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JWST observations of K2-18b can be explained by a gas-rich mini-Neptune with no habitable surface
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JWST observations of K2-18b can be explained by a gas-rich mini-Neptune with no habitable surface
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JWST observations of K2-18b can be explained by a gas-rich mini-Neptune with no habitable surface
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Paper
JWST observations of K2-18b can be explained by a gas-rich mini-Neptune with no habitable surface
2024
Overview
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.
Publisher
Cornell University Library, arXiv.org
