The Use of Transit Timing to Detect Extrasolar Planets with Masses as   Small as Earth
  Future surveys for transiting extrasolar planets, including the space-based mission Kepler (Borucki et al 2003), are expected to detect hundreds of Jovian mass planets and tens of terrestrial mass planets. For many of these newly discovered planets, the intervals between successive transits will be measured with an accuracy of 0.1--100 minutes. We show that these timing measurements will allow for the detection of additional planets in the system (not necessarily transiting), via their gravitational interaction with the transiting planet. The transit time variations depend on the mass of the additional planet, and in some cases Earth-mass planets will produce a measurable effect. 
Matthew J. Holman (Harvard-Smithsonian Center for Astrophysics) and   Norman W. Murray (Canadian Institute for Theoretical A...