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.
Access optionsAccess options
Subscribe to Journal
Get full journal access for 1 year
only $3.90 per issue
All prices are NET prices.
VAT will be added later in the checkout.
Rent or Buy article
Get time limited or full article access on ReadCube.
All prices are NET prices.
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.
About this article
Astrophysics and Space Science (2018)