Most exoplanets are too small, too faint, and too close to a bright star to photograph directly. Astronomers therefore find many of them by measuring the effect a planet has on its star or on light passing through the planetary system. Several detection methods are used because each is sensitive to different kinds of planets and orbits.
The transit method: watching a star dim
If a planet passes between its star and our line of sight, it blocks a small fraction of the starlight. Repeated dips at regular intervals can reveal a planet’s orbital period. The depth of the dip helps astronomers estimate the planet’s size relative to its star.
Transit surveys have discovered large numbers of planets because one telescope can monitor many stars. The method has a geometric limitation, however: the orbit must be aligned so the planet crosses the star from our viewpoint. Other phenomena, including eclipsing binary stars, can also imitate a transit, so candidates require careful validation.
Radial velocity: measuring a star’s wobble
A planet and its star orbit a common center of mass. As the star moves slightly toward and away from Earth, its spectrum shifts because of the Doppler effect. Precise spectrographs can measure those changes. The radial-velocity signal reveals the orbital period and provides a lower limit on the planet’s mass.
Transit and radial-velocity data are especially powerful together. If researchers can estimate both size and mass, they can calculate bulk density and begin to distinguish rocky worlds from gas-rich planets.
Direct imaging
Direct imaging tries to separate the faint light of a planet from the much brighter light of its star. Instruments can block or suppress starlight and use sophisticated image processing to reveal nearby companions. This is difficult, but it can work particularly well for large, young planets that orbit far from their stars and still glow strongly in infrared light.
Gravitational microlensing and astrometry
In gravitational microlensing, the gravity of a foreground star bends and magnifies light from a more distant background star. A planet around the foreground star can add a brief feature to the magnification pattern. Microlensing can detect planets at large distances and in orbital ranges that are difficult for some other methods, but the alignment event usually does not repeat.
Astrometry measures tiny changes in a star’s position on the sky. A sufficiently massive orbiting planet can cause a periodic positional wobble. The measurements are demanding because the angle involved can be extremely small.
How astronomers confirm a candidate
No single signal should be interpreted in isolation. Researchers examine whether the event repeats, whether nearby stars contaminate the measurement, whether stellar activity could produce the same pattern, and whether another instrument can detect a compatible signal. Statistical validation is also used for some candidates.
After discovery, astronomers may study a planet’s atmosphere using spectroscopy, especially for transiting systems. For background on these worlds themselves, see What Are Exoplanets?
