The discovery of planets around other stars was (up to now) only possible due to the development of high-precision radial-velocity techniques. These techniques have permitted astronomers to look for the tiny periodic motion of a star as it moves around the center-of-mass of the star-planet system.

Radial velocity surveys have revealed up to now about 100 extra-solar planets and 9 multi-planetary systems. The discovered planets present a wide variety of orbital element and masses, which are raising many problems and questions regarding the processes involved in their formation. But the analysis of the distributions of orbital elements, like the period and eccentricity distributions is already giving some constraints on the formation of planetary systems. Furthemore, the study of planet host stars have revealed the impressive role of the stellar metallicity on the giant planet formation. The chemical composition of the molecular cloud is probably one of the key parameters to form giant planets.

The study of extra-solar planetary systems is just giving its first steps. After only 7 years, we can say that at least 5% of the solar type stars have giant planetary companions with masses as low as the mass of Saturn and orbital periods less than about 5 years. But the understanding of how giant planets are formed is still shaded in many points and we are still lacking of any detections of analogs of terrestrial planets. To help solve some of these problems, several projects are currently
il the pipeline .Future space missions, like the photometric missions COROT,Eddington or Kepler (to search for planetary transits) or the astrometric satellites GAIA ans SIM will definitively permit to unveil hundreds (or even thousands) of "new" planets .But ground based astronomy will also give enormous steps in the next few years.