Methane on the temperate exo-Saturn TOI-199b
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, and Xi Zhang

TL;DR
This study presents the first transmission spectrum of the temperate exo-Saturn TOI-199b, revealing methane presence and providing insights into its atmospheric composition, clouds, and potential for future atmospheric characterization.
Contribution
It provides the first atmospheric characterization of a temperate gas giant, detecting methane and analyzing clouds, with implications for understanding low-temperature exoplanet atmospheres.
Findings
Methane detected with high confidence in TOI-199b's atmosphere.
Cloud and haze models show weak preference, indicating complex atmospheric conditions.
Transit depth increase near 3 μm suggests NH₃ or HCN presence.
Abstract
Temperate ( 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 ( 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 CH (Bayes factor of 700 in a cloudy atmosphere), corresponding to a metallicity of solar, although the absence of detectable CO and CO disfavors metallicities 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 $\sim…
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Taxonomy
TopicsAstrophysics and Star Formation Studies · Stellar, planetary, and galactic studies · Astronomy and Astrophysical Research
