Star clusters: Optimal targets for a photometric planetary search program

By targeting stars in some of the most populous open star clusters, the stringent requirements of a photometric search for planets as small as the Earth, i.e., that large numbers of stars must be monitored frequently and with extremely high precision, can be achieved in a ground-based observing program. Data for the cluster NGC 188 confirms previous studies that even with a telescope of modest aperture (0.9 m), high-precision relative photometry of moderately bright stars is possible; even under marginal conditions of thin clouds and variable seeing, high-frequency atmospheric fluctuations (scintillation) can be largely corrected for. By continuously monitoring the many hundreds of stars in the richest old clusters simultaneously in two colors, it should also be possible to model explicitly the slower variations in atmospheric extinction that could mask the small light variations caused by transit events. Furthermore, the stars selected will automatically be main sequence stars of known age and distance; there are several old open clusters with ages of billions of years that together contain several thousand stars of solar mass or less and which are virtually identical to the Sun in chemical composition. An effective search program will require one or more telescopes of at least 4 m in aperture dedicated to the project for a period of several years. Even very large ground-based telescopes can be built for a small fraction of the cost of a space mission and would be usable for other projects at the end of the search period. By targeting four of the ''best'' of the old open clusters, the project could uncover as many as 25 planetary systems in a few years of observing, or at least set meaningful upper limits on the frequency of planetary systems. Furthermore, the same data set will also be useful to investigate stellar surface activity as a function of stellar age, to derive precise parallaxes of the target clusters (which will help to set the cosmic distance scale) and possibly to conduct asteroseismological studies. The data will also likely lead to the discovery of serendipitous events such as cosmologically distant supernova explosions along the line of sight.