The Nebular Hypothesis
The modern theory of the solar system's formation was first put forward by the French mathematician and physicist Pierre Laplace (1749-1827). His idea was a simple idea, and one that takes into account many of the distinctive characteristics of the solar system--the rotation of the Sun, the orbits of the planets, and the distribution of mass into several large objects and lots of smaller ones. According to the model, called the nebular hypothesis, long ago (about 4.5 billion years ago based on radiometer dating)a large cloud of dust and gas clouds, called nebulae, are common throughout the galaxy, the Milky Way. They typically contain more that 99% hydrogen and helium, with lesser amounts of all the other naturally occurring elements.
Under the influence of gravity, the nebula slowly, inexorably, started to collapse on itself. As was the case for the formation of stars from a nebula, the collapse caused the cloud to spin faster and faster. The rapid spin had several consequences. For one thing, it meant that some of the material in the outer parts of the cloud began to spin out into a flat disk. This common consequence of fast rotation is familiar to anyone who has watched a pizza maker create a flat disk of dough by spinning a mass over head. If you imagine the solar system at this stage of its formation as a large pancake with a big lump in the middle, you would not be far wrong. The big lump represents the material that will eventually become the Sun and the material in the thin flatten disk will eventually become the planets and the rest of the solar system.
The flattening of the nebula into a disk explains another feature of the solar system. The planets had to form in this rotating disk of material, and hence their eventual orbits had to lie close to the disk's plane. That fact that all planetary orbits lie near the same plane, then, is a simple consequence of the solar system's rapid rotation as the nebular cloud began to contract.
In any clump of matter like the spinning disk, by chance, matter is more densely collected in some regions than elsewhere. These regions exert a stronger gravitational force than their neighbors, so that nearby matter tends to gravitate to them. Once the nearby matter has come in, the concentration of matter at that point is even greater, and it will pull even more material into it, As material accumulates, solid grains start to stick together.
This ultimate consequence of gravitational force leads to the rapid breakup of the disk into smaller objects called planetsimals, which range in size from boulders to masses several kilometers across. Once this has happened, the process of gravitational attraction goes on at a grander scale--larger objects capture smaller ones and continue to growing.
About the time that this process was going on in the early solar system the material at the center--more that 99% of the nebula's original mass--began to turn into a star. Light energy began to radiate out from the Sun, and temperature differences began to develop in the disk. Those parts nearest the Sun warmed up, while those farther out warmed only a little. As a result, the inner and outer solar systems developed differently. In the warm inner system, materials such as water, hydrogen and helium were in gaseous form, while father out they were frozen into solids.
Thus everyday physical process having to do with phases of matter and response to temperature--process as familiar as boiling water and making ice--explain one crucial fact about the solar system. The terrestrial planets--Mercury, Venus, Earth and mars--were formed from materials that remain solid at high temperature. Consequently, they are small rocky worlds.
Farther out in the solar system, we find the Jovian planets Jupiter, Saturn, Uranus, and Neptune. The composition of those planets are essentially the same material in the original nebula; that is they contain large amounts of hydrogen and helium. These planets formed from material that remained solid (or at least liquid) because of lower temperatures so far from the Sun. Consequently, they have a markedly different they have a markedly different chemical composition from the inner planets of the solar system.
In passing, we should note that the Jovian planets probably had their own complement of high-energy density materials. Scientists suspect that beneath the thousands of miles of helium, hydrogen, and other condensed gases on these planets is concealed a core like small terrestrial planet, whose composition is much like that of Earth, and its neighbors. However, this rocky matter represents only a small fraction of these planet's total mass.
Some astronomers argue that the largest planets--Juipter and Saturn--formed by a process more like that of a small star than through the accretion of planetsimals. The details of the structure and formation of the Jovian planets remain a rich ground for debate in the sciences.
Just as any construction site has a pile of leftover materials lying around when the building is finished, so too does the solar system have its "scrap pile." These leftovers take the form of the rocky asteroids and icy comets that still orbit the Sun. They represent the matter that never was taken up into the planets.
