Thirty years ago, William Borucki and Audrey Summers published a paper assessing the potential for detecting extrasolar planetary systems by transit photometry. The key concept is to simultaneously monitor the brightness of a large number of stars with a high-precision photometer. Planets are revealed by the dips in brightness they produce when they pass in front of (transit) their host stars. Successive transits by a planet are spaced by its orbit period, which helps to distinguish transits from other sources of stellar variability. Transit depths determine the ratio of the planet’s surface area to that of its host star. Borucki and Summers emphasized that detection of Earth-size planets would require observations from above the atmosphere.
Subsequently, Borucki began a long quest to develop a suitable photometer and to convince the astronomical community and the US National Space and Aeronautics Administration (NASA) that a modest space mission could discover planets potentially capable of harbouring life. Four proposals submitted between 1992 and 1998 were rejected before the fifth was selected in December 2001 as Discovery Mission #10. Mission development began in 2002 and launch occurred in March 2009.