Tidal Decay and Stable Roche-Lobe Overflow of Short-Period Gaseous Exoplanets

TL;DR

This paper models the Roche-lobe overflow process of short-period gaseous exoplanets using MESA, revealing that core mass influences the orbital evolution and potential remnant formation, with implications for observed close-in planets.

Contribution

It introduces a detailed MESA-based model of RLO in gaseous exoplanets, highlighting the role of core mass in orbital evolution and remnant formation, which was not previously well understood.

Findings

Orbital expansion during RLO often reverses at a maximum period depending on core mass.

Remnants of gas giants may be stranded near specific orbital periods predicted by the model.

Some observed short-period planets are consistent with being gas giant remnants undergoing RLO.

Abstract

Many gaseous exoplanets in short-period orbits are on the verge or are in the process of Roche-lobe overflow (RLO). Moreover, orbital stability analysis shows tides can drive many hot Jupiters to spiral inevitably toward their host stars. Thus, the coupled processes of orbital evolution and RLO likely shape the observed distribution of close-in exoplanets and may even be responsible for producing some of the short-period rocky planets. However, the exact outcome for an overflowing planet depends on its internal response to mass loss, and the accompanying orbital evolution can act to enhance or inhibit RLO. In this study, we apply the fully-featured and robust Modules for Experiments in Stellar Astrophysics (MESA) suite to model RLO of short-period gaseous planets. We show that, although the detailed evolution may depend on several properties of the planetary system, it is largely determined by the core mass of the overflowing gas giant. In particular, we find that the orbital expansion that accompanies RLO often stops and reverses at a specific maximum period that depends on the core mass. We suggest that RLO may often strand the remnant of a gas giant near this orbital period, which provides an observational prediction that can corroborate the hypothesis that short-period gas giants undergo RLO. We conduct a preliminary comparison of this prediction to the observed population of small, short-period planets and find some planets in orbits that may be consistent with this picture. To the extent that we can establish some short-period planets are indeed the remnants of gas giants, that population can elucidate the properties of gas giant cores, the properties of which remain largely unconstrained.