A gigantic cloud of hydrogen evaporating from a planet orbiting a nearby star has been discovered with the Hubble Space Telescope by astronomers.

Dubbed “The Behemoth”, the massive, comet-like object is around 50 times the size of it’s parent star. The hydrogen is venting from a warm, Neptune-sized planet, because extreme radiation from the star.

Study leader, David Ehrenreich of the Observatory of the University of Geneva in Switzerland, explains:

“This cloud is very spectacular, though the evaporation rate does not threaten the planet right now. 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 because of the strong radiation from the young star. Overall, we estimate that it may have lost up to 10 percent of its atmosphere over the past several billion years."

Due to its size and because it is much closer to its star than Neptune is to our sun, the planet, named GJ 436b, is considered to be a “Warm Neptune.”. Even though it is in no danger of having its atmosphere totally evaporated and stripped down to a rocky core, this planet could explain the existence of so-called Hot Super-Earths that are very close to their stars.

Such a phenomenon has never been observed around an exoplanet so small.

It may give clues to how other planets with hydrogen-enveloped atmospheres could have their outer layers evaporated by their parent star, leaving behind solid, rocky cores. Hot, rocky planets such as these that roughly the size of Earth are known as Hot-Super Earths.

Hot Super-Earths could be the remnants of more massive planets that completely lost their thick, gaseous atmospheres to the same type of evaporation. These hot, rocky worlds were revealed by the Convection Rotation and Planetary Transits (CoRoT) and NASA’s Kepler space telescope.

Sice Earth’s atmosphere blocks most ultraviolet light, astronomers needed a space telescope with Hubble’s ultraviolet capability and exquisite precision to find the Behemoth.

“You would have to have Hubble’s eyes,” says Ehrenreich. “You would not see it in visible wavelengths. But when you turn the ultraviolet eye of Hubble onto the system, it’s really kind of a transformation, because the planet turns into a monstrous thing."

Because the planet’s orbit is tilted nearly edge-on to our view from Earth, the planet can be seen passing in front of its star. Astronomers also saw the star eclipsed by “The Behemoth” hydrogen cloud around the planet.

David Ehrenreich, Vincent Bourrier, Peter J. Wheatley, Alain Lecavelier des Etangs, Guillaume Hébrard, Stéphane Udry, Xavier Bonfils, Xavier Delfosse, Jean-Michel Désert, David K. Sing & Alfred Vidal-Madjar A giant comet-like cloud of hydrogen escaping the warm Neptune-mass exoplanet GJ 436b Nature 522, 459–461 (25 June 2015) doi:10.1038/nature14501

_Illustration: 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. Credit: NASA, ESA, and G. Bacon (STScI) _

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