Kepler is a space observatory planned by NASA that will search for extrasolar planets. For this purpose, it will observe the brightness of about 100,000 stars over four years to detect periodical transits of a star by one of its planets. Kepler will not be in an Earth orbit but in an Earth-trailing solar orbit so that Earth will not hide the stars which are to be observed and the photometer will not be influenced by stray light from Earth. The observatory is currently scheduled for launch in November 2008, though there is a high probability of change in the launch date as it has already changed a number of times.

The probe is estimated to weigh 995 kg, have a 0.95 meter photometer, have a field of view of roughly two hands held at arm's length, will take un-focused "pictures" every three seconds, and will cost an estimated $467 million.