Insufficient evidence for DMS and DMDS in the atmosphere of K2-18 b

Context. Recent JWST observations of the temperate sub-Neptune K2-18 b have been interpreted as suggestive of a liquid water ocean with possible biological activity. Signatures of dimethyl sulfide (DMS) and dimethyl disulfide (DMDS) have been observed in the near-infrared (using the NIRISS and NIRSpec instruments) and mid-infrared (using MIRI). However, the statistical significance of the atmospheric imprints of these potential biomarkers has yet to be quantified from a joint analysis of the entire planet spectrum. Aims. We aim to test the robustness of the proposed DMS and DMDS detections by simultaneously modeling the NIRISS and NIRSpec observations jointly with the MIRI spectrum for the first time, considering different data reductions and modeling choices. Methods. We used three well-tested pipelines to re-reduce the JWST observations and two retrieval codes to analyze the resulting transmission spectra as well as previously published data. Results. The first joint analysis of the panchromatic (0.6–12 µm) spectrum of K2-18 b finds insufficient evidence for the presence of DMS and/or DMDS in the atmosphere of the planet. We find that any marginal preferences are the result of limiting the number of molecules considered in the model and oversensitivity to small changes between data reductions. Conclusions. Our results confirm that there is no statistical significance for DMS or DMDS in K2-18 b’s atmosphere. While previous works have demonstrated this on MIRI or NIRISS/NIRSpec observations alone, our analysis of the full, panchromatic transmission spectrum does not support claims of potential biomarkers. Using the best-fitting model including DMS and/or DMDS on the published data, we estimate that ∼25 more MIRI transits would be needed for a 3σ rejection of a flat line relative to DMS and/or DMDS features in the planet’s mid-infrared transmission spectrum.