The radial velocity method for finding exoplanets

This animation shows how astronomers use very precise spectrographs such as the HARPS instrument on the 3.6-metre telescope at La Silla, Chile to find exoplanets. As the planet orbits its gravitational pull causes the parent star to move back and forth. This tiny radial motion shifts the observed spectrum of the star by a correspondingly small amount because of the Doppler shift. With super-sensitive spectrographs such as HARPS the shifts can be measured and used to infer details of a planet’s mass and orbit.