2022ApJ...940L..35F

JWST is here. The early release observation (ERO) program provides us with the first look at the scientific data and the spectral capabilities. One of the targets from the ERO is HAT-P-18b, an inflated Saturn-mass planet with an equilibrium temperature of ∼850 K. We present the NIRISS/SOSS transmission spectrum of HAT-P-18b from 0.6 to 2.8 μm and reveal the planet in the infrared beyond 1.6 μm for the first time. From the spectrum, we see clear water and escaping helium tail features in an otherwise very hazy atmosphere. Our free chemistry retrievals with ATMO show moderate Bayesian evidence (3.79) supporting the presence of methane, but the spectrum does not display any clearly identifiable methane absorption features. The retrieved methane abundance is ∼2 orders of magnitude lower than that of solar composition. The methane-depleted atmosphere strongly rejects simple equilibrium chemistry forward models with solar metallicity and a C/O ratio and disfavors high metallicity (100 times) and a low C/O ratio (0.3). This calls for additional physical processes such as vertical mixing and photochemistry, which can remove methane from the atmosphere.