Water vapor has been spotted in the atmosphere of an exoplanet, roughly the size of Neptune. The alien world lies 124 light years from the Earth, and is the smallest planet ever observed with the vapor in its gaseous outer layer.

Astronomers examined the planet HAT-P-11b using data from the Hubble Space Telescope, the Spitzer Space Telescope, and the Kepler Space Telescope. They found water vapor present in the atmosphere surrounding the distant planet. If confirmed by other researchers, this world would be the smallest ever seen outside the Solar System to have measurable concentrations of atmospheric water vapor. The presence of the vapor suggests liquid water might exist on the surface, which could make the development of life there more likely.

The exoplanet was viewed as it passed in its orbit between the Earth and its own sun. Astronomers were able to study both the intensity of light, as well as gases in the atmosphere, twice during the crossing. First, as the planet passed over one rim of the star, then a second time, as it passed the other side. Absorption of light from hydrogen and oxygen revealed the presence of water vapor. This rare glimpse through the alien atmosphere showed clear skies, a rare sight in the study of exoplanets. Most alien worlds studied in this fashion have shown cloudy, murky atmospheres.

"It's the smallest planet that we've seen anything in the atmosphere besides clouds. The fact that it's clear at all is significant," Jonathan Fraine, astronomer at the University of Maryland, said.

Clouds may still exist in lower levels of the atmosphere, similar to the way the atmospheric features form on Earth.

On Neptune, a thick atmosphere of hydrogen, helium and methane surrounds an Earth-sized core covered in water ice, methane and ammonia. Life as we know it could not exist on the planet, due to frigid temperatures and crushing atmospheric pressures.

The planet HAT-P-11b has a mass 26 times greater than the Earth, although its diameter is just four times greater. Astronomers believe the planet began existence as a rocky or icy body, which slowly accumulated hydrogen and other gases in its atmosphere. The unusually clear skies could provide researchers the ability to study the surface of the world, identifying materials present on the surface of the exoplanet.

Björn Benneke of the California Institute of Technology, one of the researchers on the study, will continue investigation of other small exoplanets, looking for water vapor in their atmospheres.

Discovery of water vapor in the atmosphere of HAT-P-11b was detailed in the journal Nature.