of the solar system, consider these four: 208 Pb, 207 Pb, 206 Pb, and 204 Pb.

208 Pb, 207 Pb, 206 Pb are all radiogenic: that is to say, they stable “daughter” isotopes that are produced from the radioactive “parent” isotopes. Each is produced from a different parent, at a different rate:

204 Pb is, as far as we know, non-radiogenic. It’s relevant to this discussion because it can serve as a ‘standard’ that can allow us to compare the other lead isotopes to one another. Just as if we wanted to compare the currencies of Namibia, Indonesia, and Chile, we might reference all three to the U.S. dollar. The dollar would serve as a standard of comparison, allowing us to better see the value of the Namibian currency relative to the Indonesian currency and the Chilean currency. That’s what 204 Pb is doing for us here.

This is a plot showing the modeled evolution of our three radiogenic lead isotopes relative to 204 Pb. It is constrained by terrestrial lead samples at the young end, and projected back in time in accordance with our measurements of how quickly these three isotopes of lead are produced by their radioactive parents. Of course, if we go back far enough in time, we run out of samples to evaluate. The Earth’s rock cycle has destroyed all its earliest rocks. They’ve been metamorphosed, or weathered, or melted – perhaps many times over! What would be really nice is to find some rocks from the early end of these curves – some samples that could verify these projections back in time are accurate.

Such samples do exist! But they are not from the Earth so much as “from the Earth’s starting materials.” If the nebular theory is correct, then a few leftover scraps of the planet’s starting materials are found in the solar system’s asteroids. Every now and again, bits of these space rocks fall to earth, and if they survive their passage through the atmosphere, we may be lucky enough to collect them, and analyze them. We call these space rocks “meteors” as they streak through the atmosphere, heating through friction and oxidizing as they fall. Those that make it all the way to Earth’s surface are known as “meteorites.” They can be often be distinguished by their scalloped fusion crust, as with this sample:

Meteorites come in several varieties, including rocky and metallic versions. It is very satisfying that when measurements of these meteorites’ lead isotopes are added to the plot above, they all fall exactly where our understanding of lead isotope production would have them: at the start of each of these model evolution curves. Each lead isotope system tells the same answer for the age of the Earth, acting like three independent witnesses corroborating one another’s testimony. And the answer they all give is 4.6 billion years ago (4.6 Ga). That’s what 208 Pb says. That’s what 207 Pb says. And that’s what 206 Pb says. They all agree, and they agree with the predicted curves based on terrestrial (Earth rock) measurements. This agreement gives us great confidence in this number. The Earth, and meteorites (former asteroids), and the solar system of which they are all a part, began about 4.6 billion years ago…

…But what came before that?

The implications of meteorites

In 1969, a meteorite fell through Earth’s atmosphere and broke up over Mexico. A great many pieces of this meteorite were recovered and made available for scientific analysis. It turned out to be a carbonaceous chondrite, the largest of its kind ever documented. It was named the Allende ( “eye-YEN-day” ) meteorite, for the tiny Chihuahuan village closest to the center of the area over which its fragments were scattered.

One of the materials making up Allende’s chondrules was the calcium feldspar called anorthite. Anorthite is an extraordinarily common mineral in Earth’s crust, but the Allende anorthite was different. For some reason, it has a large amount of magnesium in it. When geochemists determined what kind of magnesium this was, they were surprised to find that it was mostly 26 Mg, an uncommon isotope. The abundances of 25 Mg and 24 Mg were found to be about the same level as Earth rocks, but 26 Mg was elevated by about 1.3%.  And after all, magnesium doesn’t even “belong” in a feldspar. The chemical formula of anorthite is CaAl 2 Si 2 O 8 – there’s no “Mg” spot in there. Why was this odd 26 Mg in this chondritic anorthite?

One way to make 26 Mg is the break-down of radioactive 26 Al. The problem with this idea is that there is no 26 Al around today . It’s an example of an extinct isotope: an atom of aluminum so unstable that it falls apart extremely rapidly. The half-life is only 717,000 years. But because these chondrules condensed in the earliest days of the solar system, there may well have been plenty of 26 Al around at that point for them to incorporate. And Al, of course, is a key part of anorthite’s Ca Al 2 Si 2 O 8 crystal structure.

So the idea is that weird extra 26 Mg in the chondrule’s anorthite could be explained by suggesting it wasn’t always 26 Mg: Instead, it started off as 26 Al ,and it belonged in that crystal’s structure. However, over a short amount of time, it all fell apart, and that left the 26 Mg behind to mark where it had once been. If this interpretation is true, it has shocking implications for the story of our solar system.

To understand why, we first need to ask, what came before the nebula? What was the ‘pre-nebula’ situation? Where did the nebula come from, anyhow?

It turns out that nebulae are generated when old stars of a certain size explode.

These explosions are called supernovae (the plural of supernova). The “nova” part of the name comes from the fact that they are very bright in the night sky – an indication of how energetic the explosion is. They look like “new” stars to the casual observer. Supernovae occur when a star has exhausted its supply of lightweight fuel, and it runs out of small atoms that can be fused together under normal conditions. The outward-directed force ceases, and gravitationally-driven inward-directed forces suddenly dominate, collapsing the star in upon itself. This jacks up the pressures to unbelievably high levels, and is responsible for the nuclear fusion of big atoms – every atom heavier than iron is made instantaneously in the fires of the supernova.

That suite of freshly-minted atoms included a bunch of unstable isotopes, including 26 Al.

And here’s the kicker: If the 26 Al was made in a supernova, started decaying immediately, and yet enough was around that a significant portion of it could be woven into the Allende chondrules’ anorthite, that implies a very short amount of time between the obliteration of our Sun’s predecessor, and the first moments of our own. Specifically, the 717,000 year half-life of 26 Al suggests that this “transition between solar systems” played out in less than 5 million years, conceivably in only 2 million years.

That is very, very quickly.

In summary, the planet Earth is part of a solar system centered on the Sun. This solar system, with its star, its classical planets, its dwarf planets, and its “leftover” comets and asteroids, formed from a nebula full of elements in the form of gas and dust. Over time, these many very small pieces stuck together to make bigger concentrations of mass, eventually culminating in a star and a bunch of planets that orbit it. Asteroids (and asteroids that fall to Earth, called meteorites), are leftovers from this process. The starting nebula itself formed from the destruction of a previous star that had exploded in a supernova. The transition from the pre-Sun star to our solar system took place shockingly rapidly.

Further reading

Marcia Bjornerud’s book Reading the Rocks . Basic Books, 2005: 226 pages.

Jennifer A. Johnson (2019), “ Populating the periodic table: Nucleosynthesis of the elements ,” Science. 01 Feb 2019 : 474-478.

Lee, T., D. A. Papanastassiou, and G. J. Wasserburg (1976), Demonstration of 26 Mg excess in Allende and evidence for 26 Al , Geophysical Research Letters , 3(1), 41-44.

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Chapter Contents

• 1 In the beginning…
• 2 Nebular theory
• 3 A star is born
• 4 Age of the solar system
• 5 The implications of meteorites
• 7 Further reading

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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

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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Nebular Theory Might Explain How Our Solar System Formed

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Our solar system contains the sun, inner rocky planets, the gas giants , or the outer planets, and other celestial bodies, but how they all formed is something that scientists have debated over time.

The nebular theory , also known as nebular hypothesis , presents one explanation of how the solar system formed. Pierre-Simon, Marquis de Laplace proposed the theory in 1796, stating that solar systems originate from vast clouds of gas and dust, known as solar nebula, within interstellar space.

Learn more about this solar system formation theory and some of the criticism it faced.

What Is the Nebular Theory?

Criticisms of the nebular theory, solar nebular disk model.

Laplace said the material from which the solar system and Earth derived was once a slowly rotating cloud, or nebula, of extremely hot gas. The gas cooled and the nebula began to shrink. As the nebula became smaller, it rotated more rapidly, becoming somewhat flattened at the poles.

A combination of centrifugal force, produced by the nebula's rotation, and gravitational force, from the mass of the nebula, left behind rings of gas as the nebula shrank. These rings condensed into planets and their satellites, while the remaining part of the nebula formed the sun.

The planet formation hypothesis, widely accepted for about a hundred years, has several serious flaws. The most serious concern is the speed of rotation of the sun.

When calculated mathematically on the basis of the known orbital momentum, of the planets, the nebular hypothesis predicts that the sun must rotate about 50 times more rapidly than it actually does. There is also some doubt that the rings pictured by Laplace would ever condense into planets.

In the early 20th century, scientists rejected the nebular hypothesis for the planetesimal hypothesis, which proposes that planets formed from material drawn out of the sun. This theory, too, proved unsatisfactory.

Later theories have revived the concept of a nebular origin for the planets. An educational NASA website states: "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."

Victor Safronov , a Russian astronomer, helped lay the groundwork for the modern understanding of the Solar Nebular Disk Model. His work, particularly in the 1960s and 1970s, was instrumental in shaping our comprehension of how planets form from a protoplanetary disk.

At a time when others did not want to focus on the planetary formation process, Safronov used math to try to explain how the giant planets, inner planets and more came to be. A decade after his research, he published a book presenting his work.

George Wetherill's research also contributed to this area, specifically on the dynamics of planetesimal growth and planetary accretion.

This article was updated in conjunction with AI technology, then fact-checked and edited by a HowStuffWorks editor.

Please copy/paste the following text to properly cite this HowStuffWorks.com article:

Observed features any origin model of the solar system/planets must explain

Atoms in your body, collapsing clouds of gas and dust in nebular hypothesis, the spinning nebula flattens, condensation of protosun and protoplanets, the composition of the sun, the two classes of planets, etc. explained by the nebula hypothesis:, evidence for the nebular hypothesis.

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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Archaeology of the Solar System

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A new model for the origin of the Solar System proposes planet building blocks formed fast from material that was transported outwards to cooler regions. It claims to be consistent with the properties of ancient meteorites.

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Where does the solar system end?

The location of the solar system's outer boundary is a point of contention among astronomers. There are three possible candidates, which "all have merit." But which one is best?

The solar system is an enormous place. Our cosmic neighborhood includes eight planets, around half a dozen dwarf planets, several hundred moons and millions of asteroids and comets, all spinning around the sun — and in many cases each other —at speeds of thousands of miles per hour, like a giant top.

But where does it end? Well, the answer may depend on whom you ask and how they define the solar system .

There are not one, but three potential boundaries to the solar system, according to NASA : the Kuiper Belt, the ring of rocky bodies beyond the orbit of Neptune ; the heliopause, the edge of the sun's magnetic field ; and the Oort Cloud, a distant reservoir of comets that are barely visible from Earth. 

The arguments for each boundary "all have merit," which makes choosing between them complicated, Dan Reisenfeld , a researcher at Los Alamos National Laboratory in New Mexico, told Live Science in an email. 

But there is one that most astronomers most commonly agree upon.

Related: Have all 8 planets ever aligned?  

Kuiper Belt

The Kuiper Belt stretches between 30 and 50 astronomical units (AU) away from the sun , according to NASA . (One astronomical unit is equal to the distance between Earth and the sun.) 

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Get the world’s most fascinating discoveries delivered straight to your inbox.

This region is filled with asteroids and dwarf planets, such as Pluto , that have been ejected from the inner solar system by one-sided gravitational tugs-of-war with the planets.

Some astronomers argue that the Kuiper Belt should be considered the edge of the solar system because it loosely represents the edge of where the sun's protoplanetary disk — the swirling ring of gas and dust that later became the planets, moons and asteroids — would have been.

"If one narrowly defines the solar system as just the sun and its planetary bodies, then the edge of the Kuiper Belt can be considered to be the edge of the solar system," Reisenfeld said. 

But this definition of the solar system is considered to be far too simple by some astronomers, such as Caltech's Mike Brown . 

"It's not really true," Brown told Live Science in an email. "Things have moved around a lot — mostly outward — since the planets were formed." This means the Kuiper Belt does not contain all of the solar system's "stuff," such as the elusive, hypothetical Planet Nine , which (if it exists) likely lies far beyond the Kuiper Belt .

In October 2023, the discovery of a dozen new objects beyond the Kuiper Belt also hinted that there may be a "second Kuiper Belt" lurking even further out. 

The uncertainty around this region's own outer edge therefore makes it an unreliable boundary for the solar system as a whole, some researchers argue.

The heliopause is the outer edge of the sun's magnetic influence, known as the heliosphere. At this point, the stream of charged particles emitted by the sun, known as the solar wind, becomes too weak to repel the oncoming stream of radiation from stars and other cosmic entities in the Milky Way . 

"Because the plasma inside the heliopause is of solar origin, and the plasma outside the heliopause is of interstellar origin, some people consider the heliopause to be the boundary of the solar system," Reisenfeld said. As a result, the space beyond the heliopause is also often referred to as "interstellar space," or the space between stars , he added.

Two spacecraft have traveled beyond the heliopause: Voyager 1 , which made the crossing in 2012, and Voyager 2, which crossed over in 2018. As the Voyager probes crossed the heliopause, they quickly detected changes in the types and levels of magnetism and radiation hitting them, signifying that they had crossed some kind of border, Brown said.

However, despite its name, the heliosphere is not a perfect sphere . Instead, it is more of an oblong blob because most of the interstellar plasma bombarding the solar system hits us from one direction, which creates a bow shock — a rounded shock wave that deflects incoming radiation around the rest of the solar system. The bow shock is located around 120 AU from the sun, and creates a long tail that stretches at least 350 AU from the sun in the opposite direction.

Using the heliopause to delineate the solar system therefore leaves us with a lopsided neighborhood, which goes against some researchers perceptions of planetary systems.

The Oort Cloud is the furthest and most expansive potential solar system boundary, extending up to around 100,000 AU from the sun, according to NASA . 

"People who define the solar system as everything that is gravitationally bound to the Sun consider the edge of the Oort cloud to be the edge of the solar system," Reisenfeld said.

For some researchers, this is the clear choice for a solar system boundary because in theory, a planetary system consists of all objects orbiting a star.

"I don't understand how anyone considers anything other than the Oort Cloud to be the edge of the solar system," Sean Raymond , an astronomer at the Bordeaux Astrophysics Laboratory in France, told Live Science in an email. "Any other definition seems ludicrous. It is literally the edge of where something can orbit the Sun."

However, other researchers believe that because the Oort Cloud is located in interstellar space, it lies beyond the solar system even if it is bound to our home star. 

There is also a large amount of uncertainty about where the Oort Cloud actually ends, which some would argue makes it just as unreliable a border as the Kuiper Belt.

Which boundary is best? 

Out of the three possible boundaries, the heliopause is the one that is most often used by researchers, and by NASA, to define the solar system's edge. This is because it is the easiest to pin down and because the magnetic properties on either side of it are significantly different.

"I would argue for the heliopause to be the boundary because it really is a boundary," Reisenfeld said. "Once you've passed it, you know it."

— How many times has Earth orbited the sun?

— How many times has the sun traveled around the Milky Way?

— What's the maximum number of planets that could orbit the sun?

But that doesn't mean that everything beyond the heliopause should be considered an interstellar object, such as the enormous space rock 'Oumuamua , Reisenfeld added. "The Oort Cloud was originally part of the same stuff that the planets were formed from, so it is composed of solar system material, not interstellar material," he said.

But while some researchers are happy to pick a side in this argument, others see no reason why the solar system cannot have multiple boundaries.

"I would say that there is no actual debate," Brown said. "There are just different ways to define it depending on what is important for the question you are trying to answer."

Harry is a U.K.-based senior staff writer at Live Science. He studied marine biology at the University of Exeter before training to become a journalist. He covers a wide range of topics including space exploration, planetary science, space weather, climate change, animal behavior, evolution and paleontology. His feature on the upcoming solar maximum was shortlisted in the "top scoop" category at the National Council for the Training of Journalists (NCTJ) Awards for Excellence in 2023. 

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• Beauxdensteiner I'd say that all three together form the boundary of the solar system because when you are out there at that very large edge of the bubble, everything that has definition to the outer edge of the bubble must be part of it. All three are part of that boundary and it is just too big for us to get our tiny human brains wrapped around it. Reply
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Astrophysics > Earth and Planetary Astrophysics

Title: vulcan: retreading a tired hypothesis with the 2024 total solar eclipse.

Abstract: The number of planets in the solar system over the last three centuries has, perhaps surprisingly, been less of a fixed value than one would think it should be. In this paper, we look at the specific case of Vulcan, which was both a planet before Pluto was a planet and discarded from being a planet before Pluto was downgraded. We examine the historical context that led to its discovery in the 19th century, the decades of observations that were taken of it, and its eventual fall from glory. By applying a more modern understanding of astrophysics, we provide multiple mechanisms that may have changed the orbit of Vulcan sufficiently that it would have been outside the footprint of early 20th century searches for it. Finally, we discuss how the April 8, 2024 eclipse provides a renewed opportunity to rediscover this lost planet after more than a century of having been overlooked.

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Someday, Earth Will Have a Final Total Solar Eclipse

The moon will drift far enough from Earth that it no longer fully obstructs the sun. But predicting when this will happen poses numerous challenges.

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By Katherine Kornei

The total solar eclipse visible on Monday over parts of Mexico, the United States and Canada was a perfect confluence of the sun and the moon in the sky. But it’s also the kind of event that comes with an expiration date: At some point in the distant future, Earth will experience its last total solar eclipse.

That’s because the moon is drifting away from Earth, so our nearest celestial neighbor will one day, millions or even billions of years in the future, appear too small in the sky to completely obscure the sun.

“We’ll only ever have annular eclipses,” said Noah Petro, a planetary scientist at NASA Goddard Space Flight Center, referring to “ring of fire” eclipses like the one that crossed the Americas in October .

But putting an exact date on Earth’s final total solar eclipse is a serious computational challenge involving a variety of scientific disciplines.

Ever since the moon formed over four billion years ago, it has been spiraling away from Earth. The moon’s retreat results from its gravitational interactions with our planet. Tides raised by that gravity send the water in our planet’s oceans sliding over the seafloor and along the edges of continents. That creates friction that causes Earth to spin more slowly on its axis, said Mattias Green, an ocean scientist at Bangor University in Wales.

The moon moves outward in its orbit in response to the slowing of the Earth. Imagine a figure skater extending her arms and slowing down, Dr. Green said. “It’s the same physical principle but backwards.”

One of the first people to predict the expanding orbit of the moon was George Darwin, one of Charles Darwin’s sons. But his hypothesis, published in 1879 , would not be verified until American astronauts and Soviet robotic rovers left devices known as retroreflectors on the moon’s surface . Researchers could fire laser pulses at mirrors on those suitcase-size instruments and time how long it took the light to make a round trip. That gave scientists a way of precisely measuring the distance to the moon. By the early 1970s, researchers had discovered that the moon was receding from Earth by about 1.5 inches each year.

That’s about the rate at which human fingernails grow. “We’re dealing with extremely small changes,” said Robert Tyler, a planetary scientist at NASA Goddard Space Flight Center.

But over hundreds of millions of years, the moon will become perceptibly smaller in the sky as it grows more distant. At some point, it will appear too small to completely blot out the sun, and total solar eclipses will become a thing of the past.

To calculate the date of the last total solar eclipse, it is important to remember that both the moon’s orbit around the Earth and Earth’s orbit around the sun are elliptical. That means that the distances between Earth and the moon and between Earth and the Sun are not constant. The apparent sizes of the moon and the sun as seen from Earth vary accordingly; the largest- and smallest-looking moons differ in size by about 14 percent, while the corresponding difference for the sun is about 3 percent.

The last total solar eclipse will occur when the largest-looking moon just barely covers the smallest-looking sun. A bit of math involving the diameter of the moon and the apparent sizes of the moon and the sun yields an estimate for that eventuality of approximately 620 million years.

But there is uncertainty in that number, researchers caution. It assumes, for starters, that the moon will recede from Earth at its current rate. And that almost certainly won’t happen, Dr. Green said.

The moon’s recession rate is affected by a slew of parameters, he said, including the length of a day on Earth, the depth of the ocean basins and the arrangement of our continents. Those things change over time, Dr. Green said, so it is too simplistic to presume that the moon will always retreat at the same pace.

Most researchers agree that the moon’s recession rate will probably decrease. “If I had to guess, the tides of the future will probably get weaker,” said Brian Arbic, a physical oceanographer at the University of Michigan. Weaker tides translate into slower lunar retreat, which would buy our planet more opportunities to bask in the moon’s umbral shadow.

There’s good evidence that the moon retreated more slowly in the past as well. Margriet Lantink, a geologist at the University of Wisconsin-Madison, has analyzed sedimentary rocks in Australia that record climatic changes caused by fluctuations in the Earth-moon distance. “I read the fingerprints of those astronomical variations,” Dr. Lantink said.

Her team’s findings , and those of other researchers, have been used in simulations that suggest the moon receded by about 0.4 to 1.2 inches per year for much of its history. Those simulations also reveal that during some periods lasting a few tens of millions of years, the moon hurtled away from Earth at more than four inches per year.

Dr. Tyler’s models take on the daunting task of forecasting the future lunar recession rate. They suggest that the moon will drift away at around 0.3 inches per year on average for the next several billion years. And the moon’s retreat in the future won’t be nearly as variable as it was in the ancient past, he said. “Most of the interesting stuff happened already.”

If Dr. Tyler’s simulations are correct, total eclipses will remain visible for about three billion years. He cautioned that there is significant uncertainty in that estimate.

And though we likely have eons left to experience total eclipses, that’s no excuse for not seeking out their splendor, Dr. Petro said. After all, they’re a celestial phenomenon that’s unique to our Earthly existence.

“No other planet in our solar system has total solar eclipses,” Dr. Petro said. “We have this wonderful opportunity.”

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10.02: Origin of the Solar System—The Nebular Hypothesis

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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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The history of scientific thought about the Formation and evolution of the Solar System begins with the Copernican Revolution. The first recorded use of the term "Solar System" dates from 1704.

1. Contemporary View

The most widely accepted theory of planetary formation, known as the nebular hypothesis, maintains that 4.6 billion years ago, the Solar System formed from the gravitational collapse of a giant molecular cloud which was light years across. Several stars, including the Sun, formed within the collapsing cloud. The gas that formed the Solar System was slightly more massive than the Sun itself. Most of the mass collected in the centre, forming the Sun; the rest of the mass flattened into a protoplanetary disc, out of which the planets and other bodies in the Solar System formed.

There are, however, arguments against this hypothesis.

2. Formation Hypothesis

French philosopher and mathematician René Descartes was the first to propose a model for the origin of the Solar System in his Le Monde (ou Traité de lumière) which he wrote in 1632 and 1633 and for which he delayed publication because of the Inquisition and it was published only after his death in 1664. In his view, the Universe was filled with vortices of swirling particles and the Sun and planets had condensed from a particularly large vortex that had somehow contracted, which explained the circular motion of the planets and was on the right track with condensation and contraction. However, this was before Newton's theory of gravity and we now know matter does not behave in this fashion. [ 1 ]

The vortex model of 1944, [ 1 ] formulated by German physicist and philosopher Baron Carl Friedrich von Weizsäcker, which harkens back to the Cartesian model, involved a pattern of turbulence-induced eddies in a Laplacian nebular disc. In it a suitable combination of clockwise rotation of each vortex and anti-clockwise rotation of the whole system can lead to individual elements moving around the central mass in Keplerian orbits so there would be little dissipation of energy due to the overall motion of the system but material would be colliding at high relative velocity in the inter-vortex boundaries and in these regions small roller-bearing eddies would coalesce to give annular condensations. It was much criticized as turbulence is a phenomenon associated with disorder and would not spontaneously produce the highly ordered structure required by the hypothesis. As well, it does not provide a solution to the angular momentum problem and does not explain lunar formation nor other very basic characteristics of the Solar System. [ 2 ]

The Weizsäcker model was modified [ 1 ] in 1948 by Dutch theoretical physicist Dirk Ter Haar, in that regular eddies were discarded and replaced by random turbulence which would lead to a very thick nebula where gravitational instability would not occur. He concluded the planets must have formed by accretion and explained the compositional difference (solid and liquid planets) as due to the temperature difference between the inner and outer regions, the former being hotter and the latter being cooler, so only refractories (non-volatiles) condensed in the inner region. A major difficulty is that in this supposition turbulent dissipation takes place in a time scale of only about a millennium which does not give enough time for planets to form.

The nebular hypothesis was first proposed in 1734 by Emanuel Swedenborg [ 3 ] and later elaborated and expanded upon by Immanuel Kant in 1755. A similar theory was independently formulated by Pierre-Simon Laplace in 1796. [ 4 ]

In 1749, Georges-Louis Leclerc, Comte de Buffon conceived the idea that the planets were formed when a comet collided with the Sun, sending matter out to form the planets. However, Laplace refuted this idea in 1796, showing that any planets formed in such a way would eventually crash into the Sun. Laplace felt that the near-circular orbits of the planets were a necessary consequence of their formation. [ 5 ] Today, comets are known to be far too small to have created the Solar System in this way. [ 5 ]

In 1755, Immanuel Kant speculated that observed nebulae may in fact be regions of star and planet formation. In 1796, Laplace elaborated by arguing that the nebula collapsed into a star, and, as it did so, the remaining material gradually spun outward into a flat disc, which then formed the planets. [ 5 ]

2.1. Alternative Theories

However plausible it may appear at first sight, the nebular hypothesis still faces the obstacle of angular momentum; if the Sun had indeed formed from the collapse of such a cloud, the planets should be rotating far more slowly. The Sun, though it contains almost 99.9 percent of the system's mass, contains just 1 percent of its angular momentum. [ 6 ] This means that the Sun should be spinning much more rapidly.

Tidal theory

Attempts to resolve the angular momentum problem led to the temporary abandonment of the nebular hypothesis in favour of a return to "two-body" theories. [ 5 ] For several decades, many astronomers preferred the tidal or near-collision hypothesis put forward by James Jeans in 1917, in which the planets were considered to have been formed due to the approach of some other star to the Sun. This near-miss would have drawn large amounts of matter out of the Sun and the other star by their mutual tidal forces, which could have then condensed into planets. [ 5 ] However, in 1929 astronomer Harold Jeffreys countered that such a near-collision was massively unlikely. [ 5 ] Objections to the hypothesis were also raised by the American astronomer Henry Norris Russell, who showed that it ran into problems with angular momentum for the outer planets, with the planets struggling to avoid being reabsorbed by the Sun. [ 7 ]

The Chamberlin-Moulton model

Forest Moulton in 1900 had also shown that the nebular hypothesis was inconsistent with observations because of the angular momentum. Moulton and Chamberlin in 1904 originated the planetesimal hypothesis [ 8 ] (see Chamberlin–Moulton planetesimal hypothesis). Along with many astronomers of the day they came to believe the pictures of "spiral nebulas" from the Lick Observatory were direct evidence of forming solar systems. These turned out to be galaxies instead but the Shapley-Curtis debate about these was still 16 years in the future. One of the most fundamental issues in the history of astronomy was distinguishing between nebulas and galaxies.

Moulton and Chamberlin suggested that a star had passed close to the Sun early in its life to cause tidal bulges and that this, along with the internal process that leads to solar prominences, resulted in the ejection of filaments of matter from both stars. While most of the material would have fallen back, part of it would remain in orbit. The filaments cooled into numerous, tiny, solid fragments, ‘planetesimals’, and a few larger protoplanets. This model received favourable support for about 3 decades but passed out of favour by the late '30s and was discarded in the '40s by the realization it was incompatible with the angular momentum of Jupiter, but a part of it, planetesimal accretion, was retained. [ 1 ]

Lyttleton's scenario [ 1 ]

In 1937 and 1940, Ray Lyttleton postulated that a companion star to the Sun collided with a passing star. Such a scenario was already suggested and rejected by Henry Russell in 1935. Lyttleton showed terrestrial planets were too small to condense on their own so suggested one very large proto-planet broke in two because of rotational instability, forming Jupiter and Saturn, with a connecting filament from which the other planets formed. A later model, from 1940 and 1941, involves a triple star system, a binary plus the Sun, in which the binary merges and later breaks up because of rotational instability and escapes from the system leaving a filament that formed between them to be captured by the Sun. Objections of Lyman Spitzer apply to this model also. [ clarification needed ]

Band-structure model

In 1954, 1975, and 1978 [ 9 ] Swedish astrophysicist Hannes Alfvén included electromagnetic effects in equations of particle motions, and angular momentum distribution and compositional differences were explained. In 1954 he first proposed the band structure in which he distinguished an A-cloud, containing mostly helium, but with some solid- particle impurities ("meteor rain"), a B-cloud, with mostly hydrogen, a C-cloud, having mainly carbon, and a D-cloud, made mainly of silicon and iron. Impurities in the A-cloud form Mars and the Moon (later captured by Earth), in the B-cloud they condense into Mercury, Venus, and Earth, in the C-cloud they condense into the outer planets, and Pluto and Triton may have formed from the D-cloud.

Interstellar cloud theory

In 1943, the Soviet astronomer Otto Schmidt proposed that the Sun, in its present form, passed through a dense interstellar cloud, emerging enveloped in a cloud of dust and gas, from which the planets eventually formed. This solved the angular momentum problem by assuming that the Sun's slow rotation was peculiar to it, and that the planets did not form at the same time as the Sun. [ 5 ] Extensions of the model, together forming the Russian school, include Gurevich and Lebedinsky (in 1950), Safronov (in 1967,1969), Safronov and Vityazeff (in 1985), Safronov and Ruskol (in 1994), and Ruskol (in 1981), among others [ 10 ] However, this hypothesis was severely dented by Victor Safronov who showed that the amount of time required to form the planets from such a diffuse envelope would far exceed the Solar System's determined age. [ 5 ]

Ray Lyttleton modified the theory by showing that a 3rd body was not necessary and proposing that a mechanism of line accretion described by Bondi and Hoyle in 1944 would enable cloud material to be captured by the star (Williams and Cremin, 1968, loc. cit.)

Hoyle's hypothesis

In this model [ 1 ] (from 1944) the companion went nova with ejected material captured by the Sun and planets forming from this material. In a version a year later it was a supernova. In 1955 he proposed a similar system to Laplace, and with more mathematical detail in 1960. It differs from Laplace in that a magnetic torque occurs between the disk and the Sun, which comes into effect immediately or else more and more matter would be ejected resulting in a much too massive planetary system, one comparable to the Sun. The torque causes a magnetic coupling and acts to transfer angular momentum from the Sun to the disk. The magnetic field strength would have to be 1 gauss. The existence of torque depends on magnetic lines of force being frozen into the disk (a consequence of a well-known MHD (magnetohydrodynamic) theorem on frozen-in lines of force). As the solar condensation temperature when the disk was ejected could not be much more than 1000 degrees K., a number of refractories must be solid, probably as fine smoke particles, which would grow with condensation and accretion. These particles would be swept out with the disk only if their diameter at the Earth's orbit was less than 1 meter so as the disk moved outward a subsidiary disk consisting of only refractories remains behind where the terrestrial planets would form. The model is in good agreement with the mass and composition of the planets and angular momentum distribution provided the magnetic coupling is an acceptable idea, but not explained are twinning, the low mass of Mars and Mercury, and the planetoid belts. It was Alfvén who formulated the concept of frozen-in magnetic field lines.

Kuiper's theory

Gerard Kuiper (in 1944) [ 1 ] argued, like Ter Haar, that regular eddies would be impossible and postulated that large gravitational instabilities might occur in the solar nebula, forming condensations. In this, the solar nebula could be either co-genetic with the Sun or captured by it. Density distribution would determine what could form: either a planetary system or a stellar companion. The 2 types of planets were assumed to be due to the Roche limit. No explanation was offered for the Sun's slow rotation which Kuiper saw as a larger G-star problem.

Whipple's theory

In Fred Whipple's 1948 scenario [ 1 ] a smoke cloud about 60,000 AU in diameter and with 1 solar mass ( M ☉ ) contracts and produces the Sun. It has a negligible angular momentum thus accounting for the Sun's similar property. This smoke cloud captures a smaller one with a large angular momentum. The collapse time for the large smoke and gas nebula is about 100 million years and the rate is slow at first, increasing in later stages. The planets would condense from small clouds developed in, or captured by, the 2nd cloud, the orbits would be nearly circular because accretion would reduce eccentricity due to the influence of the resisting medium, orbital orientations would be similar because the small cloud was originally small and the motions would be in a common direction. The protoplanets might have heated up to such high degrees that the more volatile compounds would have been lost and the orbital velocity decreases with increasing distance so that the terrestrial planets would have been more affected. The weaknesses of this scenario are that practically all the final regularities are introduced as a priori assumptions and most of the hypothesizing was not supported by quantitative calculations. For these reasons it did not gain wide acceptance.

• Williams, I.O., Cremin, A.W. 1968. A survey of theories relating to the origin of the solar system. Qtly. Rev. RAS 9: 40–62. ads.abs.harvard.edu/abs
• Woolfson, Michael Mark, The Origin and Evolution of universe and the Solar System, Taylor and Francis, 2000 ; completely considered that collision of the two suns produce the solar system and universe in the entire 100,00 years of the evolution.
• Swedenborg, Emanuel. 1734, (Principia) Latin: Opera Philosophica et Mineralia (English: Philosophical and Mineralogical Works), (Principia, Volume 1)
• See, T. J. J. (1909). "The Past History of the Earth as Inferred from the Mode of Formation of the Solar System". Proceedings of the American Philosophical Society 48 (191): 119–128. 
• Michael Mark (1993). "The Solar System: Its Origin and Evolution". Journal of the Royal Astronomical Society 34: 1–20. Bibcode: 1993QJRAS..34....1W. "Physics Department, University of New York".  http://adsabs.harvard.edu/abs/1993QJRAS..34....1W
• Woolfson, Michael Mark (1984). "Rotation in the Solar System". Philosophical Transactions of the Royal Society of London 313 (1524): 5. doi:10.1098/rsta.1984.0078. Bibcode: 1984RSPTA.313....5W.  https://dx.doi.org/10.1098%2Frsta.1984.0078
• Benjamin Crowell (1998–2006). "5". Conservation Laws. lightandmatter.com. ISBN 0-9704670-2-8. http://www.lightandmatter.com/html_books/2cl/ch05/ch05.html. 
• Sherrill, T.J. 1999. A Career of Controversy: the Anomaly of T.J.J. See. J. Hist. Astrn. ads.abs.harvard.edu/abs/1999JHA.
• Alfvén, H. 1978. Band Structure of the Solar System. In Origin of the Solar System, S.F. Dermot, ed, pp. 41–48. Wiley.
• Williams, I.O., Cremin, A.W. 1968. A survey of theories relating to the origin of the solar system. Qtly. Rev. RAS 9: 40–62. ads.abs.harvard.edu/abs.

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April 5, 2024

A Solar Eclipse Is Too Special to See Through Your Smartphone

To make the most of any total solar eclipse, put down your gadgets and bask in one of our solar system’s most glorious spectacles

By Phil Plait

Pete Marovich/Getty Images

This article is part of a special report on the total solar eclipse that will be visible from parts of the U.S., Mexico and Canada on April 8, 2024.

Unless you’ve been living in Earth’s core for the past few weeks, you’ve probably heard that there will be a total solar eclipse across a long swath of the U.S. on April 8, 2024.

Scientific American has been covering this fantastic event with a lot of articles about why this happens , where to watch , what you can expect to see , how you can participate in the science and how to watch safely .

On supporting science journalism

If you're enjoying this article, consider supporting our award-winning journalism by subscribing . By purchasing a subscription you are helping to ensure the future of impactful stories about the discoveries and ideas shaping our world today.

All of those articles are based on fact and science and observation, and they are a big help if you want to understand what’s going on in the skies over your head on the big day.

But what will you feel?

I know this isn’t the most scientific of questions—it stopped Spock cold when it was asked of him in Star Trek IV: The Voyage Home—but while an eclipse is, at its most fundamental, a scientific event, we experience it as humans. We feel.

Stories abound of people seeing a total solar eclipse for the first time. They gasp; they laugh; they jump up and down; they stand rooted to the ground and gape upward in awe; they even weep and become choked up by the overwhelming emotions flowing through them.

I understood all that as I gathered with a group of friends on a ranch in Wyoming on August 21, 2017. Well, I understood it academically , but I hadn’t experienced it for myself. I had been an astronomer for many decades, yet I hadn’t ever witnessed a total eclipse for myself. I’ve lost count of how many partial eclipses I’ve seen—more than a dozen, certainly. But partiality is a very different beast than totality. The most apt comparison I’ve heard is that it’s like reading about kissing versus actually kissing someone. You just have to experience it firsthand (or lip).

And so, on that day, when the moon slid over the sun, I experienced my first celestial kiss. And I finally understood what the commotion was about.

Still, it’s hard to describe. Worse, perhaps, is that it’s not clear why we feel the things we do when totality begins. I have my suspicions, though.

For one, the partial phase takes a while to transpire. At first you won’t note any changes around you. The sky, the lighting and everything else will seem normal. It’s not until the sun is mostly obscured, maybe around 70 to 80 percent covered, that you’ll start to notice things getting darker. The color of the sky will change subtly, and even shadows will look different. The sun is not a one-dimensional point of light, like most other stars in Earth’s sky, but rather a spread-out two-dimensional disk. Light shining from all across the sun’s disk acts to soften the edges of shadows, making them a little fuzzy. As the moon covers the sun, however, the illumination sharpens, as do shadows. You may not be overtly cognizant of it, but all this does add to the overall eeriness of the proceedings.

And excitement will be building all the while. In the final minutes before totality, the gloaming deepens. Birds stop chirping, insects may start thrumming, and the temperature drops. It feels like twilight in the middle of the afternoon.

Then at last the moon slides over the last bit of the sun. The sky truly darkens, and suddenly the solar corona appears: our star’s ethereal outer atmosphere will glow with a pearly luminescence that is vivid and literally otherworldly. We cannot see it without the benefit of our moon. It’s the climax to the growing tension you’ve been experiencing—an emotional release after having all the excitement wound up more tightly. And it’s so overwhelmingly beautiful that it’s not a surprise at all that people react so strongly.

At least, that’s my hypothesis. It’s not based on actual science, just my experience from 2017. Evidence like this is whimsically called “anecdata”: it’s something that’s better than an anecdote but probably not solid enough to be published in a journal. Still, I’d bet money I’m at least partially—if not totally—right.

And this is why it’s so important for you to actually experience it.

A digression: In the 1990s I was getting ready to head down to Florida to watch the Space Shuttle Discovery loft a camera I had worked on into space to be installed on the Hubble Space Telescope. A friend of mine who had seen dozens of shuttle launches gave me a tip. “Don’t take pictures or video,” he told me. “Just watch. Experience it. If you’re fiddling with your equipment during the launch, that’s what you’ll remember, not the launch itself.”

Being the hardheaded fool I was, of course, I ignored his advice. I tried to get video of the launch, but the camera had difficulty focusing, and all I remember now is swearing at my camera. Adding to the insult, the video I have to commemorate the event is blurry and useless. By trying to record the event, I actually wound up missing it.

I took this lesson to heart in 2017 and didn’t take a single photograph of the eclipse. Why bother? Millions of others would, and almost all those snapshots would look just like mine. I did take some pictures leading up to totality; we used a colander as a pinhole camera to make images of the partial phase on the ground, for example. That was fun. But when the moment came, I just drank it in.

And it was one of the single most profound experiences of my life. It’s imprinted on my soul, along with memories of getting married and the birth of my daughter. It truly is that overwhelming.

My advice? Learn from my past foolishness and just be present for this celestial magnificence.

What will you feel if you do? You may be flooded with joy, wonder, awe, a numinous sense of majesty or any of a dozen other emotions. Your reaction is personal and deserves to be your own. This is what makes the eclipse such a special phenomenon to us. Don’t interfere with your interaction with it! Let it happen and you’ll experience a profundity that will stay with you far longer than any memory reflected in a photograph.

Be there. You won’t regret it.

The Solar System

Introduction to the solar system, lesson objectives.

• Describe historical views of the solar system.
• Name the planets, and describe their motion around the sun.
• Explain how the solar system formed.
• geocentric model
• heliocentric model
• nebular hypothesis
• solar system

Changing Views of the Solar System

Humans’ view of the  solar system  has evolved as technology and scientific knowledge have increased. The ancient Greeks identified five of the planets and for many centuries they were the only planets known. Since then, scientists have discovered two more planets, many other solar-system objects and even planets found outside our solar system.

The Geocentric Universe

The ancient Greeks believed that Earth was at the center of the universe, as shown in  Figure   below. This view is called the  geocentric model  of the universe. Geocentric means “Earth-centered.” In the geocentric model, the sky, or heavens, are a set of spheres layered on top of one another. Each object in the sky is attached to a sphere and moves around Earth as that sphere rotates. From Earth outward, these spheres contain the Moon, Mercury, Venus, the Sun, Mars, Jupiter, and Saturn. An outer sphere holds all the stars. Since the planets appear to move much faster than the stars, the Greeks placed them closer to Earth.

The Modern Solar System

Today, we know that our solar system is just one tiny part of the universe as a whole. Neither Earth nor the Sun are at the center of the universe. However, the heliocentric model accurately describes the solar system. In our modern view of the solar system, the Sun is at the center, with the planets moving in elliptical orbits around the Sun. The planets do not emit their own light, but instead reflect light from the Sun.

Extrasolar Planets or Exoplanets

Since the early 1990s, astronomers have discovered other solar systems, with planets orbiting stars other than our own Sun (called “extrasolar planets” or simply “exoplanets”) ( Figure   below).

An introduction to extrasolar planets from NASA is available at  (1g) :  http://www.youtube.com/watch?v=oeeZCHDNTvQ  (3:14).

KQED: The Planet Hunters

Hundreds of exoplanets have now been discovered. To learn something about how planet hunters find these balls of rock they usually can’t even see, watch this QUEST video. Learn more at:  http://science.kqed.org/quest/video/the-planet-hunters/  and http://science.kqed.org/quest/audio/exoplanets/ .

Planets and Their Motions

Since the time of Copernicus, Kepler, and Galileo, we have learned a lot more about our solar system. Astronomers have discovered two more planets (Uranus and Neptune), four dwarf planets (Ceres, Pluto, Makemake, Haumea, and Eris), more than 150 moons, and many, many asteroids and other small objects.

( Figure   below) shows the Sun and the major objects that orbit the Sun. There are eight planets (Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune) and the five known dwarf planets and the five known dwarf planets (Ceres, Pluto, Makemake, Haumea, and Eris).

The Role of Gravity

Isaac Newton was one of the first scientists to explore gravity. He understood that the Moon circles the Earth because a force is pulling the Moon toward Earth’s center. Without that force, the Moon would continue moving in a straight line off into space. Newton also came to understand that the same force that keeps the Moon in its orbit is the same force that causes objects on Earth to fall to the ground.

Newton defined the Universal Law of Gravitation, which states that a force of attraction, called gravity, exists between all objects in the universe ( Figure   below). The strength of the gravitational force depends on how much mass the objects have and how far apart they are from each other. The greater the objects’ mass, the greater the force of attraction; in addition, the greater the distance between the objects, the smaller the force of attraction.

Lesson Summary

• The solar system is the Sun and all the objects that are bound to the Sun by gravity.
• The solar system has eight planets: Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Ceres, Makemake, Pluto and Eris are dwarf planets.
• The ancient Greeks and people for centuries afterwards believed in a geocentric model of the universe, with Earth at the center and everything else orbiting our planet.
• Copernicus, Kepler, and Galileo promoted a heliocentric model of the universe, with the Sun at the center and Earth and the other planets orbiting the Sun.
• Gravity holds planets in elliptical orbits around the Sun.
• The nebular hypothesis describes how the solar system formed from a giant cloud of gas and dust about 4.6 billion years ago.

Review Questions

1. What does geocentric mean?

2. Describe the geocentric model and heliocentric model of the universe.

3. How was Kepler’s version of the heliocentric model different from Copernicus’?

4. Name the eight planets in order from the Sun outward. Which are the inner planets and which are the outer planets?

5. Compare and contrast the inner planets and the outer planets.

6. What object used to be considered a planet, but is now considered a dwarf planet? What are the other dwarf planets?

7. What keeps planets and moons in their orbits?

8. How old is the solar system? How old is Earth?

9. Use the nebular hypothesis to explain why the planets all orbit the Sun in the same direction.

Further Reading / Supplemental Links

• More information about the solar system from NASA:  http://sse.jpl.nasa.gov/planets/index.cfm
• Lots of information about the solar system from the BBC: BBC Explore the solar system  http://www.bbc.co.uk/solarsystem/
• Information about solar system objects:  http://www.solarviews.com/eng/homepage.htm
• A multimedia tour of the solar system:  http://www.nineplanets.org/
• Windows to the Universe:  http://www.windows.ucar.edu/tour/link=/our_solar_system/formation.html
• Space news:  http://www.space.com/

Points to Consider

• Would you expect all the planets in the solar system to be made of similar materials? Why or why not?
• The planets are often divided into two groups: the inner planets and the outer planets. Which planets do you think are in each of these two groups? What do members of each group have in common?
• Provided by : CK12.org. Located at : http://www.ck12.org/book/CK-12-Earth-Science-For-High-School/section/25.1/ . License : CC BY-NC: Attribution-NonCommercial

First ever 'rainbow' discovered outside our solar system

The CHEOPS space telescope has found a 'rainbow-like' phenomenon outside our solar system for the first time.

The telescope has been surveying the mysterious exoplanet WASP-76b. A peculiarity on the planet is thought to be due to a ''glory'', a luminous phenomenon similar to a rainbow, which occurs if the light from the star - the ''sun'' around which the exoplanet orbits - is reflected by clouds made up of a perfectly uniform substance.

If this hypothesis is confirmed, this would be the first detection of this phenomenon outside our solar system. WASP-76b is an ultra-hot giant planet. Orbiting its host star twelve times closer than Mercury orbits our Sun, it receives more than 4,000 times the Sun's radiation on Earth.

''The exoplanet is 'inflated' by the intense radiation from its star. So, although it is 10 per cent less massive than our cousin Jupiter, it is almost twice as big,'' explains Monika Lendl, assistant professor in the Department of Astronomy of the UNIGE Faculty of Science, and co-author of the study.

Since its discovery in 2013, WASP-76b has been the subject of intense scrutiny by astronomers. A strangely hellish picture has emerged. One side of the planet is always facing its star, reaching temperatures of 2,400 degrees Celsius.

Elements that would form rocks on Earth melt and evaporate here, before condensing on the slightly cooler night side, creating clouds of iron that drip molten iron rain.

Copyright 2024 Cover Media. Distributed by Tribune Content Agency, LLC.

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On April 8, the moon’s shadow will sweep through North America, trailing a diagonal ribbon of momentary, midday darkness across parts of the continent. Those who happen to be within the “path of totality” will experience a total solar eclipse — a few eerie minutes when the sun, moon, and Earth align, such that the moon perfectly blocks out the sun.

The last solar eclipse to pass over the continental United States occurred in August 2017, when the moon’s shadow swept from Oregon down to South Carolina. This time, the moon will be closer to the Earth and will track a wider ribbon, from Mexico through Texas and on up into Maine and eastern Canada. The shadow will move across more populated regions than in 2017, and will completely block the sun for more than 31 million people who live in its path. The eclipse will also partly shade many more regions, giving much of the country a partial eclipse, depending on the local weather.

While many of us ready our eclipse-grade eyewear, scientists at MIT’s Haystack Observatory are preparing a constellation of instruments to study the eclipse and how it will affect the topmost layers of the atmosphere. In particular, they will be focused on the ionosphere — the atmosphere’s outermost layer where many satellites orbit. The ionosphere stretches from 50 to 400 miles above the Earth’s surface and is continually blasted by the sun’s extreme ultraviolet and X-ray radiation. This daily solar exposure ionizes gas molecules in the atmosphere, creating a charged sea of electrons and ions that shifts with changes in the sun’s energy.

As they did in 2017 , Haystack researchers will study how the ionosphere responds before, during, and after the eclipse, as the sun’s radiation suddenly dips. With this year’s event, the scientists will be adding two new technologies to the mix, giving them a first opportunity to observe the eclipse’s effects at local, regional, and national scales. What they observe will help scientists better understand how the atmosphere reacts to other sudden changes in solar radiation, such as solar storms and flares.

Two lead members of Haystack’s eclipse effort are research scientists Larisa Goncharenko, who studies the physics of the ionosphere using measurements from multiple observational sources, and John Swoboda, who develops instruments to observe near-Earth space phenomena. While preparing for eclipse day, Goncharenko and Swoboda took a break to chat with MIT News about the ways in which they will be watching the event and what they hope to learn from Monday’s rare planetary alignment.

Q: There’s a lot of excitement around this solar eclipse. Before we dive into how you’ll be observing it, let’s take a step back to talk about what we know so far: How does a total eclipse affect the atmosphere?

Goncharenko : We know quite a bit. One of the largest effects is, as the moon’s shadow moves over part of the continent, we have a significant decrease in electron, or plasma, density in the ionosphere. The sun is an ionization source, and as soon as that source is removed, we have a decrease in electron density. So, we sort of have a hole in the ionosphere that moves behind the moon’s shadow.

During an eclipse, solar heating shuts off and it’s like a rapid sunset and sunrise, and we have significant cooling in the atmosphere. So, we have this cold area of low ionization, moving in latitude and longitude. And because of this change in temperature, you also have disturbances in the wind system that affect how plasma, or electrons in the ionosphere, are distributed. And these are changes on large scales.

From this cold area that follows totality, we also have different kinds of waves emanating. Like a boat moving on the water, you have bow shock waves moving from the shadow. These are waves in electron density. They are small perturbations but can cover really large areas. We saw similar waves in the 2017 eclipse. But every eclipse is different. So, we will be using this eclipse as a unique lab experiment. And we will be able to see changes in electron density, temperature, and winds in the upper atmosphere as the eclipse moves over the continental United States.

Q: How will you be seeing all this? What experiments will you be running to catch the eclipse and its effects on the atmosphere?

Swoboda: We’re going to measure local changes in the atmosphere and ionosphere using two new radar technologies . The first is Zephyr, which was developed by [Haystack research scientist] Ryan Volz. Zephyr looks at how meteors break up in our atmosphere. There are always little bits of sand that burn up in the Earth’s atmosphere, and when they burn up, they leave a trail of plasma that follows the wind patterns in the upper atmosphere. Zephyr sends out a signal that bounces off these plasma trails, so we can see how they are carried by winds moving at very high altitude. We will use Zephyr to observe how these winds in the upper atmosphere change during the eclipse.

The other radar system is EMVSIS [Electro-Magnetic Vector Sensor Ionospheric Sounder], which will measure the electron or plasma density and the bulk velocity of the charged particles in the ionosphere. Both these systems comprise a distributed array of transmitters and receivers that send and receive radio waves at various frequencies to do their measurements. Traditional ionospheric sounders require high-power transmitters and large towers on the order of hundreds of feet, and can cover an area the size of a football field. But we’ve developed a lower-power and physically smaller system, about the size of a refrigerator, and we’re deploying multiple of these systems around New England to make local and regional measurements.

Goncharenko: We will also make regional observations with two antennas at the Millstone Hill Geospace Facility [in Westford, Massachusetts]. One antenna is a fixed vertical antenna, 220 feet in diameter, that we can use to observe parameters in the ionosphere over a huge range of altitudes, from 90 to 1,000 kilometers above the ground. The other is a steerable antenna that’s 150 feet in diameter, which we can move to look what happens as far away as Florida and all the way to the central United States. We are planning to use both antennas to see changes during the eclipse.

We’ll also be processing data from a national network of almost 3,000 GNSS [Global Navigation Satellite System] receivers across the United States, and we’re installing new receivers in undersampled regions along the area of totality. These receivers will measure how the ionosphere’s electron content changes before, during, and after the eclipse.

One of the most exciting things is, this is the first time we’ll have all four of these technologies working together. Each of these technologies provides a unique point of view. And for me as a scientist, I feel like a little kid on Christmas Eve. You know great things are coming, and you know you’ll have new things to play with and new data to analyze.

Q: And speaking of what you’ll find, what do you expect to see from the measurements you collect?

Goncharenko : I expect to see the unexpected. It will be first time for us to look at the near-Earth space with a combination of four very different technologies at the same time and in the same geographic region. We expect higher sensitivity that translates into better resolution in time and space. Probing the upper atmosphere with a combination of these diagnostic tools will provide simultaneous observations we never had before — four-dimensional wind flow, electron density, ion temperature, plasma motion. We will observe how they change during the eclipse and study how and why changes in one area of the upper atmosphere are linked to perturbations in other areas in space and time.

Swoboda : We’re also sort of thinking longer term. What the eclipse is giving us is a chance to show what these technologies can do, and say, what if we could have these going all the time? We could run it as a sort of radar network for space weather, like how we monitor weather in the lower atmosphere. And we need to monitor space weather, because we have so much going on in the near-Earth space environment, with satellites launching all the time that are affected by space weather.

Goncharenko : We have a lot of space to study. The eclipse is just the highlight. But overall, these systems can produce more data to get a look at what happens in the upper atmosphere and ionosphere during other disturbances, such as storms and lightning periods, or coronal mass ejections and solar flares. And all of this is part of a large effort to build up our understanding of near-Earth space to meet demands of modern technological society.

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