2005PASP..117..141V

Open clusters potentially provide an ideal environment for the search for transiting extrasolar planets, since they feature a relatively large number of stars of the same known age and metallicity at the same distance. With this motivation, over a dozen open clusters are now being monitored by four different groups. We review the motivations and challenges for open cluster transit surveys for short-period giant planets. Our photometric monitoring survey of Galactic southern open clusters, the Extrasolar Planet Occultation Research/Open Clusters (EXPLORE/OC) project, was designed with the goals of maximizing the chance of finding and characterizing planets and of providing a statistically valuable astrophysical result in the case of no detections. We use the EXPLORE/OC data from two open clusters, NGC 2660 and NGC 6208, to illustrate some of the largely unrecognized issues facing open cluster surveys, including severe contamination by Galactic field stars (>80%) and the relatively low number of cluster members for which high-precision photometry can be obtained. We discuss how a careful selection of open cluster targets under a wide range of criteria such as cluster richness, observability, distance, and age can meet the challenges, maximizing chances to detect planet transits. In addition, we present the EXPLORE/OC observing strategy to optimize planet detection, which includes high-cadence observing and continuously observing individual clusters rather than alternating between targets.