What is an exoplanet?
An exoplanet is a planet that orbits a star outside our solar system. Astronomers have confirmed more than 6,000 exoplanets, and there are likely billions in the Milky Way.
The first exoplanets were found in 1992 around a pulsar, a dead star. In 1995, Michel Mayor and Didier Queloz discovered 51 Pegasi b, the first exoplanet found around a Sun-like star, later winning a Nobel Prize. NASA's Kepler space telescope, launched in 2009, found thousands more, and NASA's tally passed 6,000 confirmed exoplanets in 2025.
Most exoplanets are found indirectly. The transit method detects the tiny dip in a star's brightness when a planet passes in front of it. The radial velocity method spots a star's slight wobble caused by a planet's gravity. Only a few have been photographed directly.
Strange new worlds
Exoplanets are very diverse. There are hot Jupiters, gas giants orbiting extremely close to their stars, and super-Earths, rocky planets larger than Earth. Scientists are especially interested in planets in the habitable zone, where liquid water could exist on the surface, such as some of the seven planets around TRAPPIST-1.
Did You Know?
Proxima Centauri b, which orbits the closest star to the Sun, is the nearest known exoplanet, about 4.2 light-years away.