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

TL;DR

This study analyzes the atmosphere of the eccentric exo-Saturn WASP-117b, revealing high metallicity, absence of methane, and potential sodium and potassium features, using combined Hubble and VLT observations.

Contribution

It provides the first robust detection of water and evidence for high metallicity and specific atmospheric features in WASP-117b, advancing understanding of exo-Saturn atmospheres.

Findings

Detection of water spectral feature at 3σ confidence.

Preference for high atmospheric metallicity [Fe/H]=2.58.

Indications of sodium and potassium in the atmosphere.

Abstract

We investigate the atmospheric composition of the long period ($P_{\rm orb}=$ 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 ($3\sigma$) 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 ${\rm [Fe/H]}=2.58^{+0.26}_{-0.37}$ and clear skies. The data are also consistent with a lower-metallicity composition ${\rm [Fe/H]}<1.75$ 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 $1 \sigma$ and $<2\cdot 10^{-1}$ volume fraction within $3 \sigma$. 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 $K_{zz}\geq 10^8$~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.