Removal of Hot Saturns in Mass-Radius Plane by Runaway Mass Loss

Abstract

The hot Saturn population exhibits a boundary in mass-radius space, such that no planets are observed at a density less than $\sim$0.1 g cm${}^{-3}$. Yet, planet interior structure models can readily construct such objects as the natural result of radius inflation. Here, we investigate the role XUV-driven mass-loss plays in sculpting the density boundary by constructing interior structure models that include radius inflation, photoevaporative mass loss and a simple prescription of Roche lobe overflow. We demonstrate that planets puffier than $\sim$0.1 g cm${}^{-3}$ experience a runaway mass loss caused by adiabatic radius expansion as the gas layer is stripped away, providing a good explanation of the observed edge in mass-radius space. The process is also visible in the radius-period and mass-period spaces, though smaller, high-bulk-metallicity planets can still survive at short periods, preserving a partial record of the population distribution at formation.

11 pages, 5 figures, Accepted to ApJ Letters. This revision includes the effect of orbital evolution resulting from Roche lobe overflow; the overall conclusions are the same

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• Discovery and characterization of a dense sub-Saturn TOI-6651b
• Inflated hot Jupiters: Inferring average atmospheric velocity via Ohmic models coupled with internal dynamo evolution
• Atmospheric Mass Loss from TOI-1259 A b, a Gas Giant Planet With a White Dwarf Companion
• TOI-1199 b and TOI-1273 b: Two new transiting hot Saturns detected and characterized with SOPHIE and TESS
• Ground-Based Reconnaissance Observations of 21 Exoplanet Atmospheres with the Exoplanet Transmission Spectroscopy Imager
• Confirmation of the hot super-Neptune TOI-672 b with NIRPS and HARPS and Insights into the Neptunian desert around M dwarfs
• Evolution of the ZTF SLRN-2020 star-planet merger
• A sub-Saturn Mass-Radius Desert for Planets with Equilibrium Temperature Less than 600 K