2021A&A...646A.168C

Aims. We investigate the atmospheric composition of the long-period (P_orb=10 days) eccentric exo-Saturn WASP-117b. WASP-117b could be similar in atmospheric temperature and chemistry to WASP-107b. In mass and radius, WASP-117b is similar to WASP-39b, which allows a comparative study of these planets.
Methods. We analyzed a near-infrared transmission spectrum of WASP-117b taken with the Hubble Space Telescope (HST) WFC3 G141, which was reduced with two independent pipelines. High-resolution measurements were taken with VLT/ESPRESSO in the optical.
Results. We report the robust (3σ) detection of a water spectral feature. In a 1D atmosphere model with isothermal temperature, uniform cloud deck, and equilibrium chemistry, the Bayesian evidence of a retrieval analysis of the transmission spectrum indicates a preference for a high atmospheric metallicity [Fe/H]=2.58_–0.37^+0.26 and clear skies. The data are also consistent with a lower metallicity composition [Fe/H]<1.75 and a cloud deck between 10^–2.2 and 10^–5.1 bar, but with weaker Bayesian preference. We retrieve a low CH₄ abundance of <10^–4 volume fraction within 1σ and <2x10^–1 volume fraction within 3σ. We cannot constrain the equilibrium temperature between theoretically imposed limits of 700 and 1000K. Further observations are needed to confirm quenching of CH₄ with K_zz≤10⁸cm²/s. We report indications of Na and K in the VLT/ESPRESSO high-resolution spectrum with substantial Bayesian evidence in combination with HST data.