Radial Velocity: Physical Mechanisms

A radial-velocity curve records line-of-sight stellar speed versus time. Its amplitude depends on planet mass, stellar mass, orbital period, eccentricity, and inclination. Because inclination is usually unknown without a transit or astrometry, the measurement yields minimum mass, often written m sin i.

Star and planet both orbit their barycenter. As the star moves toward Earth, spectral lines shift slightly toward shorter wavelengths; as it moves away they shift longer. High-resolution stabilized spectrographs measure shifts corresponding to speeds of meters per second or less.

The discovery of 51 Pegasi b and many subsequent planets came from periodic stellar Doppler signals. Independent transit detections and astrometry confirm that these signals correspond to orbiting planets in many systems.

The star alternately approaches and recedes as both bodies orbit their common center of mass. More massive or closer planets generally produce larger stellar velocity amplitudes.