Kepler is a space observatory launched by NASA to discover Earth-like planets orbiting other stars. The spacecraft, named in honor of the 17th-century German astronomer Johannes Kepler, was launched in March 2009 and has been active for 3 years, 5 months and 30 days as of today (6 September 2012) (time since launch).
The Kepler mission is "specifically designed to survey a portion of our region of the Milky Way galaxy to discover dozens of Earth-size planets in or near the habitable zone and determine how many of the billions of stars in our galaxy have such planets." Kepler's only instrument is aphotometer that continually monitors the brightness of over 145,000 main sequence stars in a fixed field of view. This data is transmitted to Earth, then analyzed to detect periodic dimming caused by extrasolar planets that cross in front of their host star.
Kepler is a project under NASA's Discovery Program of relatively low-cost, focused science missions. Construction and initial operation were managed by NASA's Jet Propulsion Laboratory, with Ball Aerospaceresponsible for developing the Kepler flight system. The Ames Research Center is responsible for the ground system development, mission operations (from December 2009), and science data analysis.
The Kepler observatory is currently in active operation, with the first main results announced on 4 January 2010. As expected, the initial discoveries were all short-period planets. As the mission continued, additional longer-period candidates were found – as of December 2011, there are a total of 2,326 candidates. Of these, 207 are similar in size to Earth, 680 are super-Earth-size, 1,181 are Neptune-size, 203 are Jupiter-size and 55 are larger than Jupiter. Moreover, 48 planet candidates were found in the habitable zones of surveyed stars. TheKepler team estimated that 5.4% of all stars host Earth-size planet candidates, and that 17% of all stars have multiple planets.