Three outstanding physical questions for K2-18 b and other temperate sub-Neptunes

TL;DR

This study uses climate and photochemical models, along with laboratory data, to analyze the atmosphere of K2-18 b, addressing debates over spectral interpretations and the presence of certain compounds, ultimately supporting a sub-Neptune scenario without DMS.

Contribution

It provides a comprehensive modeling approach combined with new laboratory measurements to clarify atmospheric composition and spectral features of K2-18 b, challenging previous claims of DMS detection.

Findings

Supports a sub-Neptune atmospheric scenario without DMS

Finds no strong evidence for a water-rich interior

Reconciles low water abundance with climate models

Abstract

Recent transmission spectra of the temperate sub-Neptune K2-18 b obtained with JWST have attracted significant attention. Debates have quickly arisen over the interpretation of the spectral data, particularly the recent MIRI observation where dimethyl sulfide (DMS) and dimethyl disulfide (DMDS) are claimed. Here we revisit K2-18 b as a case study to examine several key questions that are also broadly relevant to the temperate sub-Neptune population: i) Can the low water abundance be reconciled with water clouds driven by orbital eccentricity? ii) Are the observed and non-observed atmospheric compositions mutually consistent? iii) Is it kinetically possible to produce DMS under sub-Neptune conditions? To address these questions, we couple climate and photochemical models to obtain self-consistent climate-photochemistry states for K2-18 b with a moderate orbital eccentricity of 0.2, as suggested by radial-velocity measurements. In addition, we present new laboratory measurements of DMS and DMDS infrared opacities by HFML-FELIX and compile updated C$_2$H$_6$ (ethane) opacities that include weak overtone bands. Our results support the interpretation of a sub-Neptune scenario without invoking DMS, and we do not find strong evidence for a water-rich interior.