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NASA has selected for further study a proposal from Ames Research Center to search for Earth-size planets around stars beyond our solar system.

The Kepler mission, which will use a space telescope specifically designed to search for habitable planets, is one of three candidates for NASA’s next Discovery Program mission. If selected, Kepler will be launched in 2005.

“The Kepler mission will, for the first time, enable humans to search our galaxy for Earth-size or even smaller planets,” said principal investigator William Borucki of Ames. The mission could find habitable planets in Earth-like orbits within 4 years of launch, he said.

To date, about 50 extra-solar planets have been discovered. However, these are all giant planets similar to Jupiter, which are probably composed of hydrogen and helium. None is likely to be habitable. So far, none of the planet detection methods used has the capability of finding Earth-size habitable planets — those that are 30 to 600 times less massive than Jupiter and have liquid water on their surface.

The Kepler method is different; it will look for “transits” of planets. A transit occurs each time a planet crosses the line of sight between the observer and the planet’s parent star. When this happens, the planet blocks some of the starlight, resulting in periodic dimming, which is used to detect the planet and determine its size.

Three transits of the star, all with a consistent period, brightness change and duration, will provide a rigorous method of detection and planet confirmation. And three values — orbit, temperature and size — will be used to determine if a planet is habitable.

To measure small changes in brightness, the Kepler mission will hunt for planets using a specialized one-meter diameter telescope called a photometer that will be launched into orbit around the sun, away from the distorting effects of the Earth’s atmosphere. The light meter in a camera is another form of a photometer.

The key technology at the heart of the photometer is a set of charged coupled devices (CCDs) that measures the brightness of hundreds of thousands of stars at the same time. CCDs are the silicon light- sensitive chips that are used in every TV camera, camcorder and digital camera today. Kepler will monitor many thousands of stars simultaneously, since the chance of any one planet being aligned along the line of sight is only about 1/2 of 1 percent.

“From monitoring 100,000 stars similar to our sun for 4 years, the Kepler mission team expects to find about 640 terrestrial planets,” said David Koch of NASA Ames, the mission’s deputy principal investigator. “If many planets are found, then life could be widespread in our galaxy. If few or none are found, then life must be rare, or we might be alone.”

The Kepler mission will view an amount of sky about equal to the size of a human hand held at arm’s length, or about equal in area to two “scoops” of the sky made with the Big Dipper constellation.

The Kepler mission team also includes researchers from 15 institutions in the U.S. and Canada. The industrial partner for development of the hardware is Ball Aerospace & Technologies Corporation.

NASA’s Discovery Program is designed to provide frequent, low-cost access to space for planetary missions and missions to search for planets around other stars. The selected science missions must be ready for launch before Sept. 30, 2006, within the Discovery Program’s cap on each mission’s cost to NASA of $299 million.