Expectations from a microlensing search for planets

The statistical distribution of the masses of planets about stars between the Sun and the center of the Galaxy is constrained to within a factor of 3 by an intensive search for planets during microlensing events. Projected separations in terms of the lens Einstein ring radius yield a rough estimate of the distribution of planetary semimajor axes with planetary mass. The search consists of following ongoing stellar microlensing events involving sources in the center of the Galaxy lensed by intervening stars with high-time-resolution, 1% photometry in two colors in an attempt to catch any short-time-scale planetary perturbations of the otherwise smooth lightcurve. It is assumed that 3000 events are followed over an 8-year period, but with half of the lenses, those that are members of binary systems, devoid of planets. The remaining 1500 lenses have Solar System-like distributions of four or five planets. The expectations from the microlensing search are extremely assumption dependent, with 56, 138, and 81 planets being detected for three sets of assumptions involving how the planetary masses and separations vary with lens mass. The events can be covered from 54 to 62% of the time on average by high-time-resolution photometry from a system of three or four dedicated 2-m telescopes distributed in longitude, so 38 to 46% of the detectable small-mass planets (very short perturbations of the lightcurve) will be missed. But perturbations comparable to a day in length mean all of the detectable Jupiters and Saturns will in fact be detected as will a large fraction of the Uranuses. The groundbased observational technique is robust, and although no followup studies can be made, meaningful statistics on planetary masses and separations can be inferred from such an intensive search. These statistics, like the inferred data set, will also be dependent on the assumptions about the nature of the set of planetary systems. More precisely, the detection of few Earths gives meaningful statistics for the occurrence of Earths in the lensing zone, but does not exclude Earths very close to their stars. But, finding few Jupiters, Saturns, and Uranuses would have profound implications independent of reasonable assumptions about the nature of the systems. Finding most of many giant and sub-giant planets outside the Einstein ring radii of their respective stars may be a better indicator of the frequency of Earth-mass planets than direct detection of a few of the latter. (C) 1997 Academic Press.