{"about":{"site":"https://codewithpapers.app","non_affiliation":"Code with Papers and Syntology are not affiliated with, endorsed by, or sponsored by Papers with Code, Meta, or the pwc-archive mirror.","licence":"CC BY-SA 4.0","licence_url":"https://creativecommons.org/licenses/by-sa/4.0/legalcode","attribution":"https://codewithpapers.app/attribution","modified":"archive material modified by Syntology; see the attribution page"},"url":"/paper/jwst-observations-of-k2-18b-can-be-explained","title":"JWST observations of K2-18b can be explained by a gas-rich mini-Neptune with no habitable surface","arxiv_id":"2401.11082","date":"2024-01-20","proceeding":null,"authors":["Nicholas F. Wogan","Natasha E. Batalha","Kevin Zahnle","Joshua Krissansen-Totton","Shang-Min Tsai","Renyu Hu"],"abstract":"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 $\\sim 1\\%$ of the gas is present. Sustaining %-level CH$_4$ on a Hycean K2-18b may require the presence of a methane-producing biosphere, similar to microbial life on Earth $\\sim 3$ billion years ago. On the other hand, we predict that a gas-rich mini-Neptune with $100 \\times$ 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 non-detections. 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.","url_abs":"https://arxiv.org/abs/2401.11082v2","url_pdf":"https://arxiv.org/pdf/2401.11082v2.pdf","source":{"archive":"pwc-archive (Hugging Face), CC BY-SA 4.0","snapshot":"2025-07-28","licence_url":"https://creativecommons.org/licenses/by-sa/4.0/legalcode","row_kind":"links_only","authors_date_abstract":"arXiv metadata, CC0 1.0 (https://info.arxiv.org/help/license), from the Kaggle arXiv metadata snapshot of 2026-09-12"},"code_links":[{"paper_slug":"jwst-observations-of-k2-18b-can-be-explained","repo_url":"https://zenodo.org/record/zenodo.10525671","is_official":1,"mentioned_in_paper":0,"mentioned_in_github":0,"framework":"none","reach":null}],"tasks":[],"methods":[],"datasets_introduced":[],"methods_introduced":[],"results":[],"syntology":{"atlas_url":null,"mcp":null,"developers":"https://syntology.ai/developers"},"arxiv_metadata":null,"syntology_extracted_results":null}