Methane Planets and their Mass-Radius Relation

TL;DR

This paper explores the mass-radius relationship of methane-rich exoplanets, revealing how high-mass methane planets can differentiate into layered structures and highlighting the complexity of inferring composition from observed data.

Contribution

It introduces models of methane planets showing differentiation at high masses and considers rocky core effects, advancing understanding of their internal structure.

Findings

Massive methane planets (>15 M_Earth) can dissociate and differentiate.

Differentiated methane planets have a carbon core, methane envelope, and hydrogen atmosphere.

The study highlights the complexity of determining planetary composition from mass and radius.

Abstract

Knowledge of both the mass and radius of an exoplanet allows us to estimate its mean density, and therefore its composition. Exoplanets seem to fill a very large parameter space in terms of mass and composition, and unlike the solar-system's planets, exoplanets also have intermediate masses (~ 5 - 50 M_Earth) with various densities. In this letter, we investigate the behavior of the Mass-Radius relation for methane (CH_4) planets and show that when methane planets are massive enough (Mp >~ 15 M_Earth), the methane can dissociate and lead to a differentiated planet with a carbon core, a methane envelope, and a hydrogen atmosphere. The contribution of a rocky core to the behavior of CH_4 planet is considered as well. We also develop interior models for several detected intermediate-mass planets that could, in principle, be methane/methane-rich planets. The example of methane planets emphasizes the complexity of the Mass-Radius relation and the challenge involved in uniquely inferring the planetary composition.