Most Rocky Sub-Neptunes are Molten: Mapping the Solidification Shoreline for Gas Dwarf Exoplanets

TL;DR

Most detected sub-Neptunes are likely to have permanent magma oceans if they are gas dwarfs, due to their position relative to the solidification shoreline, impacting their interior and atmospheric characteristics.

Contribution

This study introduces the 'solidification shoreline' concept using the PROTEUS model to determine when gas dwarf exoplanets remain molten or solidify.

Findings

98% of detected sub-Neptunes are consistent with having permanent magma oceans.

Mantle oxidation state and atmospheric composition influence magma ocean cooling.

Most gas dwarf sub-Neptunes are predicted to have molten interiors based on their stellar irradiation levels.

Abstract

Sub-Neptunes are the most common type of detected exoplanet, yet their observed masses and radii are degenerate with several interior structures. One possibility is that sub-Neptunes have silicate/iron interiors and H$_2$-dominated atmospheres ($\mu$<3.8 g mol$^{-1}$), i.e., they are 'gas dwarfs'. If gas dwarfs have molten interiors, interactions between their magma oceans and atmospheres will produce distinct observational signatures. These signatures may break the degeneracy in interior structure, while providing insight into their interior processes, history, and population trends. We expect all such planets are born molten, but under what conditions do they remain molten today? We use the coupled interior-climate evolution model, PROTEUS, to estimate the 'solidification shoreline': the instellation flux boundary (as a function of stellar $T_{\rm eff}$) that separates molten gas dwarfs from solidified ones. Our results show that 98% of detected sub-Neptunes occupy a region of parameter space consistent with their having permanent magma oceans, if they are gas dwarfs. While mantle $f{\rm O}_2$ and bulk volatile C/H ratio both influence magma ocean cooling, planets with oxidising mantles and carbon-rich atmospheres are likely to have high mean-molecular weight atmospheres ($\mu$>3.8 g mol$^{-1}$) and are thus outside the scope of this study. Therefore, most detected sub-Neptunes, if they are gas dwarfs, have permanent magma oceans. This result motivates further research into the interactions between molten interiors and overlying atmospheres, and campaigns to identify unambiguous signatures of these interactions.