2024ApJ...964L..19G

Quantifying disequilibria is important to understand whether an environment could be habitable. It has been proposed that the exoplanet K2-18b has a hydrogen-rich atmosphere and a water ocean, making it a “hycean world.” The James Webb Space Telescope recently made measurements of methane, CO₂, and possibly dimethyl sulfide (DMS) in the atmosphere of this planet. The initial interpretation of these data is that they may support the occurrence of hycean conditions. Here I attempt to take a next step in exploring the prospects for habitability. I use constraints on the abundances of atmospheric gases to calculate how much chemical disequilibrium there could be, assuming that K2-18b is a hycean world. I find that the presence of oxidized carbon species coexisting with abundant H₂ (1-1000 bars) at cool to warm (25°C-120°C) conditions creates a strong thermodynamic drive for methanogenesis. More than ∼75 kJ (mol C)^–1 of free energy can be released from CO₂ hydrogenation. Partially oxidized carbon compounds such as DMS (if present) also have the potential to provide metabolic energy, albeit in smaller quantities. Because of the thermodynamic instability of CO₂ under hycean conditions, other reductive reactions of CO₂ are likely to be favored, including the synthesis of amino acids. Glycine and alanine synthesis can be energy releasing or at least much less costly on K2-18b than in Earth's ocean, even when NH₃ is scarce but not totally absent. These first bioenergetic calculations for a proposed ocean-bearing exoplanet lay new groundwork for assessing exoplanetary habitability.