[astro-ph.EP] 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 CH4 and the photochemical product CS2. For the first time in an exoplanet 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 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 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±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.
Michael Zhang, Qiao Xue, Jeehyun Yang, Vighnesh Nagpal, Michael R. Line, Guangwei Fu, Matthew C. Nixon, Jacob L. Bean, Peter Gao, Eliza M.-R. Kempton, Luis Welbanks, Edward M. Bryant, Daniel Bayliss, Madison Brady, Jean-Michel Désert, Vincent Van Eylen, Jonathan J. Fortney, Andrés Jordán, Vivien Parmentier, Caroline Piaulet-Ghorayeb, Elyar Sedaghati, Kevin B. Stevenson, Amaury H.M.J. Triaud
Comments: Submitted to ApJL
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2606.23774 [astro-ph.EP] (or arXiv:2606.23774v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2606.23774
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Submission history
From: Michael Zhang
[v1] Mon, 22 Jun 2026 18:00:00 UTC (4,225 KB)
https://arxiv.org/abs/2606.23774
Astrobiology, exoplanet,
