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# 9.9 From Planetesimals to Planets

According to the Solar Nebula Model, the early Solar System collapsed from a diffuse gas cloud to form a young star with a surrounding disk of gas and dust. In this collapsing cloud, molecules condensed into droplets, and then solid grains, as the nebula cooled. Over time, the process of condensation takes us from molecules and clusters of molecules to dust grains about a millimeter across. Rocky and metal-rich grains survived throughout the nebula, but icy and hydrogen-rich grains were able to tolerate the young Sun's bright glare from large distances, and were only present in the lower temperatures of the outer regions of the Solar System.

Artist's conception of the solar nebula. Click here for original source URL.

Over time, these seed sized grains merged with other grains and grew progressively larger. Emerging planetesimals drifted around the Sun within a dusty, gaseous, disk-shaped nebula. Most of the mass of the nebula was in the form of gas; only a few percent of the mass was solid material. This tenuous mix derived its composition from the original composition of the solar nebulae. This cloud was 98% hydrogen and helium by mass. Only about two percent of the nebula was composed of the heavier elements that more readily form solid matter. These were elements like silicon, magnesium, carbon, and oxygen, which condense into solid mineral and ice particles. The majority of the cloud's mass went into forming the Sun, with maybe as little as 10% remaining to form the planets, moons, and other small bodies in our Solar System.

The solar nebula model paints Solar System formation in very broad brush strokes. More detailed theories are still be worked on, and must be able to explain not only our solar system but the diversity of other systems being found around other stars. Some of the basic details they must explain, include:

• All the planets except two spin in the same direction that they revolve around the Sun, and they all have similar rotation periods.

• Planets do not form with equal spacing from the Sun, nor do they form at random distances from the Sun. Rather, each planet is a factor of 1½ to 2 times farther out than the previous one (Bode's Rule).

• Planets in other solar systems are seen in positions far closer to their star than can be explained without allowing them to migrate from where they formed to where they are now seen.

• Lastly, in our Solar System, the outer planets are 15 to 300 times more massive than the inner planets, and most of their mass is in the form of hydrogen and helium. Any successful theory must explain the differences in size, mass, and composition between the planets of the inner and outer Solar System.