2021ApJ...921L...8H

The recent discovery and initial characterization of sub-Neptune-sized exoplanets that receive stellar irradiance of approximately Earth's raised the prospect of finding habitable planets in the coming decade, because some of these temperate planets may support liquid-water oceans if they do not have massive H₂/He envelopes and are thus not too hot at the bottom of the envelopes. For planets larger than Earth, and especially planets in the 1.7-3.5 R_⊕ population, the mass of the H₂/He envelope is typically not sufficiently constrained to assess the potential habitability. Here we show that the solubility equilibria versus thermochemistry of carbon and nitrogen gases typically results in observable discriminators between small H₂ atmospheres versus massive ones, because the condition to form a liquid-water ocean and that to achieve the thermochemical equilibrium are mutually exclusive. The dominant carbon and nitrogen gases are typically CH₄ and NH₃ due to thermochemical recycling in a massive atmosphere of a temperate planet, and those in a small atmosphere overlying a liquid-water ocean are most likely CO₂ and N₂, followed by CO and CH₄ produced photochemically. NH₃ is depleted in the small atmosphere by dissolution into the liquid-water ocean. These gases lead to distinctive features in the planet's transmission spectrum, and a moderate number of transit observations with the James Webb Space Telescope should tell apart a small atmosphere versus a massive one on planets like K2-18 b. This framework thus points to a way to use near-term facilities to constrain the atmospheric mass and habitability of temperate sub-Neptune exoplanets.