They say you can't judge a book by its cover. But what about planets?

Take Neptune for example. For many years, especially since 1989 when Voyager 2 flew past Neptune and measured its gravity field, astronomers have known that the blue giant harbors a secret world inside. Hidden deep below the azure cloud tops lies a rocky core not much larger than Earth. Uranus has one, too! These "worlds within worlds" could have exotic properties including scorching hot oceans and diamond rain.

If only researchers could peel back the clouds for a closer look....

About 30 light years away, a Neptune-sized planetis having some of its layers peeled back.

Astronomers using NASA's Hubble Space Telescope have discovered an immense cloud of hydrogen evaporating from a Neptune-sized planet named GJ 436b.

"This cloud is spectacular," says the study's leader, David Ehrenreich of the Observatory of the University of Geneva in Switzerland. "The research team has nicknamed it 'The Behemoth.'"

The planet's atmosphere is evaporating because of extreme irradiation from its parent star-a process that might have been even more intense in the past.

"The parent star, which is a faint red dwarf, was once more active," says Ehrenreich. "This means that the planet's atmosphere evaporated faster during its first billion years of existence. Overall, we estimate that the planet may have lost up to 10 percent of its atmosphere."

GJ 436b is considered to be a "Warm Neptune" because of its size and because it is much closer to its parent star than Neptune is to our own sun. Orbiting at a distance of less than 3 million miles, It whips around the central red dwarf in just 2.6 Earth days. For comparison, the Earth is 93 million miles from the sun and orbits it every 365.24 days.

Systems like GJ 436b could explain the existence of so-called "Hot Super-Earths."

"Hot Super-Earths" are larger, hotter versions of our own planet. Space telescopes such as NASA's Kepler and the French led CoRoT have discovered hundredsof them orbiting distant stars. The existence of The Behemoth suggests that Hot Super-Earths could be the remnants of Warm Neptunes that completely lost their gaseous atmospheres to evaporation.

Finding a cloud around GJ 436b required Hubble's ultraviolet vision. Earth's atmosphere blocks most ultraviolet light so only a space telescope like Hubble could make the crucial observations.

"You would not see The Behemoth in visible wavelengths because it is optically transparent," says Ehrenreich. On the other hand, it is opaque to UV rays. "So 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."

The ultraviolet technique could be a game-changer in exoplanet studies, he adds. Ehrenreich expects that astronomers will find thousands of Warm Neptunes and Super-Earths in the years ahead. Astronomers will want to examine them for evidence of evaporation. Moreover, the ultraviolet technique might be able to spot the signature of oceans evaporating on Earth-like planets, shedding new light on worlds akin to our own.

Maybe you can't judge a book by its cover, but you can judge a planet by its Behemoth.