A new study suggests that the advanced sensors on the James Webb Space Telescope (JWST) could detect methyl halides—potential biosignatures—in the atmospheres of Hycean planets.
Study: Examining the potential for methyl halide accumulation and detectability in possible Hycean-type atmospheres. Image Credit: Vadim Sadovski/Shutterstock.com
These planets, a subset of sub-Neptunes with hydrogen-rich atmospheres and underlying liquid water oceans, are emerging as promising candidates for hosting life. Observations from the JWST suggest that planets such as K2-18b and TOI-270d could offer favorable conditions for detecting biological activity, thanks to their extended atmospheric scale heights.
Methyl halides—organic molecules formed by the combination of methyl groups (CH3) with halogens—are mainly produced by marine microbial processes on Earth. Their presence in exoplanetary atmospheres could serve as a strong biosignature. This study focuses on the potential detectability of methyl chloride (CH3Cl) and emphasizes the need for refined methodologies to identify these biosignatures as we expand our search for extraterrestrial life.
Background
Hycean planets are thought to have extended hydrogen-rich atmospheres, which can enhance the visibility of biosignatures by amplifying spectral features. The study examines how methyl halides could accumulate in these atmospheres through photochemical processes, leveraging JWST's advanced observational capabilities.
While previous research has considered methyl halides as biosignatures, their presence in H2-rich atmospheres has not been extensively explored. By integrating observational data with computational models, the researchers aim to understand the conditions that influence the accumulation and detectability of these gases, offering a framework to assess their potential as indicators of life.
The Study
To simulate the accumulation of methyl halides in Hycean atmospheres, the researchers employed the VULCAN photochemical model. This widely validated model allows for detailed simulations of planetary atmospheric chemistry, including Earth and sub-Neptune-like planets.
For this study, the model was configured to represent a Hycean planet similar to K2-18b, incorporating multiple reaction pathways for methyl halides (CH3Cl, CH3Br, and CH3I). The model included 444 chemical reactions, including 38 photodissociation reactions, to assess how biological production rates influence the atmospheric concentrations of these molecules.
To evaluate the detectability of methyl halides, the researchers used the Planetary Spectrum Generator (PSG) to simulate transmission and emission spectra under different atmospheric conditions. These simulations factored in observational parameters, atmospheric composition, and stellar characteristics, employing advanced techniques such as correlated k-tables and line-by-line opacity calculations.
Noise reduction methods were also applied to estimate the number of transits required to achieve statistically significant detections of methyl halides using JWST’s NIRSpec PRISM and MIRI-LRS instruments.
Results and Discussion
The study found that under simulated biological production scenarios, CH3Cl could accumulate in Hycean atmospheres at levels detectable by JWST, particularly at moderate to high biological flux rates.
The baseline detectability threshold was estimated at approximately 10 parts per million (ppm), with higher biological production rates—up to 1000 times Earth’s average—leading to even greater concentrations. These findings suggest that biologically active environments could produce significant accumulations of methyl halides, making them viable biosignatures.
The study identified optimal detection wavelengths for CH3Cl, particularly around 4.0 μm, where JWST’s NIRSpec PRISM instrument could provide clearer signals with reduced noise. The researchers estimated that CH3Cl could be detected in as few as five transits, depending on the confidence level required. In emission spectroscopy, methyl halide absorption features were found to be comparable in prominence to well-established biomarkers like H2O and CO2.
Looking ahead, the researchers highlighted the potential of future telescopes, particularly the upcoming Habitable Worlds Observatory (HWO), to enhance biosignature detection. HWO’s capabilities in reflected light spectroscopy could expand the range of detectable biosignatures across different planetary environments. The study also emphasized the importance of laboratory measurements to refine spectral feature characterization, ultimately improving the accuracy of biosignature detection techniques.
Conclusion
The research conducted by Leung et al. provides key insights into the potential for detecting methyl halides as biosignatures in Hycean planet atmospheres. The findings suggest that under certain biological production conditions, CH3Cl could accumulate to detectable levels, reinforcing the idea that Hycean planets represent promising targets in the search for extraterrestrial life.
The study’s modeling approach and observational strategies are helping to lay the foundation for future research, underscoring the need for continuous exploration of methylated gases as potential indicators of life. With ongoing advancements in space-based observations, the scientific community is well-positioned to capitalize on upcoming opportunities to detect biosignatures beyond our solar system.
Journal Reference
Leung M., Tsai S.-M., et al. (2025). Examining the potential for methyl halide accumulation and detectability in possible Hycean-type atmospheres. The Astrophysical Journal Letters, 982(2), L2. DOI: 10.3847/2041-8213/adb558, https://iopscience.iop.org/article/10.3847/2041-8213/adb558