C, N, O, S, and Photochemistry in a Temperate Giant Planet Orbiting a Late M Dwarf

Journal article

We report the JWST NIRSpec/PRISM transit spectrum of TOI-6894 b, an exceptional 420 K sub-Saturn that is one of the rare giant planets transiting a late M dwarf. Remarkably, both the light curve and the transit spectrum exhibit almost no stellar contamination. The spectrum is dominated by prominent absorption features from CH4 and the photochemical product CS2. For the first time in a transit spectrum, NH3 is visually evident, while subtler features from H2O and CO2 can also be seen. We significantly improve upon state-of-the-art photochemical reaction networks, and use our new network to run radiative-convective photochemical (“RCP”) models at different metallicities. These models show that the spectrum—in particular the size of the NH3 and CO2 features relative to the CH4 and H2O features—is most consistent with a metallicity of 3–10× solar. Using a semi-free retrieval framework that perturbs the RCP model’s abundance and temperature profiles to fit the data, we find that the planet’s C/O, N/O, and S/O ratios are consistent with solar values. A grid retrieval on 1D RCP equilibrium models reveals a similar result: [M/H]  = 0.46 ± 0.08 and C/O = 0.69 ± 0.06. The planet’s atmospheric metallicity, abundance ratios, and bulk metal fraction are all strikingly similar to those of Jupiter, Saturn, and other gas giant exoplanets, despite orbiting a very low-mass star.

Authors: Zhang, M; Xue, Q; Yang, J; Nagpal, V; Line, MR

• Publication status: Published
• Peer review status: Peer reviewed
• Publisher: American Astronomical Society
• Journal: The Astrophysical Journal Letters
• Volume: 1009
• Issue: 2
• Pages: L28-L28
• Article number: L28
• Publication date: 2026-09-22
• DOI: 10.3847/2041-8213/aea4ae
• EISSN: 2041-8213
• ISSN: 2041-8205
• Language: English
• Keywords: N/O ratio; metallicity; water vapor; C/O ratio; ammonia absorption; photochemistry; S/O ratio; transit spectroscopy; sub-Saturns; methane absorption; giant planet atmospheres; carbon monosulfide; carbon dioxide