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Exoplanets orbiting close to their parent stars could lose some fraction oftheir atmospheres because of the extreme irradiation. Atmospheric mass lossprimarily affects low-mass exoplanets, leading to suggest that hot rockyplanets might have begun as Neptune-like, but subsequently lost all of theiratmospheres; however, no confident measurements have hitherto been available.The signature of this loss could be observed in the ultraviolet spectrum, whenthe planet and its escaping atmosphere transit the star, giving rise to deeperand longer transit signatures than in the optical spectrum. Here we report thatin the ultraviolet the Neptune-mass exoplanet GJ 436b (also known as Gliese436b) has transit depths of 56.3 +/- 3.5% (1 sigma), far beyond the 0.69%optical transit depth. The ultraviolet transits repeatedly start ~2 h before,and end >3 h after the ~1 h optical transit, which is substantially differentfrom one previous claim (based on an inaccurate ephemeris). We infer from thisthat the planet is surrounded and trailed by a large exospheric cloud composedmainly of hydrogen atoms. We estimate a mass-loss rate in the range of~10^8-10^9 g/s, which today is far too small to deplete the atmosphere of aNeptune-like planet in the lifetime of the parent star, but would have beenmuch greater in the past.

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