Habitable Zones & Global Climate

JWST Observations Of K2-18b Can Be Explained By A Gas-rich Mini-Neptune With No Habitable Surface

By Keith Cowing
Status Report
January 24, 2024
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JWST Observations Of K2-18b Can Be Explained By A Gas-rich Mini-Neptune With No Habitable Surface
Climate and photochemical simulation of K2-18b as a mini-Neptune with no habitable surface (Model 3 in Table 1). The black dashed line is the computed P-T profile which is referenced to the upper x-axis. The horizontal grey line at 1 bar divides the lower and upper atmosphere as discussed in Section 2.1. Solid colored lines are predicted atmospheric composition from our photochemical model. For comparison, the dotted lines in the lower atmosphere are chemical equilibrium composition. If K2-18b is a 100× solar mini-Neptune with a solar C/O ratio, then we predict the observable upper atmosphere should have ∼ 4% CH4 and nearly 0.1% CO2, which is in reasonable agreement with recent JWST observations (Madhusudhan et al. 2023b). — astro-ph.EP

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.

Nicholas F. Wogan, Natasha E. Batalha, Kevin Zahnle, Joshua Krissansen-Totton, Shang-Min Tsai, Renyu Hu

Comments: Under review at Astrophysical Journal Letters
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2401.11082 [astro-ph.EP] (or arXiv:2401.11082v1 [astro-ph.EP] for this version)
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Submission history
From: Nicholas Wogan
[v1] Sat, 20 Jan 2024 02:16:18 UTC (224 KB)

Explorers Club Fellow, ex-NASA Space Station Payload manager/space biologist, Away Teams, Journalist, Lapsed climber, Synaesthete, Na’Vi-Jedi-Freman-Buddhist-mix, ASL, Devon Island and Everest Base Camp veteran, (he/him) 🖖🏻