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.
At that time only 80 exoplanets had been discovered since Gamma Cephei Ab was first found in 1989 (but confirmed in 2002), and the exoplanets being discovered with the available technology at that time were large “hot Jupiter” gas giants orbiting close to their stars, completely unsuitable for life as we know it.
The Kepler telescope was initially set to launch in 2006, but ended up being pushed back to 2009 as the spacecraft design and development proceeded and the Discovery program worked through issues like the loss of the CONTOUR spacecraft and budget difficulties. While Kepler’s development proceeded, exoplanet discovery methods and technologies improved and the exoplanet count at the time of Kepler’s launch in March ...