The Keplerplanet-hunting spacecraft was designed to scan an area of sky falling across the borders of Cygnus and Lyra, and extending slightly into Draco. Within that area, a catalogue of more than thirteen million stars was compiled for investigation, a list known as the Kepler Input Catalog or KIC. Of these, a set of some 150,000 stars were identified (a little over one per cent of the full KIC) as showing signals potentially indicative of a transiting planet. This index of 150,000 stars represents the catalogue of Kepler Objects of Interest, commonly abbreviated to 'KOI'.
Each potential transit signal within the KOI is given a separate decimal identifier, potentially representing a planet for further investigation. For example, four such signals were found associated with the star designated KOI-1236, and were individually labelled as KOI-126.01, KOI-126.02, KOI-126.03 and KOI-126.04. Three of these data patterns were later verifed to represent actual planets, and were given full 'Kepler' identities. So, the bright yellowdwarfKOI-1236 acquired the idenitity 'Kepler-279', and the first verified planetKOI-126.02 became 'Kepler-279 b'. Similarly, KOI-126.01 became Kepler-279 c,* and KOI-126.03 became Kepler-279 d. The fourth potential transit signal is still awaiting confirmation, and at the time of writing KOI-1236.04 remains a 'candidate' exoplanet without a full 'Kepler' number.
Of the signals identified as Kepler Objects of Interest, not all are confirmed as planets. Some represent actual orbiting bodies of kinds other than planets (objects of this kind typically represent unusual binary or multiple systems). In addition, an eclipsingbinary system on the same line of sight as the target star can produce an apparent transit signal that, on further examination, proves unrelated to the star itself. For these and other reasons, by no means all Kepler Objects of Interest are ultimately verified, but it is estimated that some 90% of the stars in the catalogue represent true exoplanets.
* It perhaps seems slightly strange that the order changes in this example, so that transit '.02' became planet 'b', and then transit '.01' became planet 'c'. This is because the order that the transit numbers are allocated is not necessarily the same as the order that planets are confirmed, and Kepler-279 b (or KOI-126.02) was verified before Kepler-279 c (or KOI-126.01). Once planets are confirmed, their single-letter designations are also sometimes seen as part of their KOI numbers, so 'KOI-126.01' can also be given as 'KOI-126 b'.