A Systematic Search for Trace Molecules in the Atmosphere of Exoplanet K2-18 b

TL;DR

This study systematically searches for trace molecules in K2-18 b's atmosphere using JWST data, identifying dimethyl sulfide as a potential biosignature and emphasizing the need for further observations.

Contribution

It conducts an extensive, agnostic search across 661 molecules in K2-18 b's atmosphere, highlighting DMS as a candidate biosignature and assessing the plausibility of various gases.

Findings

DMS shows consistent Bayesian preference across datasets.

Other gases have limited support or plausible sources.

Further observations are needed to confirm trace gases.

Abstract

The first transmission spectrum of the habitable-zone sub-Neptune K2-18 b with JWST has opened a new avenue for atmospheric characterisation of temperate low-mass exoplanets. The observations led to inferences of methane and carbon dioxide, as well as of dimethyl sulfide (DMS) and/or dimethyl disulfide (DMDS), both potential biosignatures. In the present work we conduct a broad and agnostic search for other chemical species in the atmosphere of K2-18 b. Our exploration includes 661 molecules, spanning a wide range of trace gases, including biotic, abiotic, and anthropogenic gases on Earth. We investigate possible preference for any of these gases, compared to a model only including the previously-detected CH$_4$ and CO$_2$, using three metrics: (a) preference in the JWST mid-infrared (MIR) spectrum, (b) preference in the JWST near-infrared spectrum, for species preferred in MIR and (c) plausible sources of production. We find that only DMS consistently results in Bayes factors $\ln B \geq 2.0$ across the datasets considered independently, though in the near-infrared this depends on detector offsets, as previously reported. The threshold of $\ln B \geq 2.0$ is motivated by the conventional threshold of $\ln B \geq 2.5$ for moderate preference, allowing for an empirical uncertainty of 0.5. A few other gases also provide comparable fits to a subset of the data or only with some of the retrieval codes used, but with limited known plausible sources. Our study highlights the need for further observations to distinguish between possible trace gases in the atmosphere of K2-18 b and theoretical work to establish their plausible sources.