The Kepler spacecraft was designed to hunt for extrasolar planets in a particular area of the sky, a region running across the border between the constellations of Cygnus and Lyra, and extending slightly into Draco to the north. The Kepler Input Catalog represents a detailed index and analysis of the stars in this part of the sky, including specific data for a total of more than thirteen million objects. This data includes factors such as each star'smass and radius, as well as physical data such as its temperature and metallicity. It includes stars as faint as magnitude +21.
Although most of the stars in the Kepler Index Catalog showed no evidence of planetary systems (and therefore did not 'graduate' to the KOI and Kepler indices), the data in the catalogue nonetheless reveals interesting properties for many other objects. Pictured is KIC 9832227, a twelfth-magnitude star in Cygnus. It has no planetary system, but the star is a contact binary, in which two component stars lie so close as to physically interact with one another. It is also an eclipsingbinary, in which the mutual orbit of the two components cause the brightness of the star to fluctuate over an eleven-hour period. Imagery provided by Aladin sky atlas
The arrangement of Kepler's view of its target region means that not all of these thirteen million stars could be observed and analysed directly. Of those that could, Kepler searched for evidence of planetary systems using the transit method, and identifed a set of about 150,000 potential targets of interest (that is, about three per cent of the observed KIC stars). These 150,000 stars are described as Kepler Objects of Interest (or KOI), used as a basis for further study. When planets are definitively identified around a star, that star is then further identified by a 'Kepler' number.