A planet circling a nearby star is losing its hydrogen atmosphere, which is bleeding off into space in a cloud so huge, astronomers have dubbed it "The Behemoth."

The hot, Neptune-sized exoplanet is having its atmosphere evaporated away by the extreme levels of radiation from its host star, researchers say.

As it boils away, it is forming a comet-like tail 50 times the size of the star, say astronomers who used the NASA/ESA Hubble Space Telescope to observe the dramatic phenomenon.

"This cloud is very spectacular, though the evaporation rate does not threaten the planet right now," explains David Ehrenreich from the University of Geneva in Switzerland, who led the study. "But we know that in the past, the star, which is a faint red dwarf, was more active."

Strong radiation from the star when it was younger probably had the planet's atmosphere evaporating even faster for the first billion years of life, and it could have been stripped of as much as 10 percent of its surrounding atmosphere during the several billion years of its existence, the researchers say in their study published in Nature.

Never before observed around an exoplanet as small as the world known as Gliese 436b, the phenomenon could provide clues to how some planets with hydrogen atmospheres could have their outer layers boiled off by their parent star, leaving behind solid, rocky cores — a class of planets about the size of Earth known as Hot-Super Earths.

Such planets have been observed by NASA's Kepler space telescope and have long been believed to be formerly massive planets completely stripped of thick gas atmospheres through some kind of evaporation process.

Because the ultraviolet light that would reveal the presence of the "The Behemoth" is mostly blocked by Earth's atmosphere, the astronomers turned to the Hubble telescope and its ultraviolet "eye" to study it.

"You would have to have Hubble's eyes," Ehrenreich says. "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 — the planet turns into a monstrous thing."

In the early history of our Solar System, when our Earth possessed an atmosphere rich in hydrogen, a similar evaporation phenomenon may have dissipated much of it, the astronomers suggest.

Gliese 436b orbits very closely around its parent star, Gliese 436, completing an orbit every 2.6 Earth days.

Believed to be around 6 billion years old, it sits only 30 light-years away from Earth, making it one of the nearest known exoplanets.