A Systematic Search for Trace Molecules in 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 (DMDS), both potential biosignatures. However, robust identification of DMS and/or DMDS requires further observations to increase the detection significances. More theoretical studies are also needed to identify potential false positives and possible abiotic sources for these molecules. In the present work we demonstrate the next step in this direction with a comprehensive and agnostic search for other chemical species in the atmosphere of K2-18 b. Our exploration includes 650 molecules, spanning a wide range of trace gases, including biotic, abiotic, and anthropogenic gases on Earth. We investigate possible evidence for any of these gases using three metrics: (a) evidence in the JWST mid-infrared spectrum, (b) evidence in the JWST near-infrared spectrum, and (c) plausible sources of production. We find three molecules, including DMS, which appear promising across the datasets considered. The two molecules besides DMS are diethyl sulfide and methyl acrylonitrile, which are more complex than DMS, biogenic on Earth, and have no significant sources known beyond Earth. A few other gases also provide comparable fits to a subset of the data considered but again with limited known plausible sources. Our study highlights the need for further observations to distinguish between possible trace gases in K2-18 b and theoretical work to establish their plausible sources if confirmed on this planet.