2025ApJ...995L..70R

In recent years, JWST has facilitated detections of carbon-bearing molecules in the atmospheres of temperate sub-Neptunes orbiting M dwarfs, ushering in a new era in the characterization of this intriguing planetary regime. We report the transmission spectrum of the temperate sub-Neptune TOI-732 c, observed with JWST NIRISS, NIRSpec G395H, and MIRI Low Resolution Spectrometer between 0.9 and 12 μm. The observations provide evidence for methane (CH₄) in a H₂-rich atmosphere, at a volume mixing ratio of ∼1%, and nondetection of NH₃ and HCN, along with nominal constraints on other prominent molecules H₂O, CO, and CO₂, which are typically expected in H₂-rich atmospheres. We conduct a comprehensive survey of 250 chemical species and find moderate to strong evidence (up to $\mathrm{ln}B\sim 5.9$, 3.9σ) for additional absorption due to one or more complex molecules including higher-order hydrocarbons and/or sulfur-bearing molecules. The spectral features are strongly degenerate among these molecules and with methane, which we find at $\mathrm{ln}B=3.2-8.8$ (up to 3.0σ-4.6σ) significance. Two complex molecules are preferred with at least moderate evidence ($\mathrm{ln}B\gtrsim 2.5$) in both the near- and mid-infrared, while several others show such evidence in at least one of the two wavelength ranges. The preferred molecules are found in trace quantities on Earth, with no significant sources identified in other planetary atmospheres, requiring future work to assess their physical plausibility in this planet. Future observations are required to resolve the degeneracies and place more robust constraints on these species. We highlight the need for further theoretical and experimental work to robustly characterize the atmospheric and internal composition of TOI-732 c and similar sub-Neptunes.