The Nebular Hypothesis and Formation of the Solar System


By Robert Hazen, Ph.D.George Mason University

The generally accepted model for the formation of the solar system is called the nebular hypothesis. It takes all the different kinds of information and merges them into one hypothesis that seems to match all the observations. The theory was proposed by French mathematician Pierre Simon Laplace more than 200 years ago.

An illustration of the solar system’s birth
The nebular hypothesis explains the way the solar system was formed. (Image: Mopic/Shutterstock)

Pierre Simon Laplace: The Mathematical Prodigy

Laplace was a French mathematician with a strong interest in physics and astronomy. He was a brilliant mathematical prodigy; he became a university professor at the age of 19. He described his theory of the nebular hypothesis in an extremely popular work, translated to be The Exposition of the System of the World, published in 1796.

Laplace was a scientist’s scientist; he seemed to speak for all scientists. On his deathbed, he said what is a sentiment that’s rarely expressed, “The knowledge we have of things is small indeed, while that of which we are ignorant is immense.”

This is a transcript from the video series The Joy of ScienceWatch it now, on Wondrium.

Role of Dust and Gas in the Nebular Hypothesis 

Well, Laplace added greatly to our knowledge by pointing out the nebular hypothesis. According to this hypothesis, gravity attracts dust and gas into an ever denser and more compact cloud. The composition of this nebula is going to be primarily hydrogen and helium because those, of course, are the primary elements of the universe. 

The very first stars had only hydrogen and helium and a little bit of lithium to work with; but after the first supernova, there were more and more heavy elements. An estimate of our solar system is about 90 percent hydrogen, and the rest are other elements. That may represent five or six or seven cycles of supernova, feeding into the material that now has gone into our solar system.

The Nebular Hypothesis and the Solar System’s Formation

We have now a collapsing cloud of dust and gas, and as it collapses, the temperature at the center gets hotter and hotter because of the gravitational potential energy that’s released as the dust comes closer and closer in. Also, the cloud begins to rotate. It has little swirls and eddies that were random at first, but as it gets more and more compact and condensed, the predominant eddy becomes in one direction, and the whole system begins to rotate and spiral. 

Eventually, the solar system begins to look like a flattened disk of matter with a central bulge, not unlike the way a galaxy looks, a spiral galaxy in cross-section, with that central bulge and the spiral arms coming out. Most of the mass gets pulled to the center; that’s where the proto-Sun, the pre-Sun, gets hotter and hotter, and pressures get higher and higher inside this hydrogen sphere.

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How the Sun and Planets Were Born

At some critical point, nuclear fusion reactions begin. The pressure and temperature get so hot that hydrogen begins to fuse to helium, and then the star is born. When that happens, energy starts radiating out from the middle of the proto-sun to the surface, and then the radiant energy begins to come from the sun to the other parts of the nebula. 

The solar wind starts blowing outward at a very high, intense rate, and this starts changing and modifying the rest of the dust cloud. The inner parts of the dust cloud are stripped of hydrogen and helium, which are much too light; they’re pushed to the outside. What’s left are more rocky, condensed materials, and this becomes the four rocky or terrestrial planets, planets that have very little hydrogen and helium relative to the other objects in the solar system.

Significance of Computer Modeling in Science

Computer modeling is so important in science today—it keeps track of the evolution of the dust cloud under gravity, under the condensing cloud, under the first nuclear-fission reactions that form the stars. What happens to a dust cloud in those situations? One surprise of these computer models is under most conditions, it appears that two stars should form, not one. 

This would form a binary star, and observations confirm this. Most of the stars we see in the heavens—in fact, two-thirds or more of the stars we see—are binary stars, with two large masses orbiting around each other. In that sense, our solar system is a little bit unusual. Not rare, by any means, but certainly less common than the binaries. 

If Jupiter had been 50 to 100 times larger than it is, we too would be living in a binary system. Jupiter too, would be a star—a brown dwarf star, to be sure—but a star. When only one star dominates in a system, calculations show that the pattern of planets we have is very likely. 

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Calculations and the Importance of Very-Late-Stage Collisions

An illustration of Venus
The manner in which Venus orbits the Sun can be the consequence of a late-stage collision. (Image: NASA images/Shutterstock)

One fascinating aspect of these calculations is the importance of very-late-stage collisions, which fix some of the details of planetary behavior. Imagine this: for example, an object like the Earth. Once a large mass forms, it starts sweeping up the other planetesimals in its vicinity; but it’s very possible to have two competing large masses near the same orbit. 

That situation is absolutely unstable; you cannot have two planets occupying the same orbit. Eventually, those two planets have to collide. When they do, in a late-stage event, it’s epic, it’s catastrophic, it’s violent, and it can change some of the details of planetary orbits.

So, for example, Venus appears to be rotating— not counterclockwise, in the same sense as the orbits—Venus has a very, very slow clockwise rotation against the orbits of all the other planets. This, it’s suggested, may have occurred because of a late-stage collision.

Common Questions about the Nebular Hypothesis and Formation of Solar System

Q: Who was Pierre Laplace? 

Pierre Simon Laplace was a French mathematician with a keen interest in physics and astronomy. Laplace was a mathematical genius and became a university professor at the age of 19. This scientist proposed a famous theory called the nebular hypothesis.

Q: What’s the nebular hypothesis? 

According to the nebular hypothesis, gravity caused gas and dust to turn into larger and more compact clouds. The composition of these clouds is only helium and hydrogen gases since they were the only raw materials of the universe.

Q: According to the nebular hypothesis, how did the solar system form? 

According to the nebular hypothesis, massive clouds of hydrogen and helium collapsed. After the collapse, the center of the cloud became hotter and hotter. Then, as it got hotter and more condensed, the system began to rotate. This was the beginning of the solar system’s formation.

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