ABSTRACT
Most of what we know about how planets form comes from studies of the terrestrial planets, the moon and meteorites, and modeling the dynamics of the early solar system. This is now being heavily inuenced by the make-up of other stellar systems, which are turning out to be very diverse. It seems that the solar system is not a model for how other planetary systems must look. Indeed, even in the solar system, of the eight major planets, several dwarf planets, asteroids, and more than 160 moons that have been studied in any detail, no two are alike. It seems that building planetary bodies is a matter of contingency.
