Free Astronomy Magazine September-October 2015

SPACE CHRONICLES 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 Sys- tem, when the Earth had a hydro- gen-rich atmosphere that dissipated. It is also possible that it could hap- pen 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 atmo- sphere, 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 bil- lion years old, but astrono- mers 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 observa- tions," explains Vincent Bourrier, also of the Observatory of the Uni- versity 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. The ultraviolet technique may also spot the signatures of oceans evap- orating on smaller, more Earth-like planets. It will be extremely chal- lenging 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 rela- tively light hydrogen atoms can escape the planet. If scientists could spot this hydrogen evapo- rating 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. T his artist's diagram shows a polar view of the GJ 436 system. The warm, Neptune-sized exo- planet GJ 436b resides very close to its star — less than 3 million miles — and whips around it in just 2.6 Earth days. A huge, comet-like cloud of hydrogen nicknamed "The Behemoth" is shown bleeding off of the planet and trailing it like the tail of a comet. The planet is just 30 light-years from Earth. [NASA, ESA, and A. Feild (STScI)] T he unusual light curve produced when the exoplanet GJ 436b and the huge, comet-like hydrogen cloud nicknamed "The Behemoth" pass in front of the parent star. Because the planet's orbit is tilted nearly edge-on to our view from Earth, the planet and cloud can be seen eclipsing its star. Astronomers see the extended dip in the light caused by the enormous cloud. That dip trails off slowly due to the cloud's comet-like tail. [NASA, ESA, and A. Feild (STScI)] n