Microlensing Surveys for Exoplanets

Unlike most other planet-detection techniques, gravitational microlensing does not rely on detection of photons from either the host or the planet. Rather, planets are discovered by their gravitational perturbation of light from a more distant source. I review the fundamental concepts of microlensing planet searches and discuss their practical application. I show how the strengths and peculiarities of the method flow from the basic manner in which planets are discovered. In particular, microlensing is sensitive to very low-mass planets on wide orbits and free-floating planets, and can be used to search for planets orbiting host stars with a broad range of masses and Galactocentric distances. However, microlensing events are rare and cannot be predicted in advance, the majority of the host stars are extremely faint, and the planetary signals typically last less than a day. These strengths motivate microlensing searches as powerful, complementary probes of unexplored parameter space that have already provided important constraints on the demographics of planets beyond the "snow line." However, the real-world challenges associated with the practical application of the method have driven the organization and evolution of the microlensing field and will continue to drive future developments from next-generation ground-based experiments through possible future space-based missions.