Kepler is a space telescope designed to survey a portion of the Milky Way galaxy in search of exoplanets, which are planets outside our solar system.
Using data from the mission, scientists have identified more than 4,500 candidate exoplanets, and have confirmed more than 1,000 of these as bona fide planets. A handful of planets are thought to be rocky like Earth (but a bit bigger), and orbit in the habitable zone of their stars, where liquid water - an essential ingredient of life as we know it - might exist.
In 2013, Kepler was assigned a new mission called "K2." Two of the spacecraft's reaction wheels had failed, so engineers came up with a clever scheme to redesign the mission. K2 still hunts for planets, but it scans a larger swath of sky than before, along the ecliptic plane. The mission has begun new types of research as well, such as the study of objects within our solar system, exploded stars, and distant supermassive black holes at the hearts of galaxies.