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JWST observations of K2-18b can be explained by a gas-rich mini-Neptune composition

Presentation #625.15 in the session Planetary Atmospheres - Terrestrial Planets and Mini-Neptunes.

Published onApr 03, 2024
JWST observations of K2-18b can be explained by a gas-rich mini-Neptune composition

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 [1] argued the data are best explained by a habitable composition, called a “Hycean” world, consisting of a thin (e.g., ~1 bar) H2-domianted 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 incompatible with the JWST observations because only < 10-5 mixing ratio CH4 is photochemically stable in the atmosphere while the data suggest about ~1% of the gas is present. Sustaining %-level CH4 on a Hycean K2-18b would likely require the presence of a methane-producing biosphere, similar to the microbial life on Earth ~3 billion years ago. On the other hand, we predict that a gas-rich mini-Neptune with high metallicity should have %-level CH4 and close to 0.1% CO2 which is compatible with the JWST data. In this case, CH4 and CO2 are produced thermochemically in the deep atmosphere and mixed upward to the low pressures sensitive to transmission spectroscopy. The model has H2O, NH3 and CO consistent with non-detections in [1]. Given the additional obstacles to maintaining a stable temperate climate on Hycean worlds due to H2 escape and potential supercriticality at depth [2], 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.

[1] Madhusudhan, Sarkar, Constantinou, Molmberg, Piette and Moses (2023). Carbon-bearing Molecules in a Possible Hycean Atmosphere. ApJ Letters. DOI: 10.3847/2041-8213/acf577

[2] Innes, Tsai, Pierrehumbert (2023). The Runaway Greenhouse Effect on Hycean Worlds. ApJ. DOI: 10.3847/1538-4357/ace346

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