Methane on the temperate exo-Saturn TOI-199b

Abstract

Temperate ($T_{\mathrm{eq}}<400$ K) gas giants represent an unexplored frontier in exoplanet atmospheric spectroscopy. Orbiting a G-type star every $\sim 100$ days, the Saturn-mass exoplanet TOI-199 b ($T_{\mathrm{eq}}=350$ 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, the spectrum reveals the presence of CH${}_4$ (Bayes factor of $\sim$700 in a cloudy atmosphere), corresponding to a metallicity of $C/H=13_{-12}^{+78}\times$ solar, although the absence of detectable CO and CO${}_2$ at the current precision disfavors metallicities $\gtrsim 50\times$ 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 $\mu$m, which our self-consistent models attribute to either NH${}_3$ or, less likely, HCN. Follow-up observations could 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 (but 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, with the detection of spectral features in its transmission spectrum indicating that temperate gas giants are promising targets for detailed atmospheric characterization.

22 pages, 13 figures, published in AJ

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