JWST observations of K2-18b can be explained by a gas-rich mini-Neptune with no habitable surface (2401.11082v2)

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.

• The paper demonstrates that JWST data for K2-18b aligns with a gas-rich mini-Neptune model rather than a habitable Hycean world.
• It employs detailed photochemical and climate models to analyze atmospheric compositions, including methane and CO2, alongside temperature-pressure profiles.
• The study underscores the need for broader spectral data and refined simulations to better understand exoplanet atmospheric processes.

Analyzing the Interpretations of JWST Observations of K2-18b

This paper focuses on analyzing recent JWST observations of the exoplanet K2-18b, aiming to explain its atmospheric composition and structure. K2-18b has been a topic of substantial interest given its position in the habitable zone and the contentious interpretation of its potential as a "Hycean" world. This paper rigorously evaluates whether the observational data of K2-18b, which indicates notable quantities of atmospheric CH$_4$ and CO$_2$, align more convincingly with it being a habitable Hycean planet, home to an Archean-like biosphere, or a simpler, non-habitable mini-Neptune with a thick, uninhabitable gaseous envelope.

The authors initially tackle the possibility of K2-18b being a Hycean world. Using photochemical and climate models, the paper assesses scenarios where K2-18b has a thin hydrogen-dominated atmosphere overlaying a surface loaded with liquid water. One finding is that a lifeless Hycean model cannot maintain more than a negligible amount of methane, primarily due to rapid photochemical destruction. This conflicts with the $\sim 1\%$ atmospheric methane inferred from JWST data unless a hypothesized biosphere is actively replenishing methane through biological processes akin to methanogenesis on early Earth, producing a methane flux comparable to half of modern Earth's biogenic output.

The paper furthers this by comparing the temperature and pressure profiles derived from climate models and concludes that sustaining a Hycean world poses additional challenges beyond biotic methane production. Specifically, a habitable surface climate is hard to imagine without invoking speculative cloud coverage to provide adequate solar shielding necessary to prevent runaway greenhouse conditions. Additionally, the susceptibility of such a thin hydrogen atmosphere to escape, exacerbated by the star's historical XUV exposure, presents another significant hurdle.

In parallel, the paper posits an alternative explanation for the observed atmospheric characteristics: K2-18b as a gas-rich mini-Neptune. Here, the authors argue for a more plausible interpretation where the exoplanet possesses a substantial hydrogen-dominated envelope, characteristic of mini-Neptunes, with high metallicity (100x solar) and deep atmospheric thermochemical processes. Their models demonstrate how such an environment can naturally allow for the quenching of methane and CO$_2$ from deeper atmospheric layers to strata sensitive to transmission spectroscopy, aligning well with the observed spectrum without invoking biological processes. This model's complexity and assumptions appear more physically grounded and systematic without the additional speculative layers required for a Hycean world.

Ultimately, the authors weigh the observational spectral fit and the intrinsic assumptions of both models. They suggest that despite the potential for an inhabited Hycean interpretation, a gas-rich mini-Neptune offers a simpler and more straightforward account aligning with nature's known processes and parameters. These insights underscore the significance of understanding atmospheric chemistry and the balance of thermochemistry versus photochemistry, especially when deducing the origins and state of exoplanetary atmospheres.

The implications of this research provoke further investigation. They emphasize the need for more detailed spectral data across broader wavelengths, and parallel modeling efforts that refine assumptions about intrinsic planetary heat, cloud formation, and atmospheric chemistry. Moreover, this work indirectly invites speculation on the prominence of CO$_2$/CH$_4$ disequilibrium not only as potential biosignatures but also as evidences of robust abiotic processes in exoplanet atmospheres.