On the mass of gas giant planets: Is Saturn a failed gas giant?

TL;DR

This paper proposes that runaway gas accretion in giant planets begins around 100 Earth masses, suggesting Saturn may be a failed gas giant due to delayed accretion influenced by heavy-element accumulation.

Contribution

It introduces a model where delayed runaway gas accretion explains the differences between Jupiter and Saturn and the properties of exoplanets, emphasizing the role of heavy-element accretion.

Findings

Runaway gas accretion likely starts at ~100 M_Earth.

Saturn may never have undergone runaway accretion.

Delayed accretion explains observed exoplanet mass-radius relations.

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_Earth and that this mass corresponds to the transition to a gas giant, a planet that its composition is dominated in hydrogen and helium. Delaying runaway accretion to later times (a few Myr) and higher masses is likely to be a result of an intermediate stage of efficient heavy-element accretion (at a rate of ~10^-5 M_Earth/yr) that provides sufficient energy to hinder rapid gas accretion. This may imply that Saturn has never reached 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 transition mass to a gas giant planets strongly depends on the exact formation history and birth environment of the planets, which are still not well constrained for our Solar System. In terms of giant exoplanets, delaying runaway gas accretion to planets beyond Saturn's mass can explain the transitions in the mass-radius relations of observed exoplanets and the high metallicity of intermediate-mass exoplanets.