Indications for very high metallicity and absence of methane in the eccentric exo-Saturn WASP-117b

Abstract

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 <2 × 10^-1 volume fraction within 3σ. We cannot constrain the equilibrium temperature between theoretically imposed limits of 700 and 1000 K. Further observations are needed to confirm quenching of CH₄ with K_zz ≥ 10⁸ cm² s^-1. We report indications of Na and K in the VLT/ESPRESSO high-resolution spectrum with substantial Bayesian evidence in combination with HST data.