paper

C, N, O, S, and photochemistry in a temperate giant planet orbiting a late M dwarf

arXiv:2606.23774

Abstract

We report the JWST NIRSpec/PRISM transit spectrum of TOI-6894b, an exceptional 420 K sub-Saturn that is the only known giant planet 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 CH $_{4}$ and the photochemical product CS $_{2}$ . For the first time in a transit spectrum, NH $_{3}$ is visually evident, while subtler features from H $_{2}$ O, and CO $_{2}$ 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 models at different metallicities. These models show that the spectrum--in particular the size of the NH $_{3}$ and CO $_{2}$ features relative to the CH $_{4}$ and H $_{2}$ O features--is most consistent with a metallicity of 3--10 $\times$ solar. Using a semi-free retrieval framework that perturbs the self-consistent 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 broadly consistent with solar values. A grid retrieval on 1D radiative-convective photochemical equilibrium (RCPE) models reveals a similar result: $[M / H] = 0.46 \pm 0.08$ and C/O= $0.69 \pm 0.06$ . The planet's atmospheric metallicity, abundance ratios, and bulk metal fraction are all strikingly similar to that of Jupiter, Saturn, and other gas giant exoplanets, despite orbiting a very low-mass star.

Updating with far more realistic error inflation scheme in semi-free retrievals, which has the side benefit of greatly shrinking N/O, C/N, S/N, M/H posteriors