What method for detecting exoplanets relies on the Doppler shift of starlight?

The exoplanet-detection method that relies on the Doppler shift of starlight is the radial velocity method. It detects a planet indirectly by measuring the small back-and-forth motion that the planet induces in its star.

A planet and its star orbit their shared center of mass. As the star moves toward Earth, its spectral lines shift slightly toward shorter, bluer wavelengths; as it moves away, the lines shift toward longer, redder wavelengths. Repeated measurements reveal this periodic wobble and can indicate the planet’s orbital period and a minimum mass.

The method is also called Doppler spectroscopy or the wobble method. It works best when the star’s motion toward and away from Earth is measurable. Because the observed motion depends on orbital inclination, radial velocity usually gives a minimum planetary mass rather than the exact mass.

This method differs from the transit method, which detects a planet crossing in front of its star and temporarily reducing the star’s brightness. Microlensing and direct imaging are also separate techniques. Radial velocity can find planets that do not transit, although stellar activity and instrument stability can complicate the measurement.