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Exoplanetary Microlensing

Gravitational microlensing occurs when a foreground star happens to pass very close to our line of sight to a more distant background star. The foreground star acts as a lens, splitting the light from the source star into two images, which are typically unresolved. However, these images are also magnified, by an amount that depends on the angular lens-source separation. The relative lens-source motion results in a time-variable source magnification: a microlensing event. If the foreground star happens to host a planet with projected separation near the paths of these images, the planet will further perturb the images, resulting in a characteristic, short-lived signature of the planet. This chapter provides an introduction to the discovery and characterization of exoplanets with gravitational microlensing. The theoretical foundation of the method is reviewed, focusing on the phenomenology of planetary perturbations. The strengths and weaknesses of the microlensing technique are discussed, highlighting the fact that it is sensitive to low-mass planets with separations just beyond the snow line, orbiting stars located throughout the Galactic disk and foreground bulge. An overview of t