The first confirmed discovery of extrasolar planets (planets around another star - also called exoplanets) occured in 1992. Radio astronomers Aleksander Wolszczan and Dale Frail announced the discovery of planets around a pulsar. In 1995, Michel Mayor and Didier Queloz of the University of Geneva announced the first definitive detection of an exoplanet orbiting an ordinary main-sequence star (51 Pegasi). This discovery was made at the Observatoire de Haute-Provence and ushered in the modern era of exoplanetary discovery.
See below for more detailed exoplanet information sources.
Technological advances allowed astronomers to detect exoplanets indirectly by determining their gravitational influence on the motion of their parent stars. The planets are usually discovered by measuring the change in Doppler shift of the star's light resulting from the star orbiting a common center of mass with a companion planet. The graphic to the right below demonstrates this technique.
Several extrasolar planets were detected by observing the variation in a star's apparent luminosity as a planet passed or transited in front of it. The occasional transit of Venus across the sun is an example from our own Solar System.
According to the Extrasolar Planet Encyclopaedia, by the middle of 2012, more than 770 exoplanets had been discovered. PlanetQuest counted 729 confirmed, with 2321 candidate planets. The difference between the two databases highlights the uncertainties involved in exoplanet detection and confirmation.