Exoplanets are discovered primarily by detecting their effects on stars or on light passing through planetary systems, because directly seeing an ordinary planet beside a bright star is extremely difficult. The major detection methods include the transit method, radial velocity method, gravitational microlensing and direct imaging. Other approaches include astrometric measurements, transit timing variations and observations of planets orbiting unusual stellar remnants. Each technique is sensitive to different planetary properties and orbital configurations. Consequently, no single method provides a complete census of exoplanets. Astronomers often seek independent confirmation using a second technique or additional observations because apparent signals can arise from stellar activity, eclipsing binary stars, instrumental effects or other astrophysical phenomena.
The transit method monitors a stars brightness over time and searches for repeated small decreases in brightness. If a planet passes between the observer and its star, it blocks a small fraction of the stellar light. A periodic sequence of similar dips can indicate an orbiting planet. The depth of the dip provides information about the ratio of the planetary radius to the stellar radius, while the interval between transits provides the orbital period.
The duration and detailed shape of the light curve provide additional information about orbital geometry. Transit surveys are especially productive because a single instrument can monitor many thousands or millions of stars, allowing statistical searches for repeated patterns. The principal limitation is that the planets orbit must be oriented so that transits are visible from Earth.
The radial velocity method detects the movement of a star caused by the gravitational influence of an orbiting planet. A planet and its star orbit their common centre of mass, so the star moves slightly back and forth rather than remaining perfectly stationary. As the star moves towards or away from Earth, the wavelengths of spectral lines shift because of the Doppler effect. Repeated measurements can reveal a periodic radial velocity signal.
The amplitude of this signal depends on the planets mass, the stars mass, the orbital period and the orbital inclination. Radial velocity measurements therefore provide valuable information about planetary mass, although the method generally measures a minimum mass unless the orbital inclination is independently known.
Gravitational microlensing uses the curvature of spacetime produced by gravity. When a foreground star passes close to the apparent line of sight to a more distant background star, the foreground systems gravity can magnify the background stars light. If the foreground star has a planet, the planets gravitational field can produce a short additional deviation in the brightness curve.
Microlensing is particularly useful for detecting planets at distances and orbital separations that are difficult to investigate using transit or radial velocity methods. However, microlensing events are generally one time occurrences because the necessary alignment does not normally repeat. This makes follow up observations and detailed characterisation more difficult than for transiting planets.
Direct imaging attempts to observe light or thermal radiation from the planet itself while suppressing the much brighter light of the host star. This is technically demanding because the contrast between a star and planet can be enormous, especially when observing reflected visible light from an old, relatively cool planet. Coronagraphs can block or suppress starlight, while adaptive optics correct atmospheric distortion in ground based observations.
Direct imaging is most effective for relatively young, massive and widely separated planets because young planets can retain substantial heat from formation and planets far from their stars are easier to separate angularly from the stellar glare. Spectroscopy of directly imaged planets can reveal atmospheric molecules and temperatures.
Astrometry searches for small changes in a stars apparent position caused by the gravitational pull of orbiting planets. Instead of measuring motion toward and away from Earth as radial velocity does, astrometry measures motion across the sky. The signal is particularly useful for massive planets on relatively wide orbits, although extremely precise measurements are required. Another method involves transit timing variations, in which the times of otherwise regular planetary transits are shifted slightly by gravitational interactions with additional planets. These deviations can reveal planets that do not themselves transit. Transit duration variations and orbital variations can provide related information about system architecture and gravitational interactions.
Exoplanet detection requires careful statistical and observational validation because many non planetary phenomena can imitate planetary signals. An eclipsing binary system can produce periodic brightness changes resembling a planetary transit, while star spots and stellar activity can create apparent variability. Instrumental systematics can also produce repeated signals. Astronomers therefore examine the signal shape, periodicity, stellar properties, possible contaminating sources and independent observations. Spectroscopy can help identify binary companions, while high resolution imaging can reveal nearby stars that could contaminate a photometric measurement. The distinction between a planet candidate and a confirmed planet depends on the available evidence and the reliability of the inferred explanation.
Modern exoplanet discovery increasingly combines several methods and large astronomical databases. Space telescopes provide exceptionally stable photometry because they avoid atmospheric fluctuations, while ground based telescopes provide extensive spectroscopic and follow up capabilities. Surveys such as Kepler and the Transiting Exoplanet Survey Satellite have identified large populations of candidates through transit observations, while radial velocity programmes have measured stellar motions and confirmed or characterised many systems. Future instruments are designed to improve sensitivity to smaller planets and atmospheric signatures. The overall scientific goal is not merely to count planets but to determine how common different types of planets are, how planetary systems form and evolve, and which environments may provide useful targets for detailed planetary and astrobiological investigation.
