Searching for Planetary Transits in Galactic Open Clusters: EXPLORE/OC

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Abstract
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.