page_title: Gravitational microlensing
section_title: Detection of extrasolar planets
hierarchical_section_title: Gravitational microlensing / Detection of extrasolar planets
context_page_description: Gravitational microlensing is an astronomical phenomenon due to the gravitational lens effect. It can be used to detect objects that range from the mass of a planet to the mass of a star, regardless of the light they emit. Typically, astronomers can only detect bright objects that emit much light or large objects that block background light. These objects make up only a minor portion of the mass of a galaxy. Microlensing allows the study of objects that emit little or no light.
  When a distant star or quasar gets sufficiently aligned with a massive compact foreground object, the bending of light due to its gravitational field, as discussed by Albert Einstein in 1915, leads to two distorted unresolved images resulting in an observable magnification. The time-scale of the transient brightening depends on the mass of the foreground object as well as on the relative proper motion between the background 'source' and the foreground 'lens' object.
  Ideally aligned microlensing produces a clear buffer between the radiation from the lens and source objects, as well as magnification, so will reveal or in size and/or brightness enhance distant sources.
context_section_description: If the lensing object is a star with a planet orbiting it, this is an extreme example of a binary lens event.  If the source crosses a caustic, the deviations from a standard event can be large even for low mass planets.  These deviations allow us to infer the existence and determine the mass and separation of the planet around the lens. Deviations typically last a few hours or a few days. Because the signal is strongest when the event itself is strongest, high-magnification events are the most promising candidates for detailed study. Typically, a survey team notifies the community when they discover a high-magnification event in progress. Follow-up groups then intensively monitor the ongoing event, hoping to get good coverage of the deviation if it occurs. When the event is over, the light curve is compared to theoretical models to find the physical parameters of the system. The parameters that can be determined directly from this comparison are the mass ratio of the planet to the star, and the ratio of the star-planet angular separation to the Einstein angle. From these ratios, along with assumptions about the lens star, the mass of the planet and its orbital distance can be estimated.
  The first success of this technique was made in 2003 by both OGLE and MOA of the microlensing event OGLE 2003–BLG–235 (or MOA 2003–BLG–53). Combining their data, they found the most likely planet mass to be 1.5 times the mass of Jupiter. As of April  2020, 89 exoplanets have been detected by this method. Notable examples include OGLE-2005-BLG-071Lb, OGLE-2005-BLG-390Lb, OGLE-2005-BLG-169Lb, two exoplanets around OGLE-2006-BLG-109L, and MOA-2007-BLG-192Lb. Notably, at the time of its announcement in January 2006, the planet OGLE-2005-BLG-390Lb probably had the lowest mass of any known exoplanet orbiting a regular star, with a median at 5.5 times the mass of the Earth and roughly a factor two uncertainty. This record was contested in 2007 by Gliese 581 c  with a minimal mass of 5 Earth masses, and since 2009 Gliese 581 e is the lightest known "regular" exoplanet, with minimum 1.9 Earth masses. In October 2017, OGLE-2016-BLG-1190Lb, an extremely massive exoplanet (or possibly a brown dwarf), about 13.4 times the mass of Jupiter, was reported.
  Comparing this method of detecting extrasolar planets with other techniques such as the transit method, one advantage is that the intensity of the planetary deviation does not depend on the planet mass as strongly as effects in other techniques do. This makes microlensing well suited to finding low-mass planets. It also allows detection of planets further away from the host star than most of the other methods. One disadvantage is that followup of the lens system is very difficult after the event has ended, because it takes a long time for the lens and the source to be sufficiently separated to resolve them separately.
caption: Gravitational microlensing of an extrasolar planet