Doppler spectroscopy, also known as Doppler radial velocity spectroscopy, is a technique used primarily in astronomy to detect and characterize exoplanets and other celestial bodies. It leverages the Doppler effect, which describes the change in frequency or wavelength of light (or other waves) in relation to an observer moving relative to the source of that light.
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The radial-velocity method detects an exoplanet through the star's orbital Doppler shift. It measures a projected planetary mass , not the planetary radius. Combining it with exoplanet transit photometry determines mass and radius when the stellar properties are known.