Astronomers announced Monday that the European Space Agency had committed to launching the Eddington satellite, which would search for Earth-like planets, by 2008.

With four telescopes, Eddington would gaze at different regions of the sky for intervals of about two months each, observing more 200,000 stars, measuring changes in light of one part of one million, and thus allowing astronomers to learn more about what stars are like inside.

The mission will then search for Earth-like planets orbiting other stars, pointing continuously at one region of the sky for three years, measuring light from more than 100,000 stars and detecting the tiny decrease in light as a planet passes in front of a star.

This so-called transit method will also be employed by NASA's Kepler satellite, originally slated to launch in 2006 but now said to be planned for 2007. The French COROT mission would also search for other Earths.

It is likely that one of these missions will be the first to provide evidence for potentially habitable planets -- those similar in size to Earth and in similar orbits around other stars. Researchers hope the missions provide a catalogue of such worlds, though they would not provide an actual images of the planets.

Other missions planned for later launches would be needed to probe the planets' atmospheres and search for signs of life.

"At last we will be able to find planets like the Earth around other stars and to understand how stars work and how they change as they get older," said Ian Roxburgh of Queen Mary, University of London. "Discovering the existence of planets like the Earth, with properties similar to those on Earth, is a first step towards searching for signs of life elsewhere in the universe."

Roxburgh led a coalition of astronomers who proposed the mission in 2000.

More than fifty research groups around Europe are involved in the Eddington Mission. It is under the overall direction of the European Space Agency. At a press conference in Paris Monday, Roxburgh said the ESA had made Eddington a firm part of a newly envisioned science program. The satellite might rocket into space as early as 2007, he said.

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