HST PanCET program: non-detection of atmospheric escape in the warm Saturn-sized planet WASP-29 b

¹ Observatoire astronomique de l’Université de Genève, Chemin Pegasi 51b, 1290 Versoix, Switzerland
e-mail: leonardo.dossantos@unige.ch
² Centro de Astrobiología (CSIC-INTA), 28692 Villanueva de la Cañada, Madrid, Spain
³ Center for Astrophysics, Harvard & Smithsonian, 60 Garden Street, Cambridge, MA 02138, USA
⁴ Department of Earth & Planetary Sciences, Johns Hopkins University, Baltimore, MD, USA
⁵ Department of Physics & Astronomy, Johns Hopkins University, Baltimore, MD, USA
⁶ AIM, CEA, CNRS, Université Paris-Saclay, Université de Paris, 91191 Gif-sur-Yvette, France
⁷ Tennessee State University, Center of Excellence in Information Systems, Nashville, TN 37203, USA
⁸ Groupe de Spectrométrie Moléculaire et Atmosphérique, Université de Reims, Champagne-Ardenne, CNRS UMR 7331, 51687 Reims Cedex 2, France
⁹ Institut d’Astrophysique de Paris, CNRS, UMR 7095 & Sorbonne Universités UPMC Paris 6, 98 bis bd Arago, 75014 Paris, France
¹⁰ Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, Cambridge, MA, USA
¹¹ School of Physics, University of Bristol, HH Wills Physics Laboratory, Tyndall Avenue, Bristol BS8 1TL, UK

Received: 3 February 2021
Accepted: 29 March 2021

Abstract

Short-period gas giant exoplanets are susceptible to intense atmospheric escape due to their large scale heights and strong high-energy irradiation. This process is thought to occur ubiquitously, but to date we have only detected direct evidence of atmospheric escape in hot Jupiters and warm Neptunes. The latter planets are particularly more sensitive to escape-driven evolution as a result of their lower gravities with respect to Jupiter-sized planets. But the paucity of cases for intermediate, Saturn-sized exoplanets at varying levels of irradiation precludes a detailed understanding of the underlying physics in atmospheric escape of hot gas giants. Aiming to address this issue, our objectives here are to assess the high-energy environment of the warm (T_eq = 970 K) Saturn WASP-29 b and search for signatures of atmospheric escape. We used far-ultraviolet observations from the Hubble Space Telescope to analyze the flux time series of H I, C II, Si III, Si IV, and N V during the transit of WASP-29 b. At 88 pc, a large portion of the Lyman-α core of the K4V-type host WASP-29 is attenuated by interstellar medium absorption, limiting our ability to probe the escape of H at velocities between −84 and +35 km s⁻¹. At 3σ confidence, we rule out any in-transit absorption of H I larger than 92% in the Lyman-α blue wing and 19% in the red wing. We found an in-transit flux decrease of 39%_−11%^+12% in the ground-state C II emission line at 1334.5 Å. But due to this signal being significantly present in only one visit, it is difficult to attribute a planetary or stellar origin to the ground-state C II signal. We place 3σ absorption upper limits of 40, 49, and 24% on Si III, Si IV, and for excited-state C II at 1335.7 Å, respectively. Low activity levels and the faint X-ray luminosity suggest that WASP-29 is an old, inactive star. Nonetheless, an energy-limited approximation combined with the reconstructed EUV spectrum of the host suggests that the planet is losing its atmosphere at a relatively large rate of 4 × 10⁹ g s⁻¹. The non-detection at Lyman-α could be partly explained by a low fraction of escaping neutral hydrogen, or by the state of fast radiative blow-out we infer from the reconstructed Lyman-α line.