New Technique Could Reveal Exoplanets with Water

When it comes to life as we know it, nothing is more important than liquid water. Now scientists have devised a way to spot water on distant planets that can only barely be seen now, which in turn could reveal whether they might be able to support life.

In the past two decades, astronomers have detected more than 300 planets orbiting alien stars. Although most of these exoplanets are gas giants similar to Jupiter, powerful space telescopes such as the one aboard NASA's recently launched Kepler Mission will make it easier to detect smaller rocky exoplanets similar to Earth.

Seen from dozens of light years away, an Earth-like exoplanet will appear in telescopes as little more than a pale dot. Now a team of astronomers and astrobiologists has come up with a method to tell whether such a planet harbors liquid water, using NASA's Deep Impact spacecraft.

The probe was set to watch Earth on two separate days, March 18 and June 4, 2008. At the time, Deep Impact was between 17 million and 33 million miles from our planet and directly above the equator. It measured the light intensity from our planet in seven bands of visible light, from longer redder wavelengths to shorter bluer wavelengths.

The scientists analyzed small deviations from the Earth's average color caused by surface features such as clouds and oceans rotating in and out of view. This was undertaken "as if we were aliens looking at Earth with the tools we might have in 10 years" and did not already know Earth's composition, explained researcher Nicolas Cowan, an astronomer and astrobiologist at the University of Washington in Seattle.

The light coming from the Earth was summed up as just as a single pixel. Since clouds cover half our planet, for the most part that dot looked very gray. However, as each day progressed, the clouds rotated in and out of view, revealing the surface underneath, "and the color of that little speck of light changed by some small amount, say 10 or 20 percent," Cowan told SPACE.com.

Cowan and his colleagues found two dominant colors emerged, one at redder wavelengths, interpreted as landmasses, and the other at bluer wavelengths, judged to be oceans.

"You could tell that there were liquid oceans on the planet," Cowan said.

Although some non-habitable planets such as Neptune can also appear blue, in the case of Neptune this color is likely caused by methane in the atmosphere. Cowan noted there are clues that one can use to tell a watery planet from other kinds of blue planet. For instance, Neptune "looks blue from every angle, the same blue all the way around," he said. "For Earth, the blue varies from one place to another, which indicates that it?s not something in the atmosphere."

Scientists have detected water in the atmosphere of hot extrasolar gas giants in the past three years, "but you can't claim those are nice places to live," Cowan said. "If you found water on a rocky planet that is actually in the habitable zone, not too close or far from the sun so that any water is entirely frozen or vaporized, that would be key, since liquid water seems to be the thing needed for life."

Cowan and his colleagues are scheduled to detail their findings in the August issue of Astrophysical Journal.

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