Exoplanet

An exoplanet is a planet outside the Solar System. Most known examples orbit other stars in the Milky Way, while some planets may travel through space without being bound to a star. Observed exoplanets span a wide range of sizes, masses, temperatures and orbital arrangements. The growing catalogue is shaped by the limits and selection effects of observing techniques: some methods are more sensitive to large planets or to particular orbital orientations, so discovered systems are not a complete census of all planetary systems.

The transit method detects a planet when it passes across the face of its host star from our line of sight. The planet blocks a small fraction of the star’s light, creating a repeating dip in a light curve. The depth of the dip can help estimate a planet’s size relative to its star, while the time between dips gives its orbital period. A transit only occurs for suitable viewing geometry, so many planets do not transit from Earth’s perspective.

A planet and its star both orbit their shared centre of mass. The star therefore moves slightly in response to the planet’s gravitational pull. Astronomers can detect this motion by measuring periodic shifts in the wavelengths of stellar light as the star moves toward and away from us. Combining radial-velocity measurements with other observations can constrain a planet’s mass; by itself, the method commonly measures a minimum mass because the orbital inclination may be unknown.

Astronomers also use techniques including direct imaging and gravitational microlensing to find or study planets. Each method works under different conditions and reveals different information. When a planet transits, some starlight may pass through its atmosphere before reaching a telescope; analysing the resulting light can provide evidence about atmospheric constituents. Such measurements require careful interpretation and do not by themselves establish that a planet is habitable or hosts life.

The first confirmed detection of a planet orbiting a Sun-like star came in the 1990s, inaugurating a rapidly expanding field. Space missions such as Kepler and the Transiting Exoplanet Survey Satellite (TESS) have used the transit method to survey stars, while ground-based observations contribute complementary measurements. The NASA Exoplanet Archive collects planet data from refereed literature and mission releases under stated inclusion criteria; its entries and counts can change as new observations and analyses are assessed. Candidate signals should not be treated as confirmed planets until their planetary status is sufficiently established.