How to find water worlds
4 Jun 2009
If you were an alien looking at Earth from a long way away, how would you know if the planet was able to harbour life or not? Answer—look for the water.
"Liquid water on the surface of a planet is the gold standard that people are looking for," said Nicolas Cowan of the University of Washington.
Since the early 1990s, astronomers have discovered more than 300 planets orbiting stars other than our Sun, nearly all of them huge, gas giant planets like Jupiter.
Powerful space telescopes, such as the one aboard NASA's recently-launched Kepler spacecraft, will soon make it easier to spot much smaller, rocky planets that are more similar to Earth.
But seen from dozens of light years away, an Earth-like planet will appear through telescopes as little more than a "pale blue dot," the term coined by the late astronomer Carl Sagan to describe how Earth appeared in a 1990 image taken by the Voyager spacecraft from near the edge of the Solar System.
< Voyager's "pale blue dot" image of the Earth from the edge of the Solar System.
Using instruments aboard the Deep Impact spacecraft, a team of astronomers and astrobiologists devised a technique to tell whether an exoplanet could harbour liquid water, which in turn could indicate whether it might be able to support life.
And they used Earth as their laboratory.
The scientists pointed the spacecraft at Earth and took two separate, 24-hour-long observations of the light reflected from our planet in different wavelength bands.
Earth appears grey at most wavelengths because of cloud cover, but it appears blue at short wavelengths because of the same atmospheric phenomenon that makes the sky look blue to us down here on the surface.
The researchers studied small deviations from the average colour caused by surface features like clouds and oceans rotating in and out of view. They found two dominant colours—one at long, or red, wavelengths and the other at short, or blue, wavelengths. They interpreted the red as land masses and the blue as oceans.
The analysis 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, Cowan said.
< A series of images from the Deep Impact spacecraft showing the Earth rotating. The first pictures of an Earth-like extrasolar planet will not be this detailed. Instead, the images will be more like the Voyager picture of Earth as a single point of light (above).
"You could tell that there were liquid oceans on the planet [Earth]," Cowan said. "The idea is that to have liquid water the planet would have to be in its system's habitable zone, but being in the habitable zone doesn't guarantee having liquid water."
The habitable zone is the region around a star where the temperature is neither too hot nor too cold for liquid water to exist on the surface of a planet.
It will be some years before the launch of space telescopes capable of making similar observations for Earth-sized planets, but devising this technique now could guide the construction of those instruments, he said. And while those planets will be much farther away, the technique still will be applicable.
Cowan notes that some non-habitable planets, such as Neptune, also can appear to be blue, but the colour is constant and, in the case of Neptune, likely caused by methane in the atmosphere.
Adapted from information issued by University of Washington / NASA / JPL.
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