An exoplanet is any planet found orbiting a star outside our solar system. The first confirmed exoplanets were discovered in the 1990s, and since then astronomers have found several thousand more, ranging from scorching gas giants huddled close to their star to small, rocky worlds sitting in their star’s habitable zone.
Most exoplanets are found indirectly rather than by seeing them outright, since they are far too faint and too close to their bright host stars to photograph easily. The two most productive methods are the transit method, watching for a tiny dip in a star’s brightness as a planet passes in front of it, and the radial velocity method, detecting the slight wobble a planet’s gravity causes in its star.
Missions like Kepler and TESS have turned exoplanet hunting into one of astronomy’s most active fields, and the search has increasingly shifted from simply finding planets to studying their atmospheres for signs of water, and eventually, for signs of life.
Some exoplanet discoveries have reshaped what astronomers expect to find out there. The TRAPPIST-1 system, just 40 light-years away, hosts seven roughly Earth sized planets packed tightly around a small, cool star, several of them sitting in the star’s habitable zone. Hot Jupiters, gas giants orbiting so close to their star that a year lasts only a few days, were among the first exoplanets ever found and surprised astronomers, since nothing like them exists in our own solar system. The James Webb Space Telescope has pushed the field further still, capable of splitting starlight filtering through an exoplanet’s atmosphere into a spectrum and picking out the fingerprints of specific molecules like water vapor, methane, and carbon dioxide, turning exoplanet science from simple detection into genuine atmospheric chemistry.
The first exoplanet confirmed around a sun-like star, 51 Pegasi b, was discovered in 1995 by Michel Mayor and Didier Queloz using the radial velocity method, a find that earned them the 2019 Nobel Prize in Physics. That single hot Jupiter, orbiting its star every four days, kicked off an explosion of exoplanet science that has since uncovered thousands of worlds unlike anything found in our own solar system.