Letter | Published:

A giant comet-like cloud of hydrogen escaping the warm Neptune-mass exoplanet GJ 436b

Nature volume 522, pages 459461 (25 June 2015) | Download Citation



Exoplanets orbiting close to their parent stars may lose some fraction of their atmospheres because of the extreme irradiation1,2,3,4,5,6. Atmospheric mass loss primarily affects low-mass exoplanets, leading to the suggestion that hot rocky planets7,8,9 might have begun as Neptune-like10,11,12,13,14,15,16, 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 spectrum17. 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 claim6 (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 108–109 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.

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This work is based on observations made with the NASA/ESA Hubble Space Telescope, obtained at the Space Telescope Science Institute, which is operated by the Association of Universities for Research in Astronomy, Inc., under NASA contract NAS 5-26555. These observations are associated with programmes #11817, #12034 and #12965. The scientific results reported in this article are based on observations made by the Chandra X-ray Observatory. This work was carried out in the framework of the National Centre for Competence in Research ‘PlanetS’ supported by the Swiss National Science Foundation (SNSF). D.E., V.B. and S.U. acknowledge the financial support of the SNSF. V.B., A.L.d.E., X.B. and X.D. acknowledge the support of CNES, the French Agence Nationale de la Recherche (ANR) under program ANR-12-BS05-0012 ‘Exo-Atmos’, the Fondation Simone et Cino Del Duca, and the European Research Council (ERC) under ERC Grant Agreement no. 337591-ExTrA.

Author information


  1. Observatoire de l’Université de Genève, 51 chemin des Maillettes, 1290 Versoix, Switzerland

    • David Ehrenreich
    • , Vincent Bourrier
    •  & Stéphane Udry
  2. Department of Physics, University of Warwick, Coventry CV4 7AL, UK

    • Peter J. Wheatley
  3. CNRS, UMR 7095, Institut d’Astrophysique de Paris, 98 bis boulevard Arago, 75014 Paris, France

    • Alain Lecavelier des Etangs
    • , Guillaume Hébrard
    •  & Alfred Vidal-Madjar
  4. Sorbonnes Universités, UPMC Univ. Paris 6, UMR 7095, Institut d’Astrophysique de Paris, 98 bis boulevard Arago, 75014 Paris, France

    • Alain Lecavelier des Etangs
    • , Guillaume Hébrard
    •  & Alfred Vidal-Madjar
  5. Observatoire de Haute-Provence, CNRS & OAMP, 04870 Saint-Michel-l’Observatoire, France

    • Guillaume Hébrard
  6. Univ. Grenoble Alpes, IPAG, F-38000 Grenoble, France

    • Xavier Bonfils
    •  & Xavier Delfosse
  7. CNRS, IPAG, F-38000 Grenoble, France

    • Xavier Bonfils
    •  & Xavier Delfosse
  8. CASA, Department of Astrophysical & Planetary Sciences, University of Colorado, 389-UCB, Boulder, Colorado 80309, USA

    • Jean-Michel Désert
  9. Astrophysics Group, School of Physics, University of Exeter, Stocker Road, Exeter EX4 4QL, UK

    • David K. Sing


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D.E. proposed and led the HST–Chandra joint observation programme, supervised data reduction and analysis, interpreted the results and wrote the paper. V.B. performed data reduction and analysis, and computer simulations to interpret the results. P.J.W. set up the Chandra X-ray observations, reduced, analysed and interpreted the X-ray data. A.L.d.E. co-designed the simulation programme with V.B. and provided computing resources to run the simulations. A.L.d.E. and G.H. contributed to the observation programme, data analysis and interpretation. S.U., X.B., X.D., J.-M.D., D.K.S. and A.V.-M. contributed to the observation programme and interpretation. All authors discussed the results and commented on the manuscript.

Competing interests

The authors declare no competing financial interests.

Corresponding author

Correspondence to David Ehrenreich.

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