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

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, 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 CH4 and CO2, using three metrics: (a) preference in the JWST mid-infrared (MIR) spectrum, (b) preference in the JWST near-infrared (NIR) spectrum, for species preferred in MIR, and (c) plausible sources of production. We find that only DMS consistently results in Bayes factors lnB≥2.0 across the datasets considered independently, though in the NIR this depends on detector offsets, as previously reported. The threshold of lnB≥2.0 is motivated by the conventional threshold of lnB≥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 as well as theoretical work to establish their plausible sources.