COMMENTS ON THE PHOTOMETRIC-METHOD FOR THE DETECTION OF EXTRASOLAR PLANETS

We offer further perspectives on the application of the photometric te chnique to the search for extrasolar planets. The principal obstacles to this approach include (1) the ambiguous interpretation of the photo metric signature of a possible transit event arising from the potentia l for confusion with stellar activity and (2) the observational intens ity of the search technique. We emphasize that the former issue can be addressed through multiband observations combined with consideration of the characteristic time scales of forms of stellar activity compare d to the time scale of a transit event. In the case of the latter conc ern, the advent of modern, automated telescope technologies offers the realistic prospect of meeting the demanding observational requirement s of this approach. We find that joint visual and near infrared observ ations are necessary to immediately distinguish between, for example, a planetary transit and a cool spot on the stellar surface. However, t hese observations by themselves do not appear sufficient to distinguis h between a brown dwarf and a jovian-size planetary companion of a dwa rf M star. In such cases, other complementary observations will be nec essary. We developed a computer simulation to estimate the average num ber of transits per year that would be presented far a stellar sample characterized by realistic properties in terms of spectral type distri bution, effective temperatures, masses, and radii. A simple thermal mo del is adopted to estimate star-planet separations. The simulation sug gests that the frequency of transit events will be dominated by M dwar f systems. This is due to the large number of these objects combined w ith the relative proximity to these cool stars within which planetary formation can occur according to current models. (C) 1995 academic Pre ss, Inc.