Radial Velocity: Advanced Connections

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.

Misconception: The radial-velocity technique measures the planet's own spectral Doppler shift directly.

Correction: In the standard method astronomers measure the star's reflex motion caused by the planet's gravity.

Radial velocity measures planetary masses, detects nontransiting planets, constrains eccentricity, and combines with transit radius to derive bulk density.