Reconnaissance of the TRAPPIST-1 exoplanet system in the Lyman-α line

¹ Observatoire de l’Université de Genève, 51 chemin des Maillettes, 1290 Sauverny, Switzerland
e-mail: vincent.bourrier@unige.ch
² Department of Physics, University of Warwick, Coventry CV4 7AL, UK
³ Laboratoire AIM Paris-Saclay, CEA/DRF − CNRS − Univ. Paris Diderot − IRFU/SAp, Centre de Saclay, 91191 Gif-sur-Yvette Cedex, France
⁴ Institut d’Astrophysique et de Géophysique, Université de Liège, Allée du 6 Août 19C, 4000 Liège, Belgium
⁵ Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139, USA
⁶ Center for Astrophysics and Space Science, University of California San Diego, La Jolla, CA 92093, USA
⁷ Cavendish Laboratory, J J Thomson Avenue, Cambridge, CB3 0HE, UK
⁸ Institute of Astronomy, Madingley Road, Cambridge CB3 0HA, UK

Received: 13 December 2016
Accepted: 4 January 2016

Abstract

The TRAPPIST-1 system offers the opportunity to characterize terrestrial, potentially habitable planets orbiting a nearby ultracool dwarf star. We performed a four-orbit reconnaissance with the Space Telescope Imaging Spectrograph onboard the Hubble Space Telescope to study the stellar emission at Lyman-α, to assess the presence of hydrogen exospheres around the two inner planets, and to determine their UV irradiation. We detect the Lyman-α line of TRAPPIST-1, making it the coldest exoplanet host star for which this line has been measured. We reconstruct the intrinsic line profile, showing that it lacks broad wings and is much fainter than expected from the stellar X-ray emission. TRAPPIST-1 has a similar X-ray emission as Proxima Cen but a much lower Ly-α emission. This suggests that TRAPPIST-1 chromosphere is only moderately active compared to its transition region and corona. We estimated the atmospheric mass loss rates for all planets, and found that despite a moderate extreme UV emission the total XUV irradiation could be strong enough to strip the atmospheres of the inner planets in a few billions years. We detect marginal flux decreases at the times of TRAPPIST-1b and c transits, which might originate from stellar activity, but could also hint at the presence of extended hydrogen exospheres. Understanding the origin of these Lyman-α variations will be crucial in assessing the atmospheric stability and potential habitability of the TRAPPIST-1 planets.