Gravitational microlensing of planets: the influence of planetary phase and caustic orientation

Recent studies have demonstrated that detailed monitoring of gravitational microlensing events can reveal the presence of planets orbiting the microle nsed source stars. With the potential of probing planets in the Galactic bu lge and Magellanic Clouds, such detections greatly increase the volume over which planets can be found. This paper expands on the original studies by considering the effect of planetary phase on the form of the resultant micr olensing light curve. It is found that crescent-like sources can undergo su bstantially more magnification than a uniformly illuminated disc, the model typically employed in studying such planets. In fact, such a circularly sy mmetric model is found to suffer a minimal degree of magnification when com pared with the crescent models. The degree of magnification is also a stron g function of the planet's orientation with respect to the microlensing cau stic. The form of the magnification variability is strongly dependent on th e planetary phase and from which direction the planet is swept by the caust ic, providing further clues to the geometry of the planetary system. As the amount of light reflected from a planet also depends on its phase, the det ection of extreme crescent-like planets requires the advent of 30-m class t elescopes, while light curves of planets at more moderate phases can be det ermined with today's 10-m telescopes.