A new channel for the detection of planetary systems through microlensing.I. Isolated events due to planet lenses

We propose and evaluate the feasibility of a new strategy to search for pla nets via microlensing observations. This new strategy is designed to detect planets in "wide" orbits, i.e., with orbital separation, a, greater than s imilar to 1.5R(E). Planets in wide orbits may provide the dominant channel for the discovery of planets via microlensing, particularly low-mass (e.g., Earth-mass) planets. This paper concentrates on events in which a single p lanet serves as a lens, leading to an isolated event of short duration. We point out that a distribution of events due to lensing by stars with wide-o rbit planets is necessarily accompanied by a distribution of shorter durati on events. The fraction of events in the latter distribution is proportiona l to the average value of q(1/2), where q is the ratio between planet and s tellar masses. The position of the peak or peaks also provides a measure of the mass ratios typical of planetary systems. We study detection strategie s that can optimize our ability to discover isolated short-duration events due to lensing by planets and find that monitoring employing sensitive phot ometry is particularly useful. If planetary systems similar to our own are common, even modest changes in detection strategy should lead to the discov ery a few isolated events of short duration every year. We therefore also a ddress the issue of the contamination due to stellar populations of any mic rolensing signal due to low-mass MACHOs. We describe how, even for isolated events of short duration, it will be possible to test the hypothesis that the lens was a planet instead of a low-mass MACHO, if the central star of t he planetary system contributes a measurable fraction of the baseline Bur.