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The recent discovery and initial characterization of sub-Neptune-sizedexoplanets that receive stellar irradiance of approximately Earth's raised theprospect of finding habitable planets in the coming decade, because some ofthese temperate planets may support liquid water oceans if they do not havemassive H2/He envelopes and are thus not too hot at the bottom of theenvelopes. For planets larger than Earth, and especially planets in the 1.7-3.5R_Earth population, the mass of the H2/He envelope is typically notsufficiently constrained to assess the potential habitability. Here we showthat the solubility equilibria vs. thermochemistry of carbon and nitrogen gasesresults in observable discriminators between small H2 atmospheres vs. massiveones, because the condition to form a liquid-water ocean and that to achievethe thermochemical equilibrium are mutually exclusive. The dominant carbon andnitrogen gases are typically CH4 and NH3 due to thermochemical recycling in amassive atmosphere of a temperate planet, and those in a small atmosphereoverlying a liquid-water ocean are most likely CO2 and N2, followed by CO andCH4 produced photochemically. NH3 is depleted in the small atmosphere bydissolution into the liquid-water ocean. These gases lead to distinctivefeatures in the planet's transmission spectrum, and a moderate number ofrepeated transit observations with the James Webb Space Telescope should tellapart a small atmosphere vs. a massive one on planets like K2-18 b. This methodthus provides a way to use near-term facilities to constrain the atmosphericmass and habitability of temperate sub-Neptune exoplanets.

Unveiling shrouded oceans on temperate sub-Neptunes via transit signatures of solubility equilibria vs. gas thermochemistry