Exoplanetary Microlensing

Gravitational microlensing occurs when a foreground star h appens to pass very close to our line of sight to a more distant background star. The foreground st ar acts as a lens, splitting the light from the background source star into two images, which are typica lly unresolved. However, these images of the source are also magnified, by an amount that depends on t he angular separation between the lens and source. The relative motion between the lens and sou rce therefore results in a time-variable magnification of the source: a microlensing event. If the for eground star happens to host a planet with projected separation near the paths of these images, the pla n t will also act as a lens, further perturbing the images and resulting in a characteristic, short-lived s ignature of the planet. This chapter provides an introduction to the discovery and characterization of ex oplanets with gravitational microlensing. The theoretical foundation of the method is reviewed, focus ing in particular on the phenomenology of planetary microlensing perturbations. The strengths and w eaknesses of the microlensing technique are discussed, highlighting the fact that it is sensitive to low -mass planetary companions to stars throughout the Galactic disk and foreground bulge, and that its sensiti vity peaks for planet separations just beyond the snow line. An overview of the practice of microlensing plane t s arches is given, with a discussion of some of the challenges with detecting and analyzing planetary pe turbations. The chapter concludes with a review of the results that have been obtained to date, and a di scussion of the near and long-term prospects for microlensing planet surveys. Ultimately, microlensin g is potentially sensitive to multiple-planet systems containing analogs of all the solar system planets e xcept Mercury, as well as to free floating planets, and will provide a crucial test of planet formation theories by determining the demographics of planets throughout the Galaxy.