Large Hydrogen Cloud 'Bleeds' from Neptune-Sized Exoplanet

First Posted: Jun 25, 2015 07:42 AM EDT

Astronomers have discovered a nearby, Neptune-sized planet "bleeding" away its atmosphere. Researchers have uncovered an immense hydrogen cloud, dubbed "The Behemoth," that's being leeched off the planet by the radiation from a nearby star.

"This cloud is very spectacular, though the evaporation rate does not threaten the planet right now," said David Ehrenreich, one of the researchers, in a news release. "But we know that in the past, the star, which is a faint red dwarf, was more active. This means that the planet 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 itself is named GJ 436b, and is considered to be a "warm Neptune," because of its size. However, it's much closer to its star than Neptune is to our sun. While there is no danger of having its atmosphere completely evaporated, it could explain the existence of so-called hot super-Earths that are very close to their stars.

The planet's orbit is tilted nearly edge-on to our view from Earth. This means that the planet can be seen passing in front of its star. In this case, astronomers spotted the star being eclipsed by "The Behemoth" hydrogen cloud around the planet.

The researchers believe that the huge cloud of gas can exist around this planet because the cloud is not rapidly heated and swept away by the radiation pressure from the relatively cool red dwarf star. This allows the cloud to stick around for a longer time.

Finding "the Behemoth" could be a game-changer for characterizing atmospheres of the whole population of Neptune-sized planets and super-Earths in ultraviolet observations. In the coming years, the researchers suspect that other astronomers will find thousands of these types of planets.

The findings are published in the journal Nature.

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