A Systematic Search for Trace Molecules in Exoplanet K2-18 b
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.
The flow chart shows our framework of assessment for possible molecules in the atmosphere of K2-18 b. Based on the inference of carbon dioxide (CO2) and methane (CH4) in the atmosphere of K2-18 b (Madhusudhan et al. 2023), retrievals adopting canonical models are carried out following M25, assuming hydrogen dominated atmospheres and including the mixing ratios of CO2, CH4, and X as free parameters, with X in this work being any of 650 species explored. The evidence for X is then computed by performing a Bayesian comparison between the relevant canonical model and a nested baseline model, with only X removed. Several independent retrieval codes can be used to increase the reliability of such a framework. In this work, retrieval codes 1, 2, and 3 are POSEIDON, VIRA, and petitRADTRANS, respectively. Retrievals were initially performed with the POSEIDON retrieval code on the two available MIRI datasets for K2-18 b, resulting from the JExoRES and JExoPipe data reduction pipelines (MIRI data 1 and 2, in the pink curved boxes at the top left and top right). Nearly all molecules in the HITRAN cross-section database were used in these initial retrievals, as shown in the tables on the lower left and lower right hand side of the figure. Additionally, some other available molecules, as well as atoms and ions, were considered, where originally included in POSEIDON. Chemical species for which moderate evidence (2.7σ±0.2σ, see, e.g., Trotta 2008) is found are identified (orange rectangles), and follow-up retrievals are performed with VIRA and petitRADTRANS. Chemical species for which moderate evidence is found across both reduction pipelines and all three retrieval frameworks (yellow diamond) are passed to the next step, where we include them in atmospheric retrievals on the near-infrared (NIR) data from the NIRISS and NIRSpec JWST transits (pink curved box in the middle) of K2-18 b (Madhusudhan et al. 2023). Finally, retrievals with VIRA, petitRADTRANS, and POSEIDON are find if there are species for which significances ≥ 2.5σ can be reached in the NIR data (green diamond). We then discuss the physical plausibility of promising species being present in the atmosphere of K2-18 b. — astro-ph.EP
Lorenzo Pica-Ciamarra, Nikku Madhusudhan, Gregory J. Cooke, Savvas Constantinou, Martin Binet
Comments: Submitted for publication
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2505.10539 [astro-ph.EP] (or arXiv:2505.10539v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2505.10539
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Submission history
From: Madhusudhan Nikku
[v1] Thu, 15 May 2025 17:48:51 UTC (563 KB)
https://arxiv.org/abs/2505.10539
Astrobiology, Astrochemistry,
