title: A giant comet-like cloud of hydrogen escaping the warm Neptune-mass exoplanet GJ 436b.
abstract: Exoplanets orbiting close to their parent stars may lose some fraction of their atmospheres because of the extreme irradiation. Atmospheric mass loss primarily affects low-mass exoplanets, leading to the suggestion that hot rocky planets might have begun as Neptune-like, but subsequently lost all of their atmospheres; however, no confident measurements have hitherto been available. The signature of this loss could be observed in the ultraviolet spectrum, when the planet and its escaping atmosphere transit the star, giving rise to deeper and longer transit signatures than in the optical spectrum. Here we report that in the ultraviolet the Neptune-mass exoplanet GJ 436b (also known as Gliese 436b) has transit depths of 56.3 ± 3.5% (1σ), far beyond the 0.69% optical transit depth. The ultraviolet transits repeatedly start about two hours before, and end more than three hours after the approximately one hour optical transit, which is substantially different from one previous claim (based on an inaccurate ephemeris). We infer from this that the planet is surrounded and trailed by a large exospheric cloud composed mainly of hydrogen atoms. We estimate a mass-loss rate in the range of about 10(8)-10(9) grams per second, which is far too small to deplete the atmosphere of a Neptune-like planet in the lifetime of the parent star, but would have been much greater in the past.
authors.affiliation: Observatoire de l'Université de Genève, 51 chemin des Maillettes, 1290 Versoix, Switzerland.
authors.forename: David
authors.initials: D
authors.lastname: Ehrenreich
authors.affiliation: Observatoire de l'Université de Genève, 51 chemin des Maillettes, 1290 Versoix, Switzerland.
authors.forename: Vincent
authors.initials: V
authors.lastname: Bourrier
authors.affiliation: Department of Physics, University of Warwick, Coventry CV4 7AL, UK.
authors.forename: Peter J
authors.initials: PJ
authors.lastname: Wheatley
authors.affiliation: 1] CNRS, UMR 7095, Institut d'Astrophysique de Paris, 98 bis boulevard Arago, 75014 Paris, France [2] Sorbonnes Universités, UPMC Univ. Paris 6, UMR 7095, Institut d'Astrophysique de Paris, 98 bis boulevard Arago, 75014 Paris, France.
authors.forename: Alain Lecavelier
authors.initials: AL
authors.lastname: des Etangs
authors.affiliation: 1] CNRS, UMR 7095, Institut d'Astrophysique de Paris, 98 bis boulevard Arago, 75014 Paris, France [2] Sorbonnes Universités, UPMC Univ. Paris 6, UMR 7095, Institut d'Astrophysique de Paris, 98 bis boulevard Arago, 75014 Paris, France [3] Observatoire de Haute-Provence, CNRS &OAMP, 04870 Saint-Michel-l'Observatoire, France.
authors.forename: Guillaume
authors.initials: G
authors.lastname: Hébrard
authors.affiliation: Observatoire de l'Université de Genève, 51 chemin des Maillettes, 1290 Versoix, Switzerland.
authors.forename: Stéphane
authors.initials: S
authors.lastname: Udry
authors.affiliation: 1] Univ. Grenoble Alpes, IPAG, F-38000 Grenoble, France [2] CNRS, IPAG, F-38000 Grenoble, France.
authors.forename: Xavier
authors.initials: X
authors.lastname: Bonfils
authors.affiliation: 1] Univ. Grenoble Alpes, IPAG, F-38000 Grenoble, France [2] CNRS, IPAG, F-38000 Grenoble, France.
authors.forename: Xavier
authors.initials: X
authors.lastname: Delfosse
authors.affiliation: CASA, Department of Astrophysical &Planetary Sciences, University of Colorado, 389-UCB, Boulder, Colorado 80309, USA.
authors.forename: Jean-Michel
authors.initials: JM
authors.lastname: Désert
authors.affiliation: Astrophysics Group, School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, UK.
authors.forename: David K
authors.initials: DK
authors.lastname: Sing
authors.affiliation: 1] CNRS, UMR 7095, Institut d'Astrophysique de Paris, 98 bis boulevard Arago, 75014 Paris, France [2] Sorbonnes Universités, UPMC Univ. Paris 6, UMR 7095, Institut d'Astrophysique de Paris, 98 bis boulevard Arago, 75014 Paris, France.
authors.forename: Alfred
authors.initials: A
authors.lastname: Vidal-Madjar