As the highly successful Kepler spacecraft ends its 9-year mission, humanity can look back at an instrument that greatly expanded our knowledge of planetary systems around other stars and discovered many Earth-sized and smaller planets, some of them in the habitable zone where liquid water, and therefore life as we know it, could exist.
The Kepler mission was conceived as a Discovery Program mission at the turn of the century and was approved as the tenth mission in the program in December 2001.

https://www.nasaspaceflight.com/2018/10/kepler-retired-after-running-out-of-fuel/