The mass of gas giant planets: Is Saturn a failed gas giant?
- Details
- Published on 10 July 2023
Vol. 675
1. Letters
The mass of gas giant planets: Is Saturn a failed gas giant?

Standard core-accretion models of giant planets invoke an initial formation of a heavy-element core, followed by gradual solid and gas accretion that continues until runaway gas accretion starts. Historically, these models have been challenged by timescale issues (given the need for massive gas accretion before disk gas dissipation occurs after ~ 3 Myr), but the problem is now alleviated by the possibility that core formation is accelerated by pebble accretion. In addition, in these traditional models, runaway accretion occurs at a typical planetary mass of ∼ 30 M⊕. Recent results on the composition and internal structure of Jupiter and Saturn, particularly the existence of "fuzzy" cores, point to an intermediate phase of heavy-element accretion; this has been shown to delay runaway gas accretion by prolonging planetary cooling. Based on these considerations, Helled proposes that for a typical heavy-element accretion rate of 10^-5 M ⊕/yr lasting a couple of megayears, the runaway phase may start only at ~100 M⊕. This scenario, in which Saturn appears as a "failed giant planet," naturally explains its much smaller mass compared to Jupiter's (95 M⊕ vs 318 M⊕), without needing to resort to a fine-tuning of the timing of the disappearance of the gas disk, as well as its greater enrichment in heavy elements. It also provides a plausible interpretation of the mass versus radius relation observed in giant exoplanets, which presents a transition near a mass of ~100 M⊕. In this view, this transition would delimit failed giants from actual gas giants. Many questions remain unresolved, such as the origin (pebbles, planetesimals, or both?) and precise value of the heavy-element accretion rate, or the existence of a diversity of metallicities in exoplanets not correlated with their mass, but the proposed concept provides a new look on the fundamental topic of giant (exo)planet formation.