Methane on the temperate exo-Saturn TOI-199b
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| Main Authors: | , , , , , , , , , , , , |
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| Format: | Preprint |
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2025
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| _version_ | 1866911276802768896 |
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| author | Bello-Arufe, Aaron Hu, Renyu Zilinskas, Mantas Yang, Jeehyun Tokadjian, Armen Welbanks, Luis Fu, Guangwei Greklek-McKeon, Michael Damiano, Mario Barrientos, Jonathan Gomez Knutson, Heather A. Sing, David K. Zhang, Xi |
| author_facet | Bello-Arufe, Aaron Hu, Renyu Zilinskas, Mantas Yang, Jeehyun Tokadjian, Armen Welbanks, Luis Fu, Guangwei Greklek-McKeon, Michael Damiano, Mario Barrientos, Jonathan Gomez Knutson, Heather A. Sing, David K. Zhang, Xi |
| contents | Temperate ($T_{\rm eq}<400$ 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 ($T_{\rm eq}=350$ 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 CH$_4$ (Bayes factor of $\sim$700 in a cloudy atmosphere), corresponding to a metallicity of $\rm{C/H}=13^{+78}_{-12}\times$ solar, although the absence of detectable CO and CO$_2$ disfavors metallicities $\gtrsim50\times$ 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 $\sim 2$ relative to the clear case). The spectrum also shows an increase in transit depth near 3 $μ$m, which our self-consistent models attribute to either NH$_3$ 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. |
| format | Preprint |
| id |
arxiv_https___arxiv_org_abs_2511_15835 |
| institution | arXiv |
| publishDate | 2025 |
| record_format | arxiv |
| spellingShingle | Methane on the temperate exo-Saturn TOI-199b Bello-Arufe, Aaron Hu, Renyu Zilinskas, Mantas Yang, Jeehyun Tokadjian, Armen Welbanks, Luis Fu, Guangwei Greklek-McKeon, Michael Damiano, Mario Barrientos, Jonathan Gomez Knutson, Heather A. Sing, David K. Zhang, Xi Earth and Planetary Astrophysics Temperate ($T_{\rm eq}<400$ 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 ($T_{\rm eq}=350$ 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 CH$_4$ (Bayes factor of $\sim$700 in a cloudy atmosphere), corresponding to a metallicity of $\rm{C/H}=13^{+78}_{-12}\times$ solar, although the absence of detectable CO and CO$_2$ disfavors metallicities $\gtrsim50\times$ 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 $\sim 2$ relative to the clear case). The spectrum also shows an increase in transit depth near 3 $μ$m, which our self-consistent models attribute to either NH$_3$ 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. |
| title | Methane on the temperate exo-Saturn TOI-199b |
| topic | Earth and Planetary Astrophysics |
| url | https://arxiv.org/abs/2511.15835 |