A planet does not just orbit a star — the star also wobbles slightly due to the planet's gravity. The radial velocity method detects this wobble by measuring tiny shifts in the star's light spectrum. As the star moves toward Earth, its light is blue-shifted; as it moves away, it is red-shifted. This reveals the planet's mass and orbital period.

The radial velocity method found the first exoplanet orbiting a Sun-like star in 1995: 51 Pegasi b, a Jupiter-sized world that circles its star in just 4 days. The discovery earned the 2019 Nobel Prize in Physics.