Space Missions for Exoplanet Science: Kepler/K2

The Kepler Mission is a space observatory launched in 2009 designed to monitor 170,000 stars over a period of 4 years to explore the structure and diversity of planetary systems, in particular to determine the occurrence frequency of Earth-size and larger planets in and near the habitable zone (HZ) of Sun-like stars, the size and orbital distributions of these planets, and the types of stars they orbit. Kepler is the tenth in the series of NASA Discovery Program missions that were competitively selected, PI-directed, medium-cost missions. The Kepler instrument is based on a 0.95 m aperture Schmidt-design telescope with a 113 sq. deg. FOV with 84 channels of CCD detectors. The spacecraft orbits the Sun in 53-week orbit and points at a single field of stars to avoid missing transits. It achieves a photometric precision of 10 ppm for bright, quiet stars for periods of several hours, i.e., the typical duration of planetary transits. During the 4 years of its operation, an analysis of its data detected over 4600 planetary candidates which include several hundred Earth-size planetary candidates, over 3458 confirmed planets, and 21 Earth-size and super-Earth-size planets in the HZ. These discoveries provide the information required for estimates of the occurrence frequency of planets in our galaxy. The mission results show that most stars have planets, many of these planets are similar in size to the Earth, and systems with several planets are common. In combination with radial velocity measurements, the Kepler results were able to distinguish rocky planets from planets that might be composed mostly of water or gas. The Kepler Mission also made major contributions to astrophysical sciences, including the detection of “heartbeat” stars, supernovae, and the interior structure and characteristics of red giant stars. At the end of its 4-year mission, the Kepler Mission was re-proposed as a community facility (“K2”) that provided high-precision photometric time-series data for tens of thousands of stars in each of 13 FOVs along the ecliptic.