The disk formation process of the previous chapter forms the basis for the “Nebular Model” for the formation of planetary systems, including our own solar system. As a proto-stellar cloud collapses under the pull of its own gravity, conservation of its initial angular momentum leads naturally to formation of an orbiting disk, which surrounds the central core mass that forms the developing star. We then explore the “ice line” between inner rocky dwarf planets and outer gas giants.

Chapter 6 describes theoretical models for the formation of giant planets, and relevant observational constraints from the Solar System. The core accretion theory for giant planet formation is introduced, including the equations describing planetary envelope structure, the concept of a critical core mass, and illustrative evolutionary tracks for giant planet growth. Current knowledge of the internal structure of Jupiter, based on measurements of the gravitational field, is summarized. The conditions under which a massive gas disk becomes unstable and fragments are described, together with the likely outcome of disk instability.