Exoplanet Detection is the process of identifying planets orbiting stars outside the Solar System through observations of their effects on light, stellar motion, gravitational fields, or other measurable phenomena. Because exoplanets are usually extremely faint compared with their host stars and lie at great distances, most detections are indirect. The principal methods include the transit method, radial velocity method, gravitational microlensing, direct imaging, and astrometric detection.
Additional techniques can detect planets through transit timing variations, orbital phase effects, stellar activity signatures, and other dynamical or photometric phenomena. A detection normally involves identifying a signal, testing whether it is consistent with a planetary interpretation, estimating orbital and physical parameters, and excluding plausible false positives. Exoplanet detection is therefore an observational and statistical problem rather than simply the visual discovery of a distant world.
The transit method detects the periodic reduction in stellar brightness that occurs when a planet passes across the apparent disk of its host star. The depth of the transit depends primarily on the ratio of planetary radius to stellar radius, approximately following the square of that radius ratio for a simple uniform stellar model. The duration and shape of the transit contain information about orbital geometry, stellar density, and impact parameter.
Repeated periodic events strengthen the planetary interpretation, while transit timing and duration can reveal additional dynamical information. Transit observations also provide opportunities for atmospheric spectroscopy because some starlight passes through the planet's atmosphere during transit. The method is geometrically selective because the planetary orbit must be aligned sufficiently close to the observer's line of sight. Consequently, transit surveys detect only a fraction of the planets that exist around their target stars, and detection rates must be corrected for this geometric selection effect when estimating populations.
The radial velocity method detects the gravitational reflex motion of a star caused by an orbiting planet. A planet and its star orbit their common centre of mass, causing the star to move periodically toward and away from the observer. This motion produces Doppler shifts in stellar spectral lines. The measured velocity amplitude depends on planetary mass, orbital period, orbital eccentricity, stellar mass, and orbital inclination.
Unlike transit observations, radial velocity measurements do not require the orbit to pass directly across the stellar disk, although the inclination introduces a mass degeneracy. The method therefore generally provides a minimum planetary mass rather than a direct mass unless inclination is independently known. High precision radial velocity spectroscopy requires extremely stable instruments, accurate wavelength calibration, careful modelling of stellar activity, and long observational baselines. Stellar spots, granulation, oscillations, and magnetic activity can produce velocity variations that mimic or obscure planetary signals.
Gravitational microlensing detects planets through the temporary magnification of a background star when a foreground planetary system passes near the line of sight. The gravity of the foreground system bends light and acts as a gravitational lens. A planet can create a short perturbation superimposed on the broader microlensing event.
The method is particularly sensitive to planets at orbital separations that are difficult to probe with transit or radial velocity surveys and can detect planets around relatively faint or distant stars. A major limitation is that microlensing events are generally one time occurrences because the alignment does not normally repeat. Follow up observations can therefore be difficult, and the detected planetary system may not remain directly observable after the event. Microlensing is nevertheless important for population studies because it probes a different region of planetary parameter space from many traditional surveys.
Direct imaging attempts to detect photons from an exoplanet itself rather than measuring only its influence on the host star. The main challenge is the enormous contrast between the planet and star, particularly at small angular separations. Coronagraphs, adaptive optics, specialised observing strategies, and image processing can suppress starlight and reveal relatively bright planets under favourable conditions.
Direct imaging is particularly suited to young, massive, self luminous giant planets at relatively large orbital separations because these planets can emit substantial infrared radiation. Spectroscopy of directly imaged planets can provide information about atmospheric composition, temperature, clouds, and chemistry. The method is less sensitive to small Earth sized planets close to Sun like stars because of the contrast and angular separation requirements. Nevertheless, direct imaging provides one of the most direct opportunities to characterise exoplanet atmospheres and potentially study reflected light from smaller planets as instrumentation improves.
Astrometry detects the tiny apparent movement of a star across the sky caused by the gravitational pull of an orbiting planet. The star moves around the system's centre of mass, producing a periodic positional wobble. Astrometric amplitude depends strongly on planetary mass and orbital distance and is less affected by the geometric limitations of radial velocity for certain orbital configurations. High precision astrometry requires exceptionally stable measurements because planetary signals can correspond to extremely small angular displacements.
Space based observations are particularly valuable because Earth's atmosphere introduces image motion and other systematic effects. Astrometry can complement radial velocity and transit observations by providing orbital inclination and therefore helping determine true planetary masses. Long observational baselines are important for planets with long orbital periods.
Exoplanet detection requires rigorous false positive analysis because many astrophysical phenomena can imitate planetary signals. In transit surveys, eclipsing binary stars, blended background systems, stellar activity, instrumental systematics, and statistical noise can produce periodic brightness variations. Radial velocity signals can arise from stellar spots, magnetic activity, pulsations, or spectral contamination. Candidate validation therefore uses multiple observations, statistical tests, stellar characterisation, and sometimes independent detection methods.
Planetary parameters also depend on properties of the host star, particularly stellar radius and mass, so inaccurate stellar measurements propagate directly into planet estimates. Detection surveys must account for observational completeness and selection effects when deriving occurrence rates. A catalogue containing many detected planets is not automatically representative of the underlying population because each detection method preferentially finds particular planet sizes, masses, orbital periods, stellar types, and geometries.
The development of Exoplanet Detection has transformed planetary astronomy from a field based largely on Solar System examples into a comparative study of planetary systems throughout the galaxy. The first widely accepted discoveries of planets around ordinary main sequence stars demonstrated that planetary systems are common and can differ substantially from the Solar System. Subsequent surveys have identified planets with a wide range of masses, radii, orbital periods, densities, and host star environments.
Detection alone, however, does not establish atmospheric composition, surface conditions, habitability, or life. Those questions require additional observations and modelling. The central scientific value of detection is that it provides the foundational population from which planetary formation, migration, evolution, and system architecture can be investigated. Combining transit, radial velocity, microlensing, astrometry, imaging, and follow up spectroscopy produces a more complete picture of planetary diversity than any single method can provide.
