Astronomers have found clear skies and steamy water vapor on a small, gaseous planet outside our solar system, using three of the world's most prominent space telescopes: Hubble, Spitzer and Kepler.

The planet, HAT-P-11b, is categorized as an exo-Neptune -- or a Neptune-sized planet in another system -- that orbits the star HAT-P-11, which is located an estimated 120 light-years away in the constellation Cygnus.

The newly-announced find is the smallest planet from which molecules of any kind have been detected.

"This discovery is a significant milepost on the road to eventually analyzing the atmospheric composition of smaller, rocky planets more like Earth," John Grunsfeld, assistant administrator of the National Aeronautics and Space Administration's Science Mission Directorate, said in a news release. "Such achievements are only possible today with the combined capabilities of these unique and powerful observatories."

Often, clouds in a planet's atmosphere can block the detection of underlying molecules that can provide vital information about a planet's composition and history.

"When astronomers go observing at night with telescopes, they say 'clear skies' to mean good luck," said Jonathan Fraine of the University of Maryland, College Park, lead author of a new study on the exo-Neptune appearing in the scientific journal Nature.

"In this case, we found clear skies on a distant planet," he said. "That's lucky for us because it means clouds didn't block our view of water molecules."

The new research says HAT-P-11b, which competes a full orbit of its star every five days, is believed to be a warm world with a rocky core and gas atmosphere.

One of the biggest challenges in analyzing the atmospheres of planets like this is their size, said a NASA statement. "Larger Jupiter-like planets are easier to see because of their impressive girth and relatively inflated atmospheres. In fact, researchers already have detected water vapor in the atmospheres of those planets. The handful of smaller planets observed previously had proved more difficult to probe partially because they all appeared to be cloudy."

In the study, astronomers set out to examine the atmosphere of HAT-P-11b, without actually knowing if its weather would be cloudy or not.

The team employed the Hubble telescope in a technique called transmission spectroscopy, where a planet is observed as it crosses in front of its parent star, the light from which filters through the rim of the planet's atmosphere.

In such observation, if molecules like water vapor are present, they absorb some of the starlight, leaving distinct signatures in the light reaching the telescopes.

Hubble was indeed able to detect water vapor in HAT-P-11b.

The scientists next turned to Kepler, which had been trained on one area of the sky for years, and Spitzer to find added proof the vapor wasn't actually caused by starspots, cooler blotches on the face of stars that can contain water vapor -- which may erroneously be read as emanating from one of the star's planets.

So, by combining visible-light information with targeted Spitzer observations taken at infrared wavelengths, the study researchers were able to figure out the starspots on star HAT-P-11 were too hot to produce any steam.

"We think that exo-Neptunes may have diverse compositions, which reflect their formation histories," said study co-author Heather Knutson of the California Institute of Technology in Pasadena. "Now with data like these, we can begin to piece together a narrative for the origin of these distant worlds."

The new findings will help theorists develop new models to explain the makeup and origins of HAT-P-11b and similar worlds.

Astronomers plan to examine more exo-Neptunes in the future, expecting to apply the same method to super-Earths -- massive, rocky cousins to our home world with up to 10 times the mass.

"We want to expand our knowledge to a diverse range of exoplanets," said Drake Deming, a co-author of the study also from University of Maryland. "We are working our way down the line, from hot Jupiters to exo-Neptunes."