Gravitational Microlensing is a technique used to detect the presence of dark matter or exoplanets using the gravitational force of stars. The concept of stems from Einstein’s theory of general relativity. Light passing by a celestial body, such as the sun, will be bent at an angle, where the angle depends on the body’s mass and size. http://www.physics.fsu.edu/Courses/spring98/AST3033/Micro/lensing.htmIn 1936 Albert Einstein published an article on the idea that a foreground star can act as a lens for light coming from a background star, making a bright ring at perfect alignment. http://books.google.com/books?id=H8Tece9FmQwC&printsec=frontcover&dq=Petters,+Levine,+Wambsganss,+Singularity+Theory+and+Gravitational+Lensing,+Birkhauser,+2001&source=bl&ots=7axzba8cfv&sig=SIiN4ptnh6OfbHi2BLCToIcQbUI&hl=en&ei=iqa8TPjFIsaAlAfi_9DrBw&sa=X&o#v=onepage&q&f=falseThis overlap can help scientists detect objects in between the two stars by studying the curvature of light that the event produces. http://bustard.phys.nd.edu/MPS/If the foreground star is substituted for anything with a star-like mass, then it could be used to find dark matter. When dark matter passes in front of a star, it creates a ring with no star in the middle, referred to as a halo http://www.physics.fsu.edu/Courses/spring98/AST3033/Micro/lensing.htm.

There are many different scientific organizations that use Microlensing for astronomical surveys, such as Las Cumbras Observatory, MACHO, OGLE, PLANET, NASA and ESA. http://sim.jpl.nasa.gov/simLiteScience/darkMatterGalaxies/http://ogle.astrouw.edu.pl/ So far, 10 exoplanets have been found using this technique. http://arxiv.org/abs/1002.0332 Unlike other methods of exoplanet detection, Microlensing is more effective at searching for planets that have lower masses and are farther from the star from the star they orbit. http://www.phys.canterbury.ac.nz/moa/extrasolar_planets.html

A group of scientists called The Optical Gravitational Lensing Experiment (OGLE) discovered the first planet using this method and is one of the main contributors of exoplanet discovery through Gravitational Lensing. OGLE is a collaboration of Carnegie University of Washington, Princeton University and Warsaw University, with its observatory in Las Campanas, Chile.http://ogle.astrouw.edu.pl/ On June 22, 2003, OGLE detected the planet, called OGLE 2003-BLG-235. Since June 12, 2001, OGLE has been observing over 400 million stars, having recorded 4000 Gravitational Lensing events, obtained 30 Terabytes of raw data per year. http://ogle.astrouw.edu.pl/

Despite yielding the smallest planets with the largest distances from their star so far, the amount of planets found by Gravitational Lensing pale in comparison to other methods, such as using the Doppler effect of a star to detect a planet. To date, 461 planets have been found using the Doppler effect method, while only 11 planets have been found using Microlensing. http://arxiv.org/abs/1002.0332 http://ogle.astrouw.edu.pl/ Furthermore, because detection through Gravitational Lensing is very indirect, many parameters of the planet cannot be determined, such as the time it takes for the planet to revolve around its star or the shape of its orbit. http://www.daviddarling.info/encyclopedia/M/microlensing.html Gravitational Lensing also is only feasible when both stars in the sky are aligned with the planet between them, making the planet detectable only for a short period of time, making it hard to confirm these discoveries. http://arxiv.org/abs/0905.3426v1