Exoplanets

An exoplanet is a planet beyond the Solar System, generally one that orbits a star other than the Sun. The term is also used alongside the study of free-floating planetary-mass objects, which do not orbit a star. Exoplanets are difficult to see directly because their light is faint compared with the glare of their host stars. As a result, astronomers have discovered most of them by detecting the planet's effects on a star or on light passing through a planetary system.

Two especially productive methods are radial velocity and transit photometry. A planet's gravity makes its star move around the system's centre of mass; radial-velocity measurements detect the star's small motion toward and away from Earth through shifts in its spectrum. A transit occurs when a planet crosses in front of its star from our viewpoint, producing a small, recurring dip in starlight. Transit observations can reveal a planet's radius and orbital period, while radial-velocity data help estimate its mass; together the measurements can constrain its average density. Other techniques include astrometry, which tracks a star's position on the sky, gravitational microlensing, which detects a planet's effect on light magnified by an intervening star, and direct imaging, which separates a planet's light from its star. Each method is sensitive to different kinds of systems, so the catalogued planets are not a simple, unbiased census of all planets in the galaxy.

Known exoplanets range from rocky bodies to gas-rich giants, and their orbits and environments vary greatly. A hot Jupiter is a gas giant on a close, short-period orbit around its star, unlike Jupiter's distant orbit around the Sun. Terms such as super-Earth and Neptune-like describe broad mass or size groupings, not confirmed surface conditions or Earth-like habitability; the boundaries between classes are not exact. Some planets orbit two stars, while other planetary-mass objects travel without a host star. This diversity has helped show that the Solar System is one example of planetary architecture rather than a universal template.

The first confirmed exoplanets were detected in 1992 around the pulsar PSR B1257+12. In 1995, Michel Mayor and Didier Queloz announced the discovery of 51 Pegasi b, the first planet found orbiting a Sun-like star. They detected it by measuring the star's radial-velocity wobble. That result demonstrated that planets could be found around ordinary stars beyond the Solar System and helped establish exoplanet research as a major area of astronomy. Later ground- and space-based surveys, notably NASA's Kepler mission and the Transiting Exoplanet Survey Satellite (TESS), used repeated measurements of starlight to identify and study many additional systems.

Finding a planet is the beginning of its study. When a planet transits its star, some starlight passes through the planet's atmosphere; examining how that light varies by wavelength can reveal absorption features associated with atmospheric molecules. Measurements of mass and radius provide clues to average density and possible bulk composition. Astronomers also compare a planet's orbit with its star's habitable zone, where temperatures could permit liquid water on a suitably conditioned surface. Location in that zone alone does not show that a planet has liquid water, a suitable atmosphere, or life. Current observations therefore support comparisons and tests of planetary conditions, not a conclusion that any particular exoplanet is inhabited.