Papers › JWST observations of K2-18b can be explained by a gas-rich mini-Neptune with no...
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
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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 %-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 non-detections. 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.
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