Methane On The Temperate Exo-Saturn TOI-199b
Temperate (Teq<400 K) gas giants represent an unexplored frontier in exoplanet atmospheric spectroscopy. Orbiting a G-type star every ~100 days, the Saturn-mass exoplanet TOI-199 b (Teq=350 K) is one of the most favorable low-temperature gas giants for atmospheric study.
Here, we present its transmission spectrum from a single transit observed with JWST’s NIRSpec G395M mode. Despite lower-than-nominal precision due to a pointing misalignment, Bayesian retrievals reveal the presence of CH4 (Bayes factor of ∼700 in a cloudy atmosphere), corresponding to a metallicity of C/H=13+78−12× solar, although the absence of detectable CO and CO2 disfavors metallicities ≳50× solar.
We also tested several haze prescriptions (Titan-like tholin, soot, and water-rich tholin), but the preference for these models is weak (Bayes factors of ∼2 relative to the clear case). The spectrum also shows an increase in transit depth near 3 μm, which our self-consistent models attribute to either NH3 or, less likely, HCN.
Follow-up observations will distinguish between these species, helping determine the planet’s vertical mixing regime. The TOI-199 system exhibits strong transit timing variations (TTVs) due to an outer non-transiting giant planet. For planet c, our TTV analysis reduces its mass uncertainty by 50% and prefers a slightly longer orbital period (still within the conservative habitable zone) and higher eccentricity relative to previous studies.
TOI-199 b serves as the first data point for studying clouds and hazes in temperate gas giants. The detection of methane supports the emerging trend that temperate low-molecular-weight atmospheres display spectral features in transmission.
Aaron Bello-Arufe, Renyu Hu, Mantas Zilinskas, Jeehyun Yang, Armen Tokadjian, Luis Welbanks, Guangwei Fu, Michael Greklek-McKeon, Mario Damiano, Jonathan Gomez Barrientos, Heather A. Knutson, David K. Sing, Xi Zhang
Comments: 23 pages, 16 figures, submitted to AAS Journals
Subjects: Earth and Planetary Astrophysics (astro-ph.EP)
Cite as: arXiv:2511.15835 [astro-ph.EP] (or arXiv:2511.15835v1 [astro-ph.EP] for this version)
https://doi.org/10.48550/arXiv.2511.15835
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Submission history
From: Aaron Bello-Arufe
[v1] Wed, 19 Nov 2025 19:44:30 UTC (12,859 KB)
https://arxiv.org/abs/2511.15835
Astrobiology,
