New Constraints on DMS and DMDS in the Atmosphere of K2-18 b from JWST MIRI (2504.12267v1)

Abstract: The sub-Neptune frontier has opened a new window into the rich diversity of planetary environments beyond the solar system. The possibility of hycean worlds, with planet-wide oceans and H$_2$-rich atmospheres, significantly expands and accelerates the search for habitable environments elsewhere. Recent JWST transmission spectroscopy of the candidate hycean world K2-18 b in the near-infrared led to the first detections of carbon-bearing molecules CH$_4$ and CO$_2$ in its atmosphere, with a composition consistent with predictions for hycean conditions. The observations also provided a tentative hint of dimethyl sulfide (DMS), a possible biosignature gas, but the inference was of low statistical significance. We report a mid-infrared transmission spectrum of K2-18 b obtained using the JWST MIRI LRS instrument in the ~6-12 $\mu$m range. The spectrum shows distinct features and is inconsistent with a featureless spectrum at 3.4-$\sigma$ significance compared to our canonical model. We find that the spectrum cannot be explained by most molecules predicted for K2-18 b with the exception of DMS and dimethyl disulfide (DMDS), also a potential biosignature gas. We report new independent evidence for DMS and/or DMDS in the atmosphere at 3-$\sigma$ significance, with high abundance ($\gtrsim$10 ppmv) of at least one of the two molecules. More observations are needed to increase the robustness of the findings and resolve the degeneracy between DMS and DMDS. The results also highlight the need for additional experimental and theoretical work to determine accurate cross sections of important biosignature gases and identify potential abiotic sources. We discuss the implications of the present findings for the possibility of biological activity on K2-18 b.

Analysis of Atmospheric Characteristics on Hycean Worlds: Evidence from K2-18b

The paper conducted by Madhusudhan et al. employs JWST MIRI observations to explore the atmospheric properties of K2-18b, a candidate hycean exoplanet located in the habitable zone around a nearby M dwarf. This work represents a significant advancement in the atmospheric characterization of sub-Neptune exoplanets, focusing specifically on detecting potential biosignature gases in an H$_2$-rich atmosphere.

Data Acquisition and Methodology

The research utilizes mid-infrared transmission spectroscopy with the JWST MIRI LRS instrument to observe spectral features indicative of molecular compositions in the atmosphere of K2-18b. The paper reports clear spectral features between 6-12 μm, inconsistent with a featureless spectrum at a confidence level of 3.4-$\sigma$. Two distinct methodologies (JExoRES and JexoPipe) were employed to ensure the robustness of the spectral data interpretation. This approach highlights the importance of methodological rigor in exoplanetary atmospheric analyses, particularly when exploring potential biosignatures.

Key Findings

Madhusudhan et al. report strong evidence for the presence of dimethyl sulfide (DMS) and dimethyl disulfide (DMDS), with both gases presenting significant spectral absorption features. These molecules were detected independently by different pipelines at a significance of 2.9-3.2 $\sigma$. The reported abundances surpass 10 ppmv, which is notably higher than Earth-based levels typically associated with biological processes. However, the current statistical significance falls short of confirming the presences of these molecules unequivocally, emphasizing the need for further observational verification.

Implications

The detection of DMS and DMDS raises intriguing possibilities regarding the biotic processes that might exist in the atmospheres of hycean worlds. These molecules are considered robust biosignatures due to a lack of plausible abiotic pathways for their production at such high concentrations. While the findings bolster arguments for habitability, the authors highlight the necessity of improved cross-sectional data for these gases under the specific atmospheric conditions prevalent on K2-18b. This underlines the complexities involved in accurately characterizing exoplanet atmospheres and the potential pitfalls in biosignature detection.

Future Directions

While the paper represents a promising stride in identifying biosignature gases in an exoplanet atmosphere, the authors advocate for additional observations using JWST to corroborate these findings. The refinement of molecular absorption models through laboratory simulations tailored to exoplanet atmospheres could mitigate uncertainties in abundance estimates. Furthermore, exploring diverse molecules beyond DMS and DMDS could provide complementary evidence supporting or refuting biological activity on K2-18b. Investigations into potential abiotic sources of these molecules are equally crucial in addressing false positives in biosignature identification.

Conclusion

Madhusudhan et al.'s exploration of K2-18b's atmosphere with JWST MIRI represents a pivotal development in exoplanetary atmospheric science, particularly within the context of identifying habitable environments beyond our solar system. While significant challenges remain, the paper demonstrates the potential of hycean worlds as candidates for further paper regarding the presence of life and the imperative advancements needed in biosignature detection methodologies.