An exoplanet is a planet that orbits a star outside of our solar system. The first exoplanets were discovered in the 1990s, and as of February 2020, there are 4,134 confirmed planets in 3,070 systems, with an additional 5,075 unconfirmed planets. The vast majority of these planets have been found using the radial velocity method, in which the gravitational pull of an unseen planet causes its host star to wobble slightly.
The first worlds beyond our own were identified as such back in the 1990s. In October 1995, astronomers Michel Mayor and Didier Queloz announced they had discovered an extrasolar planet orbiting 51 Pegasi—a Sun-like star some 50 light-years away in the constellation Pegasus. This was soon followed by similar announcements regarding several other stars; it became apparent that our planetary system was not unique after all but one among many others sprinkled across the Milky Way galaxy. By early 2020 more than 4100 confirmed extrasolar planets have been found in over 3000 planetary systems (including multiple-planet systems). A reasonable estimate suggests there may be tens to hundreds of billions of such worlds orbiting Sun-like stars and red dwarf stars within our Milky Way galaxy alone! And since galaxies are themselves numerous beyond counting—billions upon billions of them populate the observable universe—the total number of alien planets very likely runs into the trillions or more throughout cosmic space.
The vast majority of known extrasolar planets have been detected indirectly by observing how they affect their parent stars’ motions or brightnesses as seen from Earth. Doppler spectroscopy (also called radial velocity) has proven to be an especially fruitful technique for finding close-in giant planets like Jupiter despite their relatively small size and faintness compared to their much larger host stars; this method detects periodic shifts in a star’s spectral lines due to the back-and-forth tugging gravity exerted by an unseen world as it orbits around its parent star on a generally elliptical path once every few days or weeks or years (see artist’s impression at top right). More than 2500 exoplanets have now been found using this radial velocity technique. Transit photometry has also proved effective for finding extrasolar worlds, especially smaller ones comparable in size to terrestrial worlds like Earth (see artist’s impression at lower left). As a transiting planet crosses directly between us and its parent star along our line of sight from Earth once every orbit—causing a slight dip or dimming effect in the latter’s brightness that can be monitored over time by sensitive telescopes—it provides information about both its physical size and orbital distance from its central sun