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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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Historical Geology

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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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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.

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7.6: The Nebular Theory

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A protostar is an object in which no nuclear fusion has occurred, unlike a star that is undergoing nuclear fusion. A protostar becomes a star when nuclear fusion begins. Most likely the next step was that the nebula flattened into a disk called the Protoplanetary Disk ; planets eventually formed from and in this disk.

Three processes occurred with the nebular collapse:

• The orderly motions of the solar system today are a direct result of the solar system’s beginnings in a spinning, flattened cloud of gas and dust.
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8 The Rise and Fall of the Nebular Hypothesis

• Published: August 2023
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The first to explain the origin of the planets and moons was Pierre-Simon Laplace in his 1796 book, Exposition for the System of the World . His theory would dominate science throughout the next century and come to be accepted as a given. He held that the solar system had begun as a hot, rotating gas cloud. As it spun, centrifugal force threw off blobs of gas that coagulated into planets. The planets then repeated the process to create their moons. By the last few decades of the eighteenth century, enough evidence had come to light to call the nebular hypothesis into question, if not to falsify it. This opened the way for three different theories for the origin of the Moon. The fission theory resembled the nebular hypothesis in holding that the gravity of the Sun had pulled off a bulge in the proto-Earth which became the Moon. The co-accretion theory held that the Moon and the Earth had formed near each other and thus were like sister planets. The capture theory imagined that the Moon had started out in some distant region of the solar system but drew near enough to be captured into orbit by the Earth’s gravity.

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Key Concepts:

• What are the key characteristics of our Solar System?
• What do they tell us about the origin of our Solar System?
• How do astronomers discover other solar systems?

Jovian Planets

Solar nebular theory, observational clues, quiz 13a: dating earth..., quiz 13b: killer asteroids, planetary diversity.

Should all planetary systems be like our?

What variations should one expect?

• Planets can eclipse the central star along the line of sight in some cases, and one can measure a brief dimming of the star and infer the presence of a planet.
• A planet around a star passing in front of another star along the line of sight can cause signatures of light change to the prediction by Einstein.

Want to create or adapt books like this? Learn more about how Pressbooks supports open publishing practices.

46 The Nebular Theory: Other Important Evidence

Introduction to Astronomy Copyright © by Lumen Learning is licensed under a Creative Commons Attribution 4.0 International License , except where otherwise noted.

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Turn Your Curiosity Into Discovery

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15 intriguing facts about nebular hypothesis.

Written by Vivyanne Lussier

Modified & Updated: 02 Mar 2024

Reviewed by Sherman Smith

• Physical Sciences
• Cosmic Dust Facts
• Stellar Evolution Facts

The Nebular Hypothesis is a fascinating concept that attempts to explain the formation of our solar system. Proposed in the 18th century by Immanuel Kant and further developed by Pierre-Simon Laplace, this theory suggests that our solar system originated from a massive rotating cloud of gas and dust known as a nebula.

In this article, we will delve into the intricacies of the Nebular Hypothesis and uncover 15 intriguing facts that shed light on our understanding of how our solar system came into existence. From the creation of the sun and planets to the formation of asteroids and comets , each fact presents a unique perspective on the inner workings of the nebular theory.

So, buckle up and prepare to embark on a cosmic journey as we explore the mysteries of the universe and unravel the secrets hidden within the Nebular Hypothesis.

Key Takeaways:

• The Nebular Hypothesis explains how our solar system formed from a spinning cloud of gas and dust, giving rise to the planets and the sun. It also helps us understand planet formation in other star systems.
• This fascinating theory has shaped our understanding of the universe and continues to inspire scientists to explore the origins of solar systems, pushing the boundaries of our knowledge.

The Nebular Hypothesis is a widely accepted explanation for the formation of the solar system.

The Nebular Hypothesis suggests that the solar system originated from a cloud of gas and dust, known as the solar nebula.

It was first proposed by philosopher Immanuel Kant in the 18th century.

Kant hypothesized that a rotating, flattened disk of gas and dust gradually formed the planets and the sun .

The Nebular Hypothesis was further developed by French mathematician and astronomer Pierre-Simon Laplace in the late 18th century.

Laplace expanded on Kant’s idea, suggesting that the solar nebula contracted due to gravitational forces, causing it to spin faster and flatten into a disk.

According to the Nebular Hypothesis, the sun and planets formed from the collapse of a rotating cloud of gas and dust.

As the solar nebula contracted, it began to spin faster, and the majority of the material collected at the center, forming the sun.

The remaining material in the disk gradually accumulated to form protoplanetary bodies, known as planetesimals.

These planetesimals collided and merged over time, eventually forming the planets we see today.

The Nebular Hypothesis explains why the planets in our solar system orbit the sun in the same direction and roughly in the same plane.

The rotation of the original cloud of gas and dust determined the direction and orientation of the planetary orbits.

It also accounts for the fact that the inner planets (Mercury, Venus, Earth, and Mars) are rocky, while the outer planets (Jupiter, Saturn, Uranus, and Neptune) are composed mostly of gas.

As the solar nebula cooled, volatile compounds accumulated further from the sun, allowing the gas giants to form in the outer regions.

The Nebular Hypothesis suggests that the moon formed from the debris left over after a giant impact between Earth and another celestial body.

This collision ejected material into space , which eventually coalesced to form the moon.

The concept of the Nebular Hypothesis is not restricted to our solar system.

Astronomers have observed similar disk formations around other stars, providing evidence that the process of planet formation is common in the universe.

The Nebular Hypothesis has evolved over time and is continually refined as new observations and data become available.

Advancements in technology and space missions have allowed scientists to gather more information about the formation of planets and the evolution of solar systems.

This hypothesis has gained support from various scientific disciplines, including astronomy, astrophysics, and planetary science.

The wealth of evidence collected from telescopic observations, meteorite analysis, and computer simulations have bolstered the credibility of the Nebular Hypothesis.

The Nebular Hypothesis provides insights into the early stages of planet formation, helping scientists understand the conditions necessary for life to exist.

By studying how planets form within a solar nebula, researchers can better grasp the potential habitability of exoplanets in other star systems.

It can also explain the presence of asteroids and comets in our solar system.

These celestial objects are remnants from the early stages of planetary formation and have been preserved in their original forms.

The Nebular Hypothesis has been instrumental in shaping our understanding of the universe and our place within it.

By providing a framework for how solar systems form, it has laid the foundation for further investigations into planetary science and exoplanet research.

The Nebular Hypothesis continues to spark curiosity and drive scientific inquiry, pushing the boundaries of our knowledge about the origins of the universe.

As technology advances and our understanding deepens, we can expect further advancements and refinements to this intriguing theory.

In conclusion, the Nebular Hypothesis has revolutionized our understanding of the formation and evolution of our universe. Through extensive research and observation, scientists have unraveled the mysteries of planetary systems, including our own solar system . The Nebular Hypothesis proposes that the solar system originated from a giant rotating cloud of gas and dust called the nebula. Over time, gravity caused this nebula to collapse, giving birth to the Sun and forming a rotating disk of material around it. Within this disk, planets, moons, and other celestial objects formed.The study of the Nebular Hypothesis has provided us with intriguing facts about the origins of our solar system and beyond. From the formation of planetary rings to the presence of exoplanets, the Nebular Hypothesis continues to shape our understanding of the vast universe we inhabit.As we delve deeper into the mysteries of the universe, the Nebular Hypothesis serves as a guiding principle, shedding light on the intricate mechanisms that govern the formation and evolution of galaxies , stars, and celestial bodies. Through ongoing research and exploration, we continue to uncover new insights and expand our knowledge of the mesmerizing cosmos.

Q: What is the Nebular Hypothesis?

A: The Nebular Hypothesis is a scientific theory that proposes the formation of our solar system from a giant rotating cloud of gas and dust called the nebula.

Q: Who proposed the Nebular Hypothesis?

A: The Nebular Hypothesis was first proposed by the French mathematician and astronomer Pierre-Simon Laplace in the late 18th century.

Q: How does the Nebular Hypothesis explain the formation of planets?

A: According to the Nebular Hypothesis, as the nebula collapses under gravity, it forms a rotating disk of material around a central protostar, known as the Sun. Within this disk, planetesimals, small rocky bodies, gradually merge and accrete to form planets.

Q: Does the Nebular Hypothesis apply to other planetary systems?

A: Yes , the Nebular Hypothesis is a widely accepted explanation for the formation of planetary systems beyond our own solar system, known as exoplanetary systems.

Q: What evidence supports the Nebular Hypothesis?

A: There is substantial evidence supporting the Nebular Hypothesis, including the observations of protoplanetary disks around young stars, the presence of exoplanetary systems with similar characteristics to our own, and the isotopic composition of meteorites that aligns with predictions made by the hypothesis.

Q: Can the Nebular Hypothesis explain the formation of other celestial objects?

A: Yes, in addition to planets, the Nebular Hypothesis can also explain the formation of moons, asteroids, comets, and other celestial bodies within our solar system and beyond.

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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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15.2: Origin of the Solar System—The Nebular Hypothesis

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

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

The American Cyclopædia (1879)/Nebular Hypothesis

Edition of 1879.  See also Nebular hypothesis on Wikipedia , and the disclaimer .

NEBULAR HYPOTHESIS , the celebrated speculation of Sir William Herschel, adopted and developed by Laplace, assigning the genesis of the heavenly bodies to the gradual aggregation and condensation of a highly attenuated self-luminous substance diffused through space. (See “Philosophical Transactions,” 1802 and 1811.) To this hypothesis Herschel was led by his conclusion that there were nebulosities not composed of stars. The Rosse telescope having decomposed nebulæ hitherto considered to be irresolvable, and exhibited symptoms of resolvability in others still more intractable, it was assumed that all nebulæ are stellar, their nebulosity being solely a question of distance; and thus, the basis of Herschel's reasoning failing, the fabric of his hypothesis was thought to be demolished. Mr. Herbert Spencer came to its support in the “Westminster Review,” No. cxxxvii. (July, 1858). The argument in its favor is substantially as follows. The assumption that all nebulæ are remote galaxies does not invalidate the indications furnished by the structure of the solar system, which still points to a nebular origin just as significantly as before. But the assumption is inadmissible. The mode of distribution of the nebulæ furnishes evidence of a physical connection with our stellar system; and this evidence is confirmed by the fact of their resolvability with telescopic power which fails to make individually visible the most distant stars of our own milky way. If they are remote galaxies, it may be assumed that, speaking generally, the largest are the nearest, and therefore the most resolvable. But the fact is, the smallest are the most resolvable. Another difficulty is presented by the Magellanic clouds. (See Nebula .) Sir John Herschel, considering the structure of the larger of these clouds, concludes that “it must be taken as a demonstrated fact that stars of the seventh or eighth magnitude, and irresolvable nebula, may coexist within limits of distance not differing in proportion more than as 9 to 10.” (“Outlines of Astronomy,” London, 1851, p. 615.) This clearly supplies a reductio ad absurdum of the popular doctrine. Assuming, for the sake of the argument, a rare, homogeneous, nebulous matter, widely diffused through space, the following successive changes will, on physical principles, take place in it: 1, mutual gravitation of its atoms; 2, atomic repulsion; 3, evolution of heat, by overcoming this repulsion; 4, molecular combination, at a certain stage of condensation, followed by, 5, sudden and great disengagement of heat; 6, lowering of temperature by radiation and consequent precipitation of binary atoms, aggregating into irregular flocculi and floating in the rarer medium, just as water when precipitated from air collects into clouds; 7, each flocculus will move toward the common centre of gravity of all; but being an irregular mass in a resisting medium, this motion will be out of the rectilinear, that is to say, not directly toward the common centre of gravity, but toward one or other side of it; and thus, 8, a spiral movement will ensue, which will be communicated to the rarer medium through which the flocculus is moving; and, 9, a preponderating momentum and rotation of the whole mass in some one direction, converging in spirals toward the common centre of gravity. Certain subordinate actions are to be noticed also. Mutual attraction will tend to produce groups of flocculi concentrating around local centres of gravity, and acquiring a subordinate vertical movement. These conclusions are shown to be in entire harmony with the observed phenomena. In this genetic process, when the precipitated matter is aggregating into flocculi, there will be found here and there detached portions, like shreds of cloud in a summer sky, which will not coalesce with the larger internal masses, but will slowly follow without overtaking them. These fragments will assume characteristics of motion strikingly correspondent to those of the comets, whose physical constitution and distribution are seen to be completely accordant with the hypothesis. — The physical characters resulting from the hypothesis are found totally with the facts. In a rotating spheroid of aëriform matter in the latter stages of concentration, but before it has begun to take a liquid or solid form, the following actions will go on: 1, more and more rapid aggregation of its atoms into a smaller and denser mass, as the common centre of gravity is approached; 2, development of oblateness; 3, evolution of heat, greatest at the central parts; and, as a consequence, 4, circulation — currents setting from the centre toward the poles and thence to the equator, and counter currents from the equator to the centre. In the course of this round there will be, 5, an oscillation of temperature: first, from the centre outward — expansion by diminished pressure and other causes, and consequent lowering of temperature; secondly, from the equator inward — rise in temperature for converse reasons. 6. As a corollary to 4 and 5, external condensation will occur according to the laws of precipitation from gases, resulting in a belt of vapor about the equator, gradually widening and condensing into a fluid; 7, this fluid film will gradually extend itself till it eventually closes over at the poles, thus forming a thin hollow spheroid filled with gaseous matter; 8, at length the liquid shell will become very thick, the outer surface will experience a fall of temperature and begin to harden into a solid crust. This hypothesis explains the relative specific gravities of the planetary bodies, the formation of the asteroids, the earth's sup- posed interior structure, indications of past or present high temperature throughout the solar system, and the sun's incandescence. — These considerations relate chiefly to the physical changes undergone by a forming system. Laplace's nebular hypothesis deals with the changes of arrangement in the distribution of matter forming into a system under the action of dynamical laws. He takes as the basis of his theory certain features of our solar system which are not explained by the theory of gravitation. Gravity accounts for Kepler's laws, which are shown to be among its necessary consequences. No system could circulate in any manner around a centre, for instance, without the law holding that the numbers representing the cubes of the mean distances would be proportional to the numbers representing the squares of the periodic times. But a system could exist under gravity in which the planets would travel in widely eccentric orbits or in planes largely inclined to each other. Nor has it been proved that the planets might not safely circulate in different directions. Assuredly, if revolution in different directions, or in planes largely inclined to each other, or in very eccentric paths, might in the long run result in collisions and therefore in the destruction of the system as such, there is yet no reason to believe that all the axial rotations need take place in the same direction as the motions of revolution. But, to say the truth, none of those laws of harmony in our solar system, except the laws depending directly on gravity, can be regarded as essential to the well being of the system; nor, as will presently appear, would the difficulty of regarding the system as other than a product of evolution be appreciably diminished by supposing that without those laws the destruction of the system must inevitably have occurred in the course of time. For it would be manifestly unreasonable to regard our system as one in which the original arrangements were fortuitously so happy that it has continued to exist as a system, if we find that the probability of these arrangements so existing by mere coincidence is exceedingly minute. Now, how small this probability is may be inferred by considering only the motion of the planets in one common direction. There are known at the present time 8 major planets and 137 minor planets (the number of these is increasing year by year). Thus there are 145 known planets. Taking the earth's direction of revolution as a standard direction, the chance that any one of the remaining 144 planets would have this direction as a result of mere chance is of course one half, since a planet must revolve in one of two ways. Therefore, by the laws of probability, the chance that all the 144 other planets would revolve in that direction is represented by a fraction whose numerator is unity and its denominator 2 raised to the 144th power. Now 144 times the logarithm of 2, (or .3010300) = 43.3483200, showing that the above mentioned denominator is a number of 44 digits, beginning 2230077 with 37 digits to follow. This inconceivably enormous num- ber represents the odds to 1 against the observed arrangement being the result of chance, even considering only one relation out of several mentioned above, all of which present the same order of antecedent improbability. Thus Laplace was led to his conception of a vast rotating nebulous disk, from the gradual contraction of which, and the consequent throwing off of rings, breaking up into globes, all revolving and rotating in one common direction and nearly in the same level, the solar system was formed. This hypothesis, however, does not explain the distribution of the masses of the solar system; one planet (Jupiter), for example, containing nearly 5/7 of all the matter outside the sun, and Saturn and Jupiter together containing about 11/12 of all that matter. Accordingly, the present writer has suggested a modification of it, in which, starting from some such primary condition as that assumed by Herbert Spencer, the various parts of the solar system were formed by processes of aggregation such as are still going on (though now with extreme slowness). For the motions of the flocculi of Spencer (or of the parts, whatever their nature, from which the system was to be formed) would be more and more rapid with proximity to the central aggregation, according to well known dynamical laws. Accordingly, subordinate aggregations would form with difficulty close by the sun; and hence we can understand the smallness of the members of the interior family of planets, comprising Mercury, Venus, the earth, and Mars. Again, with extreme distance from the centre, the gravity of available material whence aggregations could form would be so far reduced, that for that reason the planets so formed would be smaller. Hence we can understand why Uranus and Neptune are so far inferior to Jupiter and Saturn. These two giant orbs are thus seen to occupy the space where the conditions were most favorable to the rapid development of subordinate aggregations. In this intermediate region there was abundance of material while yet the motions were not so rapid that a subordinate aggregation could not readily master, so to speak, the matter rushing past toward the aphelion of its orbital motion round the sun. The theory also explains well the existence of a zone of discrete bodies next within the path of Jupiter, that is, in a region disturbed at once by his attraction and that of the sun. — It is not improbable, as remarked in the article Meteor , that the study of cometic and meteoric astronomy may before long throw considerable light on the interesting question of the evolution of our solar system, and may enable us to form a nebular hypothesis on safer grounds than those on which the theories now in vogue have been based.

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