A Comprehensive Reanalysis of K2-18 b's JWST NIRISS+NIRSpec Transmission Spectrum

Abstract: Sub-Neptunes are the most common type of planet in our galaxy. Interior structure models suggest that the coldest sub-Neptunes could host liquid water oceans underneath their hydrogen envelopes - sometimes called 'hycean' planets. JWST transmission spectra of the $\sim$ 250 K sub-Neptune K2-18 b were recently used to report detections of CH$4$ and CO$_2$, alongside weaker evidence of (CH$_3$)$_2$S (dimethyl sulfide, or DMS). Atmospheric CO$_2$ was interpreted as evidence for a liquid water ocean, while DMS was highlighted as a potential biomarker. However, these notable claims were derived using a single data reduction and retrieval modeling framework, which did not allow for standard robustness tests. Here we present a comprehensive reanalysis of K2-18 b's JWST NIRISS SOSS and NIRSpec G395H transmission spectra, including the first analysis of the second-order NIRISS SOSS data. We incorporate multiple well-tested data reduction pipelines and retrieval codes, spanning 60 different data treatments and over 250 atmospheric retrievals. We confirm the detection of CH$_4$ ($\approx$ 4$\sigma$), with a volume mixing ratio of log CH$_4$ = $-1.15^{+0.40}{-0.52}$, but we find no statistically significant or reliable evidence for CO$_2$ or DMS. Finally, we quantify the observed atmospheric composition using photochemical-climate and interior models, demonstrating that our revised composition of K2-18 b can be explained by an oxygen-poor mini-Neptune without requiring a liquid water surface or life.

• The paper confirmed methane at ~4σ significance with a volume mixing ratio of log CH4 = -1.15, supporting a hydrogen-rich atmospheric model for K2-18 b.
• The paper refuted previous evidence of CO2 and DMS, finding no statistically significant presence of these molecules in the reanalyzed data.
• The paper employed multiple reduction pipelines and retrieval codes, demonstrating robust cross-validation in exoplanet atmospheric characterization.

A Comprehensive Reanalysis of K2-18 b's JWST NIRISS+NIRSpec Transmission Spectrum

The study titled "A Comprehensive Reanalysis of K2-18 b's JWST NIRISS+NIRSpec Transmission Spectrum" aimed to reassess the atmospheric composition of the sub-Neptune K2-18 b using data from JWST transmission spectra. The researchers employed multiple reduction pipelines and retrieval codes, advancing beyond previous analyses that were based on single frameworks. This strategy allowed for a more robust assessment of atmospheric components and aimed to confirm or refute earlier claims of potential biomarkers such as dimethyl sulfide (DMS).

Key Findings

1. Confirmation of Methane (CH4): The reanalysis confirmed the presence of methane at approximately 4σ significance, with a volume mixing ratio of log CH4 = -1.15 with uncertainties of +0.40 and -0.52. This indicates that methane constitutes a significant portion of K2-18 b's atmosphere, consistent with the potential for a hydrogen-rich mini-Neptune classification.
2. Reevaluation of Carbon Dioxide (CO2) and DMS: Contrary to previous reports, the study found no statistically significant evidence for carbon dioxide or DMS. The presence of CO2, often associated with potential liquid water oceans, could not be corroborated within the sensitivity limits of the data analyzed.
3. Atmospheric Modeling: The study employed photochemical and climate models that suggest the revised composition might be explained by an oxygen-poor mini-Neptune model without necessitating surface water or biological activity. This result aligns with the new finding of prevalent CH4 and effectively rules out significant amounts of CO2 and other previously claimed molecules.

Implications

• Exoplanet Characterization: The confirmation of substantial CH4 reshapes our understanding of K2-18 b’s atmospheric chemistry, supporting theories that place it firmly in the category of hydrogen-dominated atmospheres but challenge its potential habitability as initially perceived when DMS was considered as a biomarker.
• Astrobiological Considerations: With the absence of CO2 and DMS signals, the argument for biological activity indicated by DMS is weakened, urging caution against inferring habitability based solely on such biomarkers.
• Model Validation: The incorporation of multiple data pipelines and retrieval methodologies in this study underscores the importance of verification in exoplanet atmospheric studies, ensuring that conclusions are not artifacts of a single analytical path.
• Future Observations and Model Improvements: The study highlights areas for future JWST observations to refine CO2 abundance estimates and probe deeper into the atmospheric composition of K2-18 b. Additional high-sensitivity data could help distinguish between the atmosphere supported by photochemical processes alone or one altered by more complex geochemical or biological processes.

Speculation on Future Developments

This reanalysis sets a precedent for how exoplanet atmospheres could be studied using the JWST's rich datasets. As data reduction and retrieval techniques continue to evolve, our understanding of mini-Neptunes and potentially habitable planets will likely improve. More robust assessments of atmospheres could aid in narrowing down targets with true biosignature potential, ultimately influencing the future direction of exoplanetary science and exploration.