Washington

A space telescope that would search for Earth-sized planets orbiting other stars is among three mission concepts picked by NASA last week to compete for a launch slot in 2005 or 2006.

The Kepler mission and two other proposals — Dawn to orbit the asteroids Vesta and Ceres, and INSIDE Jupiter to investigate the giant planet's interior by studying its magnetic and gravitational fields — were chosen as finalists from 26 ideas submitted to the agency's Discovery programme in August. Each team will receive $450,000 to define its concept further, and a winner will be selected late this year. All three missions are priced at just under $300 million.

The choice of Kepler as a finalist is a vindication of sorts for principal investigator William Borucki of NASA's Ames Research Center in California. He has long proposed a novel and controversial method of searching for Earth-sized planets.

Placed into orbit around the Sun, and equipped with a 0.95-metre telescope and ultra-sensitive charge-coupled device (CCD) detectors, Kepler would fix its gaze on 100,000 stars simultaneously, looking for extremely small dips in their light output. An Earth-sized planet passing in front of a Sun-like star would eclipse its light by only a few parts in 100,000.

But that would be enough to betray the planet's existence, says Borucki, if the pattern is regular. If Earth-sized planets are common, Kepler could detect more than 500 during its four-year lifetime.

Borucki has had trouble convincing NASA review committees that a spacecraft can achieve the sensitivity needed to register such minute changes in light. But he has since successfully tested his system in the laboratory, even when introducing the types of 'noise' that might be expected in space.

Planet hunters have found dozens of Jupiter-sized objects, but nothing like Earth. Borucki and his colleagues claim that their technique is the only one that can find Earth-sized planets in the zone near a star where water would remain liquid at the surface.

But Kepler would not be the first such instrument to fly. The French-led Corot mission is scheduled to launch in late 2004, and will use a CCD array to look for transiting extrasolar planets.

→ http://discovery.nasa.gov/news.html