Unusually Hot Interiors Could Reconcile the Missing Methane Problem for Warm-to-Hot Exoplanets with Hydrogen Atmospheres

TL;DR

This paper introduces a rapid analytical framework to assess whether elevated internal temperatures can explain the observed methane depletion in warm-to-hot exoplanets with hydrogen atmospheres, providing insights into their interior heating processes.

Contribution

The study presents a simple, geochemistry-inspired analytical model that constrains minimum intrinsic temperatures of exoplanets from JWST atmospheric data, revealing potential additional heating mechanisms.

Findings

Several exoplanets require higher internal temperatures than standard models predict.

Some targets are consistent with lower intrinsic temperatures or show degeneracies.

A subset of planets deviate from the empirical Teq-Tint trend, indicating extra heating processes.

Abstract

JWST is revolutionizing the field of exoplanet atmospheres by delivering unprecedented spectroscopic constraints on their chemical compositions. It has provided tight constraints on the abundances of dominant carbon- and oxygen-bearing species on numerous warm-to-hot exoplanets with hydrogen-dominated atmospheres. Under thermochemical equilibrium, many of these exoplanets should have abundant methane (CH4); however, CH4 has, so far, only been spotted in a few cases. Here, we present a simple, geochemistry-inspired framework to explore whether elevated intrinsic temperatures (T$\rm_{int}$) can account for the CH4 depletions. Instead of using computationally expensive, forward grid models, our fast analytical framework focuses on two key chemical equilibria: CO-CH4 and CO-CO2, allowing us to quickly constrain the minimum Tint that is consistent with JWST-observed abundances of H2O, CO, CH4, and CO2. Applying this framework to 12 warm-to-hot exoplanets, we find that several targets require minimum Tint values exceeding those predicted by standard evolution models, while others remain consistent with lower Tint solutions or exhibit degeneracies with other solutions. Our sample enables an initial exploration of population-level trends: while many exoplanets broadly follow an empirical Teq-Tint relation derived from the hot Jupiter mass-radius population, a subset of targets lie well above this trend. The apparent need for hotter interiors suggests that while Ohmic dissipation is probably an important heat source among the general population, additional heating processes, such as tidal heating, may also play important roles for some planets. Our results demonstrate the diagnostic power of atmospheric chemistry as a complementary probe of exoplanet interiors.