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

**Authors:** Stephen P. Schmidt, Ryan J. MacDonald, Shang-Min Tsai, Michael Radica, Le-Chris Wang, Eva-Maria Ahrer, Taylor J. Bell, Chloe Fisher, Daniel P. Thorngren, Nicholas Wogan, Erin M. May, Piero Ferrari, Katherine A. Bennett, Zafar Rustamkulov, Mercedes López-Morales, David K. Sing  
**arXiv:** [2501.18477](https://arxiv.org/abs/2501.18477)  
**Date:** 2025-01-30  
**Summary page:** https://www.summarizepaper.com/en/paper/2501.18477v1/

## Key points

- Sub-Neptunes are the most common type of planet in our galaxy
- Potential to host liquid water oceans beneath their hydrogen envelopes
- Concept of 'hycean' planets
- K2-18 b is a sub-Neptune with a temperature of approximately 250 K
- Transmission spectra obtained by the James Webb Space Telescope (JWST) revealed methane (CH$_4$)
- Initial reports suggested presence of carbon dioxide (CO$_2$) and dimethyl sulfide ((CH$_3$)$_2$S or DMS)
- Comprehensive reanalysis confirmed detection of CH$_4$
- No statistically significant evidence found for CO$_2$ or DMS in K2-18 b's atmosphere after reanalysis
- Revised composition suggests an oxygen-poor mini-Neptune explanation without requiring liquid water surface or indications of life
- Study conducted by Schmidt et al. provides valuable insights into atmospheric composition and potential habitability factors of sub-Neptunes

## Summary

Sub-Neptunes are the most common type of planet in our galaxy and have recently been the subject of study due to their potential to host liquid water oceans beneath their hydrogen envelopes. This has led to the concept of 'hycean' planets. One such planet is K2-18 b, a sub-Neptune with a temperature of approximately 250 K, which has garnered attention for its transmission spectra obtained by the James Webb Space Telescope (JWST). Initial reports suggested the presence of methane (CH$_4$) and carbon dioxide (CO$_2$), as well as hints of dimethyl sulfide ((CH$_3$)$_2$S or DMS). CO$_2$ was interpreted as evidence for a liquid water ocean and DMS as a potential biomarker. However, a comprehensive reanalysis conducted by Stephen P. Schmidt and colleagues has shed new light on these findings. By utilizing multiple data reduction pipelines and retrieval codes, incorporating various data treatments and conducting over 250 atmospheric retrievals, the team confirmed the detection of CH$_4$ at a significance level of approximately 4$\sigma$. The volume mixing ratio of CH$_4$ was determined to be log CH$_4$ = $-1.15^{+0.40}_{-0.52}$. Contrary to previous claims, no statistically significant or reliable evidence was found for the presence of CO$_2$ or DMS in K2-18 b's atmosphere through this rigorous reanalysis. The revised composition suggests an oxygen-poor mini-Neptune explanation for the observed atmospheric composition without necessitating a liquid water surface or indications of life. This detailed study conducted by Schmidt et al., involving advanced modeling techniques and thorough analysis methods, provides valuable insights into the atmospheric composition and potential habitability factors of sub-Neptunes like K2-18 b. The findings contribute significantly to our understanding of exoplanetary atmospheres and their implications for planetary science research.