Formation of Giant Planets

This is a descriptive and non‐mathematical summary of giant planet formation theories. There are two end‐member models, core accretion and disk instability. In the core accretion model, several to ten Earth masses of solid (ice and rock) accumulate and this promotes subsequent gas accretion. In the disk instability model, a self‐ gravitating sphere of gas forms (somewhat analogous to star formation) and a core may arise through condensation and rainout from its low density envelope. The core accretion model may have a time‐scale difficulty because the formation of the embryo and infall of gas is not fast relative to the time at which the gaseous disk is removed. The disk instability model suffers from the current theoretical inability to follow the development of these putative instabilities through to the formation of a planet. Observational data on core masses in Jupiter and Saturn do not clearly favor one model. However, the existence and nature of Uranus and Neptune strongly suggest that the formation of appropriate embryos occurs. Moreover, there is considerable flexibility in the elapsed time required to form Jupiter and Saturn by the core accretion model, including times of only a few million years, compatible with disk lifetimes. This suggests that core accretion remains the favored mode in our solar system. It is possible that both mechanisms operate in general and that many extrasolar planetary systems make use of the disk instability mode. Future theory and observations are essential for deciding this issue.