Considerations for Photochemical Modeling of Possible Hycean Worlds

TL;DR

This study uses independent photochemical models to evaluate the atmospheric compositions of K2-18 b, supporting the Hycean world hypothesis over the mini-Neptune scenario, and emphasizes the sensitivity of results to model assumptions.

Contribution

It provides a comprehensive analysis of photochemical scenarios for K2-18 b, demonstrating the importance of model assumptions in interpreting atmospheric data for Hycean worlds.

Findings

Hycean scenarios better fit observed abundances than mini-Neptune models.

Uninhabited Hycean scenario explains most molecular abundances.

Inhabited Hycean scenario aligns with all observed abundances if CH₄ is biogenic.

Abstract

JWST is revolutionising the study of temperate sub-Neptunes, starting with the first detection of carbon-bearing molecules in the habitable-zone sub-Neptune K2-18 b. The retrieved abundances of CH$_4$ and CO$_2$ and non-detection of NH$_3$ and CO in K2-18 b are consistent with prior predictions of photochemical models for a Hycean world with a habitable ocean. However, recent photochemical modeling raised the prospect that the observed abundances may be explained by a mini-Neptune scenario instead. In this study, we explore these scenarios using independent photochemical modeling with K2-18 b as a case study. We find the previous results to be sensitive to a range of model assumptions, such as the photochemical cross sections, incident stellar spectrum, surface pressure, UV albedo, and metallicity, significantly affecting the resulting abundances. We explore a wide model space to investigate scenarios that are compatible with the retrieved molecular abundances for K2-18 b. Our analysis shows that the previously favoured mini-Neptune scenario is not compatible with most of the retrieved abundances, while the Hycean scenarios, both inhabited and uninhabited, provide better agreement. An uninhabited Hycean scenario explains most of the abundance constraints, except CH$_4$ which is generally underabundant but dependent on the model assumptions. The inhabited Hycean scenario is compatible with all the abundances if the observed CH$_4$ is assumed to be predominantly biogenic. Our results underscore the importance of systematic photochemical modeling and accurate interpretation of chemical abundance constraints for candidate Hycean worlds.