Evidence of a gap in the envelope mass fraction of sub-Saturns

¹ University Observatory Munich, Faculty of Physics, Ludwig-Maximilians-Universität München, Scheinerstr. 1, 81679 Munich, Germany
² Max-Planck Institute for Extraterrestrial Physics, Giessenbachstrasse 1, 85748 Garching, Germany
³ Max Planck Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany

^★ Corresponding author: lthomas@mpe.mpg.de

Received: 23 July 2025
Accepted: 17 September 2025

Abstract

Under the core-accretion model, gas giants form via runaway accretion. This process starts when the mass of the accreted envelope becomes equal to the mass of the core. We modeled a population of warm sub-Saturns to search for imprints of their formation history in their internal structure. Using the GAS gianT modeL for Interiors (GASTLI), we calculated a grid of interior structure models on which we performed retrievals for our sample of 28 sub-Saturns to derive their envelope mass fractions (f_env). For each planet, we ran three different retrievals, assuming low (−2.0<log (Fe/H)<0.5), medium (0.5<log (Fe/H)<1.4), and high (1.4<log (Fe/H)<1.7) atmospheric metallicity. The distribution of f_env in our sample was then compared to outcomes and predictions of planet formation models. When our results are compared to the outcomes of a planetesimal accretion formation model, we find that we require a high atmospheric metallicity for intermediate-mass sub-Saturns to reproduce the simulated planet population. For higher planetary masses, a medium atmospheric metallicity provides the best agreement. Additionally, we find a bimodal distribution of f_env in our sample with a gap that is located at different values of f_env for different atmospheric metallicities. For the high atmospheric metallicity case, the gap in the f_env distribution is located between 0.5 and 0.7, which is consistent with assumptions of the core-accretion model in which runaway accretion starts when M_env ≈ M_core(f_env is ∼ 0.5). We also find a bimodal distribution of the hydrogen and helium mass fraction (f_H/He) with a gap at f_H/He=0.3. The location of this gap is independent of the assumed atmospheric metallicity. Lastly, we compared the distributions of our sub-Saturns in the Neptunian savanna to a population of sub-Saturns in the Neptune desert and ridge. We find that the observed f_env distribution of savanna and ridge sub-Saturns is consistent with the planets coming from the same underlying population.