An alien planet circling a star 33 light-years away is mesmerising scientists with its tail.

The atmosphere of the Neptune-sized planet, named Gliese 436B, is evaporating. As Gliese sloughs off its hydrogen atmosphere into space as it orbits close to its sun, it spins out a tail that resembles a comet, explains Mashable.

The tail is so massive that it obscures some 50 percent of the planet's star when seen in ultra-violet light, according to University of Geneva astronomer David Ehrenreich, a co-author of a study of the phenomenon published in Nature.

The planet's atmosphere is evaporating because it's so close to its star, yet the radiation of the "Red Dwarf" star (meaning it's smaller and weaker than the Sun) isn't powerful enough to totally clear off the trailing hydrogen tail.

The planet orbits its star so closely — 33 times closer than the Earth orbits the Sun — that a year on Gliese 436B is only 2.5 days long.

"Gliese 436B is the first 'warm Neptune' detected in transit" in front of its star, Ehrenreich told Mashable. "Transiting planets are extremely precious because it is possible to study their atmosphere, which filters the star light when the planet is in front of it."

Scientists suspect that the planet is representative of the origin of a different class of planet, known as a super-Earth, Ehrenreich said. Super-Earths are larger than our Earth but still rocky.

"These solid small planets burnt by their stars may be remnants of an initially more massive planet that have lost their gas envelope to space, as a result of the tremendous stellar irradiation they receive," Ehrenreich explained.

NASA's Kepler telescope detected a potentially evaporating planet three years ago, and the Hubble found another — about the size of Jupiter — orbiting a star more than 100 light-years away.

The astronomers used the Hubble Space Telescope to detect the shadow of Gliese 436B's hydrogen cloud when it passed in front of its star. The observation couldn't have been made from Earth because Earth's atmosphere blocks most ultraviolet light. Astronomers needed a space telescope with Hubble's ultraviolet capability.

"You wouldn't be able to see it at visible wavelengths. But when you turn the ultraviolet eye of Hubble onto the system, it's really quite a transformation — the planet turns into a monstrous thing," said Ehrenreich.