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 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.
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. 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.