Hubble sees atmosphere being stripped from Neptune-sized exoplanet [heic1515]
24 June 2015Astronomers using the NASA/ESA Hubble Space Telescope have discovered an immense cloud of hydrogen dispersing from a warm, Neptune-sized planet orbiting a nearby star. The enormous gaseous tail of the planet is about 50 times the size of the parent star. The findings will be published in the 25 June issue of the journal Nature.
A phenomenon this large has never before been seen around such a small exoplanet . It may offer clues as to how hot super-Earths – massive, hot versions of Earth – are born around other stars.
"This cloud of hydrogen is very spectacular!" says David Ehrenreich of the Observatory of the University of Geneva in Switzerland, lead author of the study. "Although the evaporation rate doesn't threaten the planet right now, we know that the star, a faint red dwarf, was more active in the past. This means that the planet's atmosphere evaporated faster during its first billion years of existence. Overall, we estimate that it may have lost up to 10 percent of its atmosphere."
The planet, named Gliese 436b, is considered to be a "warm Neptune", because it is similar in size to Neptune, but much closer to its star Gliese 436 than Neptune is to the Sun. Although in this case the planet is in no danger of having its atmosphere completely stripped away – leaving just a solid, rocky core – this behaviour could explain the existence of hot super-Earths, which orbit very close to their stars and are typically more massive than Earth, although smaller than the seventeen Earth masses of Neptune.
Hot super-Earths could be the remaining cores of more massive planets that have completely lost their thick, gaseous atmospheres to the same type of evaporation that Hubble observed around Gliese 436b.
As the Earth's atmosphere blocks most ultraviolet light, astronomers needed a space telescope with Hubble's ultraviolet capability and exquisite precision to view the cloud. "You wouldn't be able to see it at visible wavelengths," says Ehrenreich. "But when you turn the ultraviolet eye of Hubble onto the system, it's really quite a transformation – the planet turns into a monstrous thing."
Ehrenreich and his team suggest that such a huge cloud of gas can exist around this planet because the cloud is not rapidly heated and swept away by the radiation from the relatively cool red dwarf star. This allows the cloud to stick around for a longer time.
Evaporation such as this may also have happened in the earlier history of the Solar System, when the Earth had a hydrogen-rich atmosphere that dissipated. It is also possible that it could happen to Earth's atmosphere at the end of our planet's life, when the Sun swells up to become a red giant and boils off our remaining atmosphere, before engulfing our planet completely.
Gliese 436b resides very close to Gliese 436 – just about 4 million kilometres away – and whips around it in just 2.6 Earth days . At the very youngest, this exoplanet is at least 6 billion years old, but astronomers suspect that it is somewhat older. About the size of Neptune, it has a mass of around 23 Earths. At just 30 light-years from Earth, it is one of the closest known exoplanets.
"Finding the cloud around Gliese 436b could be a game-changer for characterising atmospheres of the whole population of Neptunes and Super-Earths in ultraviolet observations ," explains Vincent Bourrier, also of the Observatory of the University of Geneva in Switzerland and co-author of the study. In the coming years, Bourrier expects that astronomers will find thousands of this kind of planet.
 Hubble observed this feature before around more massive exoplanets. The first detection of an evaporating extended atmosphere was around HD 209458b in 2003 (heic0303, heic0403). Heavy elements escaping the hot gas giant WASP-12b were studied in 2010. The lead author of the current paper performed a study on the evaporating atmosphere of the warm gas giant 55 Cancri b in 2012. In 2012 Hubble also observed a powerful burst of evaporation from the planet HD 189733b (heic1209). All observations for these discoveries were performed in the ultraviolet.
 For comparison, the Earth lies just under 150 million kilometres from the Sun and orbits it every 365.24 days, while Mercury, the innermost planet orbits the Sun every 88 days within an average distance to it of only 58 million kilometres.
 The ultraviolet technique may also spot the signatures of oceans evaporating on smaller, more Earth-like planets. It will be extremely challenging for astronomers to directly see water vapour on these worlds, because the vapour would be too low in the atmosphere (and thus shielded from telescopes). However, when stellar radiation breaks water molecules up into hydrogen and oxygen, the relatively light hydrogen atoms can escape the planet. If scientists could spot this hydrogen evaporating from a planet that is a bit more temperate and little less massive than Gliese 436b, it is a good indicator that an ocean may be present on the surface.
Notes for editors
The Hubble Space Telescope is a project of international cooperation between ESA and NASA.
The study, entitled "A giant comet-like cloud of hydrogen escaping the warm Neptune-mass exoplanet GJ 436b", will be published in the 24 June issue of the journal Nature.
The international team of astronomers in this study consists of D. Ehrenreich (The Geneva Observatory, Switzerland), V. Bourrier (The Geneva Observatory, Switzerland), P. J. Wheatley (University of Warwick, United Kingdom), A. Lecavelier des Etangs (Institut d'astrophysique de Paris, CNRS; UPMC Univ. Paris 6, France), G. Hébrard (Institut d'astrophysique de Paris, CNRS; UPMC Univ. Paris 6; Observatoire de Haute-Provence, CNRS & OAMP, France), S. Udry (The Geneva Observatory, Switzerland), X. Bonfils (Université Grenoble Alpes; CNRS, Grenoble), X. Delfosse (Université Grenoble Alpes; CNRS, Grenoble), J-M. Désert (University of Colorado, USA), D. K. Sing (University of Exeter, United Kingdom), and A. Vidal-Madjar (Institut d'astrophysique de Paris, CNRS; UPMC Univ. Paris 6, France).
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