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

Abstract

We investigate the atmospheric composition of the long period ($P_{\mathrm{orb}}=$ 10 days), eccentric exo-Saturn WASP-117b. WASP-117b could be in atmospheric temperature and chemistry similar to WASP-107b. In mass and radius WASP-117b is similar to WASP-39b, which allows a comparative study of these planets. We analyze a near-infrared transmission spectrum of WASP-117b taken with Hubble Space Telescope/WFC3 G141, which was reduced with two independent pipelines. High resolution measurements were taken with VLT/ESPRESSO in the optical. We report the robust ($3\ddot{I}ƒ$) detection of a water spectral feature. Using 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 $10^{-2.2}-10^{-5.1}$ bar, but with weaker Bayesian preference. We retrieve a low CH${}_4$ abundance of $<10^{-4}$ volume fraction within $1\ddot{I}ƒ$ and $<2\cdot 10^{-1}$ volume fraction within $3\ddot{I}ƒ$. We cannot constrain the equilibrium temperature between theoretically imposed limits of 700 and 1000~K. Further observations are needed to confirm quenching of CH${}_4$ with $K_{zz}\ge 10^8$~cm${}^2$/s. We report indications of Na and K in the VLT/ESPRESSO high resolution spectrum with substantial Bayesian evidence in combination with HST data.

Accepted by A & A, 23 pages, 21 figures, + appendix (now transmission data appended)