JWST Observations of K2-18b Can Be Explained by a Gas-rich Mini-Neptune with No Habitable Surface

The James Webb Space Telescope (JWST) recently measured the transmission spectrum of K2-18b, a habitable-zone sub-Neptune exoplanet, detecting CH₄ and CO₂ in its atmosphere. The discovery paper argued the data are best explained by a habitable "Hycean" world, consisting of a relatively thin H₂-dominated atmosphere overlying a liquid water ocean. Here, we use photochemical and climate models to simulate K2-18b as both a Hycean planet and a gas-rich mini-Neptune with no defined surface. We find that a lifeless Hycean world is hard to reconcile with the JWST observations because photochemistry only supports <1 part-per-million CH₄ in such an atmosphere while the data suggest about ∼1% of the gas is present. Sustaining percent-level CH₄ on a Hycean K2-18b may require the presence of a methane-producing biosphere, similar to microbial life on Earth ∼3 billion years ago. On the other hand, we predict that a gas-rich mini-Neptune with 100× solar metallicity should have 4% CH₄ and nearly 0.1% CO₂, which are compatible with the JWST data. The CH₄ and CO₂ are produced thermochemically in the deep atmosphere and mixed upward to the low pressures sensitive to transmission spectroscopy. The model predicts H₂O, NH₃, and CO abundances broadly consistent with the nondetections. Given the additional obstacles to maintaining a stable temperate climate on Hycean worlds due to H₂ escape and potential supercriticality at depth, we favor the mini-Neptune interpretation because of its relative simplicity and because it does not need a biosphere or other unknown source of methane to explain the data.