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Oct21-04, 06:38 PM
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It largely has to do with the distance from the star at which the material becomes cool enough to solidify. While still a gas, the atoms are moving too fast and are too diffuse to clump or be gravitationally captured in great quantities and radiation pressure from the star pushes them away. Once they solidify, clumping and gravitational attraction takes over. Thus, inner planets are mainly composed of high melting point materials [rock] while outer planets are mainly composed of low melting point materials [gas].