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

@misc{zhang2026photochemistry,
  title = {{C, N, O, S, and photochemistry in a temperate giant planet orbiting a late M dwarf}},
  author = {Michael Zhang and Qiao Xue and Jeehyun Yang and Vighnesh Nagpal and Michael R. Line and Guangwei Fu and Matthew C. Nixon and Jacob L. Bean and Peter Gao and Eliza M. -R. Kempton and Luis Welbanks and Edward M. Bryant and Daniel Bayliss and Madison Brady and Jean-Michel Désert and Vincent Van Eylen and Jonathan J. Fortney and Andrés Jordán and Vivien Parmentier and Caroline Piaulet-Ghorayeb and Elyar Sedaghati and Kevin B. Stevenson and Amaury H. M. J. Triaud},
  howpublished = {arXiv},
  year = {2026},
  month = sep,
  doi = {10.48550/arxiv.2606.23774},
  eprint = {2606.23774},
  archivePrefix = {arXiv},
  url = {https://arxiv.org/abs/2606.23774},
  note = {CC BY 4.0},
}

TY  - UNPB
TI  - C, N, O, S, and photochemistry in a temperate giant planet orbiting a late M dwarf
AU  - Michael Zhang
AU  - Qiao Xue
AU  - Jeehyun Yang
AU  - Vighnesh Nagpal
AU  - Michael R. Line
AU  - Guangwei Fu
AU  - Matthew C. Nixon
AU  - Jacob L. Bean
AU  - Peter Gao
AU  - Eliza M. -R. Kempton
AU  - Luis Welbanks
AU  - Edward M. Bryant
AU  - Daniel Bayliss
AU  - Madison Brady
AU  - Jean-Michel Désert
AU  - Vincent Van Eylen
AU  - Jonathan J. Fortney
AU  - Andrés Jordán
AU  - Vivien Parmentier
AU  - Caroline Piaulet-Ghorayeb
AU  - Elyar Sedaghati
AU  - Kevin B. Stevenson
AU  - Amaury H. M. J. Triaud
PY  - 2026
DA  - 2026/09/17
PB  - arXiv
DO  - 10.48550/arxiv.2606.23774
UR  - https://arxiv.org/abs/2606.23774
AB  - We report the JWST NIRSpec/PRISM transit spectrum of TOI-6894b, 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--10x 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/N ratios are consistent with solar values. A grid retrieval on 1D radiative-convective photochemical equilibrium (RCPE) 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 that of Jupiter, Saturn, and other gas giant exoplanets, despite orbiting a very low-mass star.
ER  -