Astronomers not associated with the Kepler project are now combing through data gathered by the probe to find exoplanets based solely on the worlds' gravitational pull on exoplanets caught by Kepler as they transit the disks of their parent stars.
The theory behind the effort is basic. As planets orbit a star, they also tug on each other gravitationally. By observing wiggles in the planets' motion during a transit, astronomers can work out the position, orbit, and mass of a world they have yet to see. So far, using this method, astronomers have "found" one Saturn-sized world and possibly a super-Earth in the same system.
The approach is reminiscient of how Neptune was discovered. In the mid nineteenth century, mathematicians noticed quirks in the orbit of Uranus, decided another, so far unknown planet beyond Uranus was responsible, and predicted its position. Astronomers using the best telescopes of the day found Neptune at precisely that position.