Radial velocity
also called the wobble method
Finding a planet by the small gravitational wobble it tugs in its star, seen as a rhythmic shift in the star's light.
A planet does not simply orbit its star: the two swing around their shared center of mass, so the star traces a small circle of its own. The radial-velocity method detects that stellar wobble by watching the star's light shift slightly redder as it moves away and bluer as it comes back, a rhythmic beat that repeats once per orbit. The size of the wobble reveals the planet's mass.
Because the wobble depends on the orbit's unknown tilt, radial velocity usually gives a minimum mass rather than an exact one. It was the method behind the first planet ever found around a Sun-like star, and it is still the best way to weigh worlds that never transit. gravityfinder takes many of its masses from these measurements and is careful to flag the ones that are only lower bounds.
Related terms: Transit method · Minimum mass (M sin i)