Free Astronomy Magazine September-October 2015

47 SEPTEMBER-OCTOBER 2015 SPACE CHRONICLES 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 Nep- tune, 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 sev- enteen Earth masses of Neptune. Hot super-Earths could be the re- maining cores of more massive pla- nets that have completely lost their thick, gaseous atmospheres to the same type of evaporation that Hub- ble 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 Eh- renreich. "But when you turn the ultraviolet eye of Hubble onto the system, it's really quite a transfor- mation — 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. T his artist's concept shows "The Behemoth," an enormous comet-like cloud of hydrogen bleeding off of a warm, Neptune-sized planet just 30 light-years from Earth. Also depicted is the parent star, which is a faint red dwarf named GJ 436. The hydrogen is evaporating from the planet due to extreme radiation from the star. A phenomenon this large has never before been seen around any exoplanet. [NASA, ESA, and G. Bacon (STScI)]