Exoplanet K2-18 b, a candidate hycean world, continues to be a focal point in the search for life beyond our solar system, with new observations from the James Webb Space Telescope (JWST) providing further intriguing evidence. A recent study reports new constraints on dimethyl sulfide (DMS) and dimethyl disulfide (DMDS) in K2-18 b’s atmosphere, molecules that are primarily associated with biological activity on Earth.

Previous JWST observations hinted at the presence of DMS, a potential biosignature gas, but the statistical significance was low and affected by instrumental factors. This new research utilized JWST’s Mid-Infrared Instrument (MIRI) Low-Resolution Spectrograph (LRS) to obtain a mid-infrared transmission spectrum of K2-18 b, covering the \sim6-12~\mu m range. This is the first mid-infrared transmission spectrum obtained for a habitable-zone exoplanet.

The MIRI spectrum reveals distinct features that are inconsistent with a featureless spectrum at a 3.4-sigma significance. While a wide range of molecules were considered to explain these features, the spectrum was best explained by the presence of DMS and/or DMDS. The study reports new independent evidence for the presence of DMS and/or DMDS at a 3-sigma significance, indicating high abundances (greater than 10 ppmv) of at least one of these molecules.

The research highlights a strong degeneracy between the spectral features of DMS and DMDS in the MIRI wavelength range, making it challenging to distinguish definitively between the two with current data quality. When both molecules are included in the atmospheric model, DMDS is somewhat preferred, but models including only one of the two molecules at a time also show high detection significances (2.9 to 3.2 sigma).

The observed spectral features in the MIRI data, with amplitudes of 300-400 ppm, are significantly larger than those seen in previous near-infrared observations. Notably, methane (CH_{4}) and carbon dioxide (CO_{2}), previously detected in K2-18 b’s atmosphere, are not detectable in the new MIRI spectrum, as their features are either weak in this range or dwarfed by the stronger contributions from DMS and/or DMDS. The retrieved abundances of DMS and/or DMDS, at volume mixing ratios of approximately 10^{-5} to 10^{-3}, are consistent with previous DMS constraints and could imply stronger biological activity than on Earth.

The authors emphasize the need for more observations to enhance the robustness of these findings and to resolve the degeneracy between DMS and DMDS. Furthermore, additional experimental and theoretical work is crucial to determine accurate cross-sections of these potential biosignature molecules in hydrogen-rich environments and to identify any potential abiotic sources. While some abiotic formation pathways have been explored, sustaining the observed high concentrations of DMS and/or DMDS abiotically is currently considered implausible.

The study concludes that these findings further raise the prospect of possible biological activity on K2-18 b and underscore the importance of continued interdisciplinary efforts in the search for life beyond Earth.

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