An exoplanet (extrasolar planet) is any planet orbiting a star beyond our Solar System, and includes rogue planets that wander without a host star. Over 6,000 exoplanets have been confirmed, showing incredible diversity in size, composition, and orbital behavior.
These worlds range from Earth-sized rocky planets to gas giants larger than Jupiter, as well as sub-Neptunes, mini-Neptunes, super-Earths, and hot Jupiters—massive planets in very close orbits. Some exoplanets orbit two stars or orbit so close to their stars that years last only days.
Exoplanets are detected using methods such as:
Transit photometry: observing small dips in a star’s brightness as a planet passes in front (used by missions like Kepler and TESS).
Radial velocity: detecting wobble in a star’s motion due to gravitational pull by an orbiting planet.
Direct imaging: taking pictures—rare, but used for large planets far from their stars.
Gravitational microlensing: observing light amplification from a background star when a planet passes in front.
Characterization techniques—spectroscopy of transits and direct imaging—allow assessment of atmospheric composition, temperature, and weather patterns. Hubble, Spitzer, and TESS have detected water vapor, sodium, and other chemicals.
Exoplanet systems are widespread in the Milky Way, located thousands of light-years from Earth. Kepler alone showed that planets are more common than stars in our galaxy.
ESA’s mission fleet—including CHEOPS, PLATO, and future missions like Ariel—aims to precisely measure planetary radii, masses, orbits, and atmospheric properties, advancing our understanding of planet formation and the potential for life in the Universe.