Which detection method finds exoplanets by measuring Doppler shifts in a star's spectrum?

Radial velocity finds exoplanets by measuring Doppler shifts in a star’s spectrum as the star is pulled back and forth by an orbiting planet.

A planet and its star orbit their shared center of mass. The star’s motion toward Earth shifts its spectral lines slightly toward blue, while motion away shifts them toward red. Repeated measurements of this alternating pattern reveal the orbital period and the star’s radial-velocity amplitude.

The method was proposed for planet hunting by Otto Struve in 1952, but early instruments were not precise enough. Better spectrographs, especially ELODIE at France’s Haute-Provence Observatory, helped Michel Mayor and Didier Queloz identify 51 Pegasi b in 1995. Their discovery was the first confirmed planet around a Sun-like main-sequence star.

Radial velocity does not usually measure a planet’s radius directly. Instead, it gives a minimum mass because the true mass depends on the orbit’s inclination. Combining it with transit photometry provides a much fuller picture.