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

¹ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
e-mail: carone@mpia.de
² Department of Astronomy/McDonald Observatory, The University of Texas, 2515 Speedway, Austin, TX, 78712, USA
³ Institut für Astrophysik, Georg-August-Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany
⁴ Instituut voor Sterrenkunde, KU Leuven, Celestijnenlaan 200D, 3001 Leuven, Belgium
⁵ Steward Observatory, The University of Arizona, 933 N. Cherry Avenue, Tucson, AZ 85721, USA
⁶ Lunar and Planetary Laboratory, The University of Arizona, 1629 E. Univ. Blvd., Tucson, AZ 85721, USA
⁷ Space Telescope Science Institute, 3700 San Martin Drive, Baltimore, MD, USA
⁸ Department of Earth, Atmospheric and Planetary Sciences, and Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA 02139, USA
⁹ Facultad de Ingeniería y Ciencias, Universidad Adolfo Ibáñez, Av. Diagonal las Torres 2640, Peñalolén, Santiago, Chile
¹⁰ Millennium Institute for Astrophysics, Chile
¹¹ ETH Zürich, Inst. f. Teilchen- und Astrophysik, Zürich, Switzerland
¹² Center for Space and Habitability, University of Bern, Bern, Switzerland
¹³ Blue Marble Space Institute of Science, Seattle, USA
¹⁴ Observatoire de Genève, chemin des maillettes 51, 1290 Sauverny, Switzerland
¹⁵ Laboratoire Interuniversitaire des Systèmes Atmosphériques (LISA), UMR CNRS 7583, Université Paris-Est-Créteil, Université de Paris, Institut Pierre Simon Laplace, Créteil, France

Received: 9 June 2020
Accepted: 2 December 2020

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.