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

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ID: 315757
2026
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Abstract
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 (μ<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 Teff) 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 fO2 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 (μ>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.
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Authors Robb Calder, Oliver Shorttle, Harrison Nicholls, Tim Lichtenberg, Claire Marie Guimond
Journal monthly notices of the royal astronomical society
Year 2026
DOI
10.1093/mnras/stag1007
URL
https://doi.org/10.1093/mnras/stag1007
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