Astronomers usually cannot see an exoplanet as a bright disk next to its star. Instead, they detect the planet by measuring how its gravity or motion changes the light we receive from the host star. Different methods are sensitive to different kinds of planets, so discoveries are strongest when several observations agree.
The transit method
The transit method looks for a small, repeated dip in a star’s brightness when a planet crosses in front of the star from our point of view. The depth of the dip helps estimate the planet’s size relative to the star, while the time between transits gives the orbital period. Transits work only when the orbit is aligned so that the planet passes across the stellar disk as seen from Earth.
Because other effects can also change a star’s brightness, astronomers compare many transits, study the shape and timing of each dip and check nearby stars or instrumental effects before declaring a planet confirmed.
Radial velocity: measuring the star’s wobble
A planet and its star both orbit their common center of mass. As the star moves slightly toward and away from Earth, its spectral lines shift because of the Doppler effect. Precision spectrographs measure those changes as radial velocity. This method can estimate a planet’s minimum mass and is especially useful for confirming candidates found by transits.
When transit and radial-velocity measurements are both available, astronomers can often estimate both radius and mass. Those values provide a bulk density, which helps distinguish a rocky world from a lower-density planet with a large envelope of gas.
Direct imaging
Direct imaging attempts to separate a planet’s faint light from the much brighter light of its star. Coronagraphs and related techniques suppress starlight, while adaptive optics can reduce distortions caused by Earth’s atmosphere. Direct imaging currently favors large, young planets that are relatively far from their stars because those planets are brighter and easier to separate from stellar glare.
Gravitational microlensing
When one star passes close to the line of sight to a more distant star, its gravity can temporarily magnify the background star’s light. A planet orbiting the foreground star can add a smaller feature to that lensing signal. Microlensing can reveal planets at orbital distances and in systems that are difficult for transit or radial-velocity surveys, but the alignment event usually does not repeat.
Astrometry and timing methods
Astrometry measures tiny changes in a star’s position on the sky caused by orbiting companions. Timing methods search for regular signals, such as pulsar pulses or transit times, that arrive slightly early or late because of gravitational interactions. These techniques are important complements to the better-known transit and radial-velocity methods.
How an exoplanet candidate becomes convincing
Detection is not the same as interpretation. Starspots, stellar pulsations, eclipsing binary stars and instrument systematics can imitate some planetary signals. Researchers therefore test alternative explanations, collect additional observations and often use independent methods or statistical validation before treating a signal as a robust planet detection.
Why detection methods have biases
Every survey has a selection effect. Transit surveys are more likely to find planets with short orbital periods and favorable alignments. Radial-velocity signals are easier to measure for more massive planets close to their stars. Direct imaging favors bright planets at wider separations. Combining methods helps astronomers build a less biased picture of planetary systems.
What happens after detection?
Once a planet is found, follow-up observations may measure its orbit, mass, radius, temperature or atmosphere. Scientists can then compare many systems to study how planets form and evolve, how common different planet types are and which worlds are promising targets for deeper atmospheric observations.
