A new analysis suggests a distant planet outside our solar system has water in its atmosphere.

The discovery of evidence of water coincides with theories predicting its presence in the atmosphere of the gas giant HD209458b — nicknamed Osiris after the Egyptian god — which lies 150 light years from Earth in the constellation Pegasus.

Artist's view of the evaporating planet HD209458b, also known as Osiris.
(European Space Agency, Institut d'Astrophysique de Paris and NASA)

Those predictions were called into question in February when three teams of scientists studying the planet with the aid of NASA's Spitzer Space Telescope failed to turn up any evidence of water.

But now Travis Barman, an astronomer at Lowell Observatory in Flagstaff, Ariz., said he's found the missing water by analyzing the light from a nearby star as it passes through the planet's atmosphere.

Barman, working with Harvard University student Heather Knutson, measured the light coming from the planet each time it orbited its star, which it does every 3.5 days.

Using data from the Hubble Space Telescope, Barman analyzed the planet's radius across a number of wavelengths of light. Across one part of the infrared spectrum, the planet appeared slightly larger than in wavelengths in the visible spectrum, a distortion consistent with the absorption of light by water molecules, he said in a statement Wednesday.

"It is encouraging that theoretical predictions of water in extrasolar planets seem to agree reasonably well with observations," he said in the statement.

"We now know that water vapour exists in the atmosphere of one extrasolar planet and there is good reason to believe that other extrasolar planets contain water vapour," he said.

Osiris was the first planet discovered outside our own solar system to have an atmosphere, making it the focus of intense study.

Not that it is suitable for humans: the gas giant is similar to Jupiter in mass and size but orbits the star HD209458 at a distance one-eighth that of the Mercury's orbit around our own Sun. Because it is so close to its star, scientists have actually been able to observe its atmosphere evaporating from the heat.

The presence of water outside our solar system is encouraging for scientists, however, because it suggests the compound's presence might be found elsewhere in the galaxy. Liquid water and a stable heat source are two conditions deemed necessary for supporting an environment favourable to life.

Barman's findings will be published in a paper in an upcoming issue of The Astrophysical Journal.