As the hunt for Earth-size planets around other stars ratchets up, some researchers are already investigating how to test those planets for habitability once they are found.

A team of U.S. and Australian scientists reports successfully testing an approach that could be used to look for water on so-called extrasolar planets, or exoplanets. The researchers tried out their technique, described this week in a paper submitted to to be published in Astrobiology, on the only Earth-like planet we know of: Earth.

University of Melbourne astrophysics graduate student Sally Langford, along with her adviser Stuart Wyithe and Princeton University astrophysics professor Edwin Turner, monitored a phenomenon known as "Earthshine," wherein a portion of the moon facing away from the sun is illuminated by sunlight reflecting off our planet. Due to the angles of reflection and viewing, Earthshine comes primarily from a relatively small patch of the planet at any given time, the researchers write.

By tracking the brightness of Earthshine as the planet rotates, Langford and her colleagues were able to detect a change in Earth's reflectance that takes place when that dominant patch moves between land and water. Specular, or mirrorlike, reflection characterizes oceanic regions, whereas land reflects sunlight more diffusely. The researchers were able to detect this difference as a dip in Earthshine when the reflecting portion of the Earth shifted from the Indian Ocean to Africa's east coast.

By monitoring the light reflected by potentially habitable exoplanets, future generations of planet-hunting telescopes might be able to detect the mirrorlike reflectance of liquid water.

"Our results highlight the importance of considering specular reflection from oceans in the modeling and analysis of light curves from Earth-like extrasolar planets," the study's authors write, "and suggest that it is a useful tool in determining the presence of liquid water on a planet."

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