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 H2-dominated atmospheres, i.e., they are `Gas Dwarfs’. If gas dwarfs have fully molten interiors, interactions between their magma oceans and atmospheres would produce distinct observational signatures. These signatures may break the degeneracy in interior structure. We use the coupled interior-climate evolution model, PROTEUS , to estimate the ‘solidification shoreline’: the instellation flux boundary 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. 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.