The GAPS Programme at TNG

XXXII. The revealing non-detection of metastable He I in the atmosphere of the hot Jupiter WASP-80b^★

¹ Space Research Institute, Austrian Academy of Sciences, Schmiedlstrasse 6, 8042 Graz, Austria
e-mail: Luca.Fossati@oeaw.ac.at
² INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025, Pino Torinese, Italy
³ Observatoire Astronomique de l’Université de Genève, Chemin Pegasi 51b, 1290 Versoix, Switzerland
⁴ Institute of Laser Physics, SB RAS, Novosibirsk 630090, Russia
⁵ Institute of Astronomy, Russian Academy of Sciences, Moscow 119017, Russia
⁶ Novosibirsk State Technical University, Novosibirsk 630087, Russia
⁷ Astronomy Department, 96 Foss Hill Drive, Van Vleck Observatory 101, Wesleyan University, Middletown, CT, 06459, USA
⁸ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, 35122 Padova, Italy
⁹ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate (LC), Italy
¹⁰ INAF – Osservatorio Astrofisico di Arcetri, Largo E. Fermi 5, 50125 Firenze, Italy
¹¹ INAF – Osservatorio Astronomico di Palermo, P.zza Parlamento 1, 90134 Palermo, Italy
¹² Lomonosov Moscow State University, Skobeltsyn Institute of Nuclear Physics, 119992 Moscow, Russia
¹³ Department of Physics, University of Warwick, Gibbet Hill Road, Coventry, CV4 7AL, UK
¹⁴ Centre for Exoplanets and Habitability, University of Warwick, Gibbet Hill Road, Coventry, CV4 7AL, UK
¹⁵ INAF – Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123 Catania, Italy
¹⁶ Department of Physics, University of Rome Tor Vergata, Via della Ricerca Scientifica 1, 00133 Roma, Italy
¹⁷ Max Planck Institute for Astronomy, KÖnigstuhl 17, 69117 Heidelberg, Germany
¹⁸ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00040 Monte Porzio Catone (RM), Italy
¹⁹ INAF – Osservatorio Astronomico di Trieste, via Tiepolo 11, 34143 Trieste, Italy
²⁰ Fundación G. Galilei – INAF (Telescopio Nazionale Galileo), Rambla J. A. Fernàndez Pèrez 7, 38712 Breña Baja (La Palma), Spain
²¹ INAF – Osservatorio Astronomico di Capodimonte, Salita Moiariello 16, 80131 Naples, Italy
²² Dipartimento di Fisica e Astronomia Galileo Galilei, Università di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
²³ INAF – Osservatorio di Cagliari, via della Scienza 5, 09047 Selargius, CA, Italy

Received: 30 September 2021
Accepted: 20 December 2021

Abstract

Context. Because of its proximity to an active K-type star, the hot Jupiter WASP-80b has been identified as a possible excellent target for detecting and measuring He I absorption in the upper atmosphere.

Aims. Our aim was to look for, and eventually measure and model, metastable He I atmospheric absorption.

Methods. We observed four primary transits of WASP-80b in the optical and near-infrared using the HARPS-N and GIANO-B high-resolution spectrographs attached to the Telescopio Nazionale Galileo telescope, focusing the analysis on the He I triplet. We further employed a three-dimensional hydrodynamic aeronomy model to understand the observational results.

Results. We did not find any signature of planetary absorption at the position of the He I triplet with an upper limit of 0.7% (i.e. 1.11 planetary radii; 95% confidence level). We re-estimated the high-energy stellar emission, which we combined with a stellar photospheric model, to generate the input for the hydrodynamic modelling. We determined that, assuming a solar He to H abundance ratio, He I absorption should have been detected. Considering a stellar wind 25 times weaker than solar, we could reproduce the non-detection only by assuming a He to H abundance ratio about 16 times smaller than solar. Instead, considering a stellar wind ten times stronger than solar, we could reproduce the non-detection only with a He to H abundance ratio about ten times smaller than solar. We attempted to understand this result by collecting all past He I measurements and looking for correlations with high-energy stellar emission and planetary gravity, but without success.

Conclusions. WASP-80b is not the only planet with an estimated sub-solar He to H abundance ratio, which suggests the presence of efficient physical mechanisms (e.g. phase separation, magnetic fields) capable of significantly modifying the He to H content in the upper atmosphere of hot Jupiters. The planetary macroscopic properties and the shape of the stellar spectral energy distribution are not sufficient for predicting the presence or absence of detectable metastable He in a planetary atmosphere, since the He abundance also appears to play a major role.