The mass of gas giant planets: 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.

The Fuzzy Cores of Jupiter and Saturn

“…In addition, recent models suggest that significant gas accretion is initiated only when a protoplanet reaches a mass of ∼100 M ⊕ and that this is a result of an intermediate stage of efficient heavy‐element accretion after core formation which provides sufficient energy to hinder rapid accretion of H–He (Alibert et al., 2018; Venturini & Helled, 2020). If rapid gas accretion is indeed delayed and occurs at much higher masses, this implies that Saturn is a “failed giant planet” and has never reached runaway gas accretion (Helled, 2023). This formation path also naturally explains the fuzzy cores of Jupiter and Saturn, the differences between the bulk metallicities of Jupiter and Saturn and the mass‐radius relations of observed exoplanets.…”

“…Although model dependent, the mass of the dilute core is of the order of ∼100 M ⊕ for Jupiter (about one third of the total planetary mass) comparable to the mass of Saturn; it is up to ∼60 M ⊕ for Saturn. The different nature of the diluted cores of the planets may be linked to their formation history (Helled, 2023; Helled & Stevenson, 2017). We suggest that further efforts should be made to determine under what conditions such compact cores are required.…”

Multiple Probe Measurements at Uranus Motivated by Spatial Variability

“… 2018 ), and some protostellar ice components could have even remained pristine within large (100 m) grains (Bergner and Ciesla 2021 ). Strategic Research in the decadal survey includes measurements “especially for the ice giants” focusing on “elemental and stable isotopic compositions of refractory and volatile elements.” Here, comparing the composition of all four giant planets is key, since it seems that Jupiter and Saturn easily crossed the threshold for runaway gas accretion, while Uranus and Neptune may have approached it only as the nebula dispersed (Helled 2023 ). This drives the Strategic Research focused on “in situ measurement of the volatile elemental compositions” of the planets.…”

TOI-5344 b: A Saturn-like Planet Orbiting a Super-solar Metallicity M0 Dwarf

“…Efficient heat dissipation is necessary for rapid gas accretion to continue-Saturns that have high-metallicity disks fail to dissipate heat rapidly enough to support runaway gas accretion. Helled (2023) recently suggests that an intermediate stage of efficient heavyelement accretion fuels the planet with energy that hinders rapid gas accretion. In their model, a prolonged intermediate metal accretion phase extends to a few Myr at a rate of 10 −5 M ⊕ yr −1 along with a slow gas accretion.…”