Astronomers say the search of water on planets outside our own solar system has reached a milestone with the discovery of the smallest exoplanet to show signs of atmospheric water.

While the planet, HAT P-11b, does possess water vapor in its atmosphere, the world itself is a superheated ball of gas around a small rocky core and its water would not support life in any form that we know of it, the scientists take pains to point out.

"We're talking about water at over 1,000 degrees, so this isn't a case for life on the planet," says Jonathan Fraine, astronomy doctoral candidate at the University of Maryland and lead author of the study. "This isn't a liquid ocean or breathable air. In fact, the planet is a gaseous object -- there's no surface that liquid water could sit on."

The planet is orbiting a star 124 light years from Earth in the constellation Cygnus.

The search for water remains at the center of the search for extraterrestrial life, something more easily done with large exoplanets the size of Jupiter but more difficult with planets closer to the Earth in size.

That's because the bigger planets have clear atmospheres easy to study, while Earth-size planets found to date have all exhibited too much cloud cover for telescopes to penetrate -- until HAT P-11b, only about four times the size of Earth, about the diameter of Neptune.

"We can look at those [other planets] and find some cool information, but not what we're looking for," Fraine says. "This is the first time we've seen a clear atmosphere on a Neptune-sized planet."

Finding out about exoplanet atmospheres and what they're comprised of will help in understanding how planets form, especially smaller ones like the Earth, he says.

"We want to figure out how unique or normal we are, as a planet," he explains.

Current theories about planet formation are based on what we know of our own solar system, but it is unclear if other planetary systems have had similar origins, which is why the study of HAT P-11b is significant since it appears to be consistent with current ideas on the formation of planets, the researchers say.

More planets like it would yield more data, they add, and toward that end upcoming research using the Hubble telescope will examine around 50 similar exoplanets.

"The great thing is that now we know that clouds aren't blocking all of the light on every single one of these planets," Fraine says.