During the first six months of the journey to Hartley 2, the Extrasolar Planet Observations and Characterization (EPOCh) mission will use the larger of the two telescopes on the Deep Impact spacecraft to search for Earth-sized planets around five stars selected as likely candidates for such planets.
By subtracting light from just the star from the combined light when a planet is next to a star from the spacecraft's point of observation, this system should at least be able to detect a planet as little as three times as massive as Earth. This phase of searching for extrasolar planets is scheduled to begin in late January.
Article:
http://www.eurekalert.org/pub_releases/2007-12/uom-die121807.php