The unequivocal signature of water vapor has been found on a planet beyond our solar system.

Using NASA’s Spitzer Space Telescope, astronomers detected the steamy signature of water vapor in the light coming from a large exoplanet circling around a star about 63 light-years from Earth. Though it’s not the first sign of water vapor around this planet, it’s the strongest evidence yet.

The planet, HD 189733b, is what’s called a "Hot Jupiter" — a boiling, gigantic gas planet more akin to our own Jupiter or Saturn than to a terrestrial planet like Earth. It’s not a good candidate itself for alien life, but the successful detection of water vapor here, in the location and quantities that theorists predicted, bodes well for further studies of more promising locales for extraterrestrial life.

"It means we’re starting to understand these objects a little bit better than we did when we first started," astrophysicist Adam

Burrows of Princeton University told Wired.com. "It’s a trial run for the much more detailed investigations that will be possible in the years to come as we take this stepping stone from giant planets to terrestrial planets."

Though water vapor is thought to be fairly common on planets — even our own Jupiter has it — the discovery of its presence on another world is significant and points the way toward future discoveries, scientists say. Yesterday scientists announced that the Hubble Space Telescope had found carbon dioxide, which under the right circumstances could be connected to life, on the same planet. The presence of methane has also been detected.

Burrows and the research team, led by Carl Grillmair of the

California Institute of Technology, used a technique known as the secondary-eclipse method to observe the infrared light coming from the planet. The world is so close to its star that normally the light from the two objects cannot be distinguished. But when the planet orbits behind the star, only the light from the star is visible. By subtracting the star’s light from the total light of star plus planet, the scientists were able to isolate only the planet’s light. When they separated the image into its constituent colors in a process called spectroscopy, they found the characteristic dip in light of a certain wavelength that results from water in the planet’s atmosphere absorbing that light.

"We’re looking at the brightness of the planet, as opposed to the effect of the stellar light," Burrows said. "The data we have is the best spectrum ever taken of a planet outside the solar system."

In an article accompanying the research paper, published Wednesday in the journal Nature, astronomer Drake Deming of the NASA

Goddard Space Flight Center in Greenbelt, Maryland, wrote, "We must first learn how to detect abundant molecules such as water before we can advance to identifying the more subtle signatures that scarcer molecules such as molecular oxygen leave in exoplanet spectra. Grillmair and colleagues have taken that first step."

Citation: "Strong water absorption in the dayside emission spectrum of the planet HD 189733b" by Carl J. Grillmair, Adam Burrows, David Charbonneau, Lee Armus, John Stauffer, Victoria Meadows, Jeffrey van Cleve, Kaspar von Braun and Deborah Levine. Nature: doi:10.1038/nature07574.

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Image: NASA/JPL-Caltech/T. Pyle (SSC)