How many transiting exoplanets might NASA’s Kepler spacecraft find?

Kepler stares at about 160,000 stars continuously — we don’t change targets because we don’t want to miss a transit that might last only 10 hours and occur only once per year. Half a percent of 160,000 stars is 800 stars, so you might think that this is about how many transiting planets with Earth-sized orbits Kepler should find. But nothing is simple when working at such high precision; we must consider several important factors.

Two factors tend to increase the numbers: (1) Planets closer to their stars than Earth is to the Sun are easier to detect. (2) Many planets live in multiworld systems, so several transiting planets could orbit one star.

Two factors will decrease the number: (1) Not all stars are the same. Most are more active (“noisier”) than the Sun, and this makes it more difficult to detect the transits. Small planets can become lost in the stellar noise, even to Kepler. And the fainter the star, the harder it is to see any transits. (2) Kepler has been collecting data since May 2009. We want to observe at least three or four transits before we have confidence that any event is real, which requires four or more years for planets in large orbits.
 
As of now, Kepler has detected 2,321 planet candidates, which are objects that show transitlike features. Most of these are likely exoworlds, but at this time we don’t have enough information to state with confidence that they are all planets.