JWST Observations of K2-18b Can Be Explained by a Gas-rich Mini-Neptune with No Habitable Surface
Nicholas F. Wogan, Natasha E. Batalha, Kevin Zahnle, Joshua Krissansen‐Totton, Shang‐Min Tsai, Renyu Hu
Abstract
Abstract The James Webb Space Telescope (JWST) recently measured the transmission spectrum of K2-18b, a habitable-zone sub-Neptune exoplanet, detecting CH 4 and CO 2 in its atmosphere. The discovery paper argued the data are best explained by a habitable “Hycean” world, consisting of a relatively thin H 2 -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 CH 4 in such an atmosphere while the data suggest about ∼1% of the gas is present. Sustaining percent-level CH 4 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% CH 4 and nearly 0.1% CO 2 , which are compatible with the JWST data. The CH 4 and CO 2 are produced thermochemically in the deep atmosphere and mixed upward to the low pressures sensitive to transmission spectroscopy. The model predicts H 2 O, NH 3 , and CO abundances broadly consistent with the nondetections. Given the additional obstacles to maintaining a stable temperate climate on Hycean worlds due to H 2 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.