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A Search for Habitable Planets
An Artist's Illustration of Kepler
An Artist's Concept of Kepler
Kepler, a NASA Discovery mission, is a spaceborne telescope designed to look for Earth-like planets around stars beyond our solar system.

According to principal investigator William Borucki, the Kepler Mission will, for the first time, allow humans to search our galaxy for planets of Earth-size or smaller. Borucki works at NASA's Ames Research Center, Moffett Field, California. "With this cutting-edge capability, Kepler may help us answer one of the most enduring questions humans have asked throughout history: Are there others like us in the Universe?"

Kepler will detect planets indirectly, using the "transit" method. A transit occurs each time a planet crosses the line-of-sight between its parent star that it is orbiting and the observer. When this happens, the planet blocks some of the light from its star. This results in a periodic dimming. This periodic signature is used to detect the planet and to determine its size and its orbit.

Three transits of a star, all with a consistent period, brightness change, and duration provide an effective method of detecting and confirming a planet. Using the measured orbit of the planet and the known properties of the parent star, scientists can determine if each planet discovered is in the habitable zone. The habitable zone is the distance from its star where liquid water could exist on the surface of the planet.

The industrial partner for development of the hardware is Ball Aerospace & Technologies Corp., based in Boulder, Colorado. The mission is managed by NASA's Jet Propulsion Laboratory.

Kepler is scheduled to launch in 2007. It will hunt for planets using a specialized one-meter diameter telescope called a photometer. The photometer measures the small changes in brightness caused by the transits.

The key technology at the heart of the photometer is a set of charged coupled devices (CCDs). These CCDs measure the brightness of hundreds of thousands of stars at the same time. CCDs are the silicon light-sensitive chips used in TV cameras, camcorders, and digital cameras. Kepler must monitor many thousands of stars at the same time. The reason for simultaneous monitoring is that the chance of any one planet being aligned along the line-of-sight is only about 1/2 of a percent.

Over a 4-year period, Kepler will continuously view a small amount of sky. This amount is about equal to the size of a human hand held at arm's length. Another way to think of this amount is that it is about equal in area to two "scoops" of the sky made with the Big Dipper. In comparison, the Hubble Space Telescope can view only the amount of sky equal to a grain of sand held at arms length. And Hubble can only view it for about a half-hour at a time.

NASA selected Kepler as one of two Discovery missions from 26 proposals made in early 2001. The missions must stay within the Discovery Program's development cost cap of about $299 million. The Discovery Program emphasizes lower-cost, highly focused scientific missions.

Excerpt from Planet Quest