Radial velocity is the speed at which an object moves toward or away from the observer, measured from the Doppler shift of its spectral lines. In exoplanet hunting it detects the small wobble a planet induces in its host star as the two orbit their common center of mass. The method reveals a planet's minimum mass and orbital period, and it found many of the first worlds discovered around Sun-like stars. Combined with a transit, which gives the radius, it yields the planet's density and therefore a clue to what it is made of.