Launched from Cape Canaveral on 7 March 2009, the Kepler Space Telescope was NASA's first mission designed to answer a single question: "How often do Earth-sized planets occur in the habitable zones of Sun-like stars?" The first exoplanet around a Sun-like star — 51 Pegasi b — had been discovered in 1995 by Michel Mayor and Didier Queloz in Switzerland, work that would receive the 2019 Nobel Prize in Physics. But by 2009 only a few hundred exoplanets were known, almost all of them large, Jupiter-class or in unusually close orbits. Kepler was built to turn that scattered catalogue into a statistical sample.

The method was direct: the transit technique. When a planet crosses in front of its star, the star's brightness dips by a few hundred parts per million for a few hours. Kepler was built to monitor about 530,000 stars in a single patch of sky in the Cygnus-Lyra region for such repeating dips. The prime mission ran from 2009 to 2013; in 2013 the failure of two of its four reaction wheels seemed to end the mission. Engineers then devised an ingenious workaround, using the pressure of sunlight itself as a third stabilizer. The extended mission, called K2, ran from 2014 to 2018. The telescope was retired on 30 October 2018 when it finally ran out of fuel.

The scientific balance sheet is historic. More than 2,700 confirmed exoplanets and thousands of additional candidates — roughly half of all exoplanets known today come from Kepler data. The real contribution, however, is not the count but the distribution: Kepler's statistics implied that the Milky Way contains hundreds of billions of stars and at least as many planets, with roughly one in five Sun-like stars carrying an Earth-sized planet in its habitable zone.

Kepler is over; the mission is not. TESS (Transiting Exoplanet Survey Satellite), launched in 2018, now scans the whole sky for exoplanets around nearby bright stars; the James Webb Space Telescope has begun measuring the atmospheres of those planets directly. Within a single generation the question shifted from "are there other worlds out there?" to "what is there on those worlds?"