Kepler is a space observatory launched by NASA to discover Earth-like planets orbiting other stars. The spacecraft, named for the Renaissance astronomer Johannes Kepler, was launched on 7 March 2009.

The Kepler observatory 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". A photometer 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 part of NASA's Discovery Program of relatively low-cost, focused primary science missions. The telescope's construction and initial operation were managed by NASA's Jet Propulsion Laboratory, with Ball Aerospace responsible for developing the Kepler flight system. The Ames Research Center is responsible for the ground system development, mission operations since December 2009, and science data analysis. The initial planned lifetime was 3.5 years,, but greater than expected noise in both the stars and the spacecraft means additional time will be needed to fulfill all mission goals. Therefore in 2012 the mission was extended to 2016, but this would only be possible if all remaining reaction wheels (used for pointing the spacecraft) stayed healthy. On May 15, 2013, a second of four reaction wheels had failed, threatening the continuation of the mission.

As of January 2013[update], there were a total of 2,740 candidate exoplanets. As of May 22, 2013, a total of 889 exoplanets (in 694 planetary systems, including 133 multiple planetary systems) have been confirmed. In January 2013, astronomers at the Harvard-Smithsonian Center for Astrophysics (CfA) used Kepler's data to estimate that "at least 17 billion" Earth-sized exoplanets reside in the Milky Way Galaxy.

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