Examining the Potential for Methyl Halide Accumulation and Detectability in Possible Hycean-Type Atmospheres

TL;DR

This study explores the potential for detecting methyl halide biosignatures, especially methyl chloride, in Hycean-type atmospheres using JWST, highlighting their detectability and implications for biosignature searches on sub-Neptune exoplanets.

Contribution

The paper presents the first self-consistent photochemical and spectral modeling of methyl halides in H2-dominated atmospheres of Hycean planets, assessing their detectability with JWST.

Findings

Methyl chloride could be detected in Hycean atmospheres within tens of JWST transits.

Detectability threshold is around 10 ppm atmospheric mixing ratio.

Global fluxes similar to Earth's moderately productive environments could lead to detectable levels.

Abstract

Some sub-Neptune planets may host habitable conditions; for example "Hycean" worlds with H2 envelopes over liquid water oceans can maintain potentially hospitable pressures and temperatures at their surface. Recent JWST observations of K2-18b and TOI-270d have shown that such worlds could be compelling targets for biosignature searches, given their extended scale heights and therefore large atmospheric signatures. Methylated biosignatures, a broad group of gases that can be generated by biological attachment of a CH3 group to an environmental substrate, have been proposed as candidate signs of life for Earth-like exoplanets. However, methyl halides (CH3 + halogen) have not yet been robustly examined with self-consistent photochemical and spectral models for planets with H2-dominated atmospheres. Here we demonstrate that methyl chloride (CH3Cl), predominantly produced by marine microbes, could be detected using JWST in tens of transits or fewer for Hycean planets, comparable to detection requirements for other potential atmospheric biosignatures. The threshold atmospheric mixing ratio for detectability is $\sim$10 ppm, which can accumulate with global fluxes comparable to moderately productive local environments on Earth.