Hunt for Earth-Like Worlds Looks in the Mirror

To find Earth-like worlds around other stars, scientists should take a page from our own planet, a new study found.

In particular, observers should look for glints of sunlight reflected by oceans or lakes, such as what's seen by spacecraft looking back on Earth. These glints are tell-tale signals of liquid, which could indicate the presence of liquid water, considered one of the fundamental necessities for life.

The glints can be seen in new set of videos taken of Earth by NASA's Deep Impact spacecraft, currently about 11 million miles (17.7 million km) away. The Extrasolar Planet Observations and Characterization (EPOCh) project aims to use the tapes to help characterize other worlds beyond the solar system, called extrasolar planets.

Some of the most revealing features in the new data are the flashes of sunlight reflected off Earth's oceans and lakes. The planet and spacecraft must be positioned just right for light to bounce off in this way. And it only happens with liquid and ice, which are smooth enough to produce the effect. Land masses are generally too rough to create glints.

"These sun glints are important because, if we saw an extrasolar planet which had glints that popped up periodically, we would know that we were seeing lakes, oceans or other large bodies of liquid, such as water," said Drake Deming, deputy principal investigator of EPOCh, who is a researcher at NASA's Goddard Space Flight Center in Greenbelt, Md. "And if we found large bodies of water on a distant planet, we would become much more optimistic about finding life."

The EPOCh team just released a set of videos from the spacecraft that show the first view of Earth for a full rotation from the north pole and the south pole. The researchers expected to see sun glints, but were surprised by the intensity and small focus of some, said Richard K. Barry, also of Goddard.

The glints appeared over oceans, and in some cases, over land masses where there were large lakes. Barry is compiling a catalog that will connect each observed glint to an exact location on Earth.

The researchers are also connecting other signals seen from a distant view of Earth with known features of the planet. For example, they found that when a large expanse of bare land, like the Sahara Desert, rotates into view, the planet's overall color appears to change slightly, because continents reflect light differently from oceans. This information could help to characterize other planets discovered in the universe.

Deep Impact?s EPOCh project is part of an effort to recycle the spacecraft?s surviving component after it fulfilled its original mission to observe the intentional crash of an impactor probe on the surface of a comet in 2004.

"This is just the first step in trying to understand the nature of the surfaces of extrasolar planets," said Michael A'Hearn, principal investigator for Deep Impact's extended mission.

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• Image Gallery: Deep Impact Collides With a Comet