Toward the Detection of Transiting Hot Earths and Hot Neptunes in Open Clusters

Radial velocity searches for extrasolar planets have recently detected several very low mass (7-20 M_oplus) planets in close orbits with periods less than 10 days. We consider the prospects for detecting the analogs of these planets in Galactic open clusters via transits. We outline the requirements for constructing a transit survey that would allow one to probe such ``Hot Earths'' and ``Hot Neptunes.'' Specifically, we present a simple criterion for detection that defines the minimum aperture required to detect planets of a given radius in a cluster at a given distance. Adopting photometric precisions that have been demonstrated in state-of-the-art variability surveys, we then predict the number of planets one could potentially detect with ambitious transit surveys toward several open clusters. Dedicated surveys lasting more than 20 nights with Pan-STARRS toward the Hyades and Praesepe could detect a handful of Hot Earths, if the majority of stars host such planets. Similar surveys with larger aperture telescopes (eg CFHT, MMT), toward M67, M35, M50, and M37 could detect Hot Neptunes, provided that their frequency is greater than 1%. The majority of planets will be detected around M dwarfs; detecting Hot Neptunes around such primaries requires photometric precisions of approx 1%, whereas Hot Earths require approx 0.1 %. We discuss potential hurdles in detecting and confirming small planets in ground-based surveys, including correlated noise, false positives, and intrinsic stellar variability.