Is Saturn a failed gas giant?

Abstract

The formation history of giant planets inside and outside the Solar System remains unknown. We suggest that runaway gas accretion is initiated only at a mass of ∼100 M⊕ and that this mass corresponds to the transition to a gas giant, a planet whose composition is dominated by hydrogen and helium. Delayed runaway accretion (by a few million years) and having it occurring at higher masses is likely a result of an intermediate stage of efficient heavy-element accretion (at a rate of ∼10−5 M⊕ yr−1) that provides sufficient energy to hinder rapid gas accretion. This may imply that Saturn has never reached the stage of runaway gas accretion and that it is a “failed giant planet”. The transition to a gas giant planet above Saturn’s mass naturally explains the differences between the bulk metallicities and internal structures of Jupiter and Saturn. The mass at which a planet transitions to a gas giant planet strongly depends on the exact formation history and birth environment of the planet, which are still not well constrained for our Solar System. In terms of giant exoplanets, the occurrence of runaway gas accretion at planetary masses greater than Saturn’s can explain the transitions in the mass-radius relations of observed exoplanets and the high metallicity of intermediate-mass exoplanets.