WASP-117 b: an eccentric hot-Saturn as a future complex chemistry laboratory

TL;DR

This study analyzes the atmosphere of the eccentric hot-Saturn exoplanet WASP-117b using Hubble data, detecting water vapor and clouds, and discusses prospects for future observations with Ariel and JWST.

Contribution

The paper presents the first spectral analysis of WASP-117b, detecting water vapor and clouds, and explores how future telescopes can better characterize its atmospheric chemistry.

Findings

Detection of water vapor with positive but not strong statistical significance.

Presence of a fully opaque cloud layer in the atmosphere.

Future telescopes like JWST and Ariel can reveal more complex chemistry.

Abstract

We present spectral analysis of the transiting Saturn-mass planet WASP-117b, observed with the G141 grism of Wide Field Camera 3 (WFC3) on the Hubble Space Telescope. We reduce and fit the extracted spectrum from the raw transmission data using the open-source software Iraclis before performing a fully Bayesian retrieval using the publicly available analysis suite TauREx 3.0. We detect water vapour alongside a layer of fully opaque cloud, retrieving a terminator temperature of 833 K. In order to quantify the statistical significance of this detection, we employ the Atmospheric Detectability Index (ADI), deriving a value of 2.30, which provides positive but not strong evidence against the flatline model. Due to the eccentric orbit of WASP-117b, it is likely that chemical and mixing timescales oscillate throughout orbit due to the changing temperature, possibly allowing warmer chemistry to remain visible as the planet begins transit, despite the proximity of its point of ingress to apastron. We present simulated spectra of the planet as would be observed by the future space missions Ariel and JWST and show that, despite not being able to probe such chemistry with current HST data, these observatories should make it possible in the not too distant future.