How do astronomers use the Doppler effect to detect planets around other stars?
Astronomers discover distant planets by detecting the subtle wobbles they cause in their host stars.
As a planet orbits its star, gravity pulls the star toward the planet, making the star move in a small circle. This motion shifts the star's light toward blue as it moves toward Earth and toward red as it moves away—an effect called the Doppler shift. By measuring these color changes, scientists can determine the planet's mass and orbital characteristics.
Nerd Mode
The Radial Velocity Method, also known as Doppler Spectroscopy, was the first successful technique for discovering exoplanets. It is based on the principle that a planet does not orbit a star in isolation. Instead, both the planet and star orbit their shared center of mass, called the barycenter. Because the star is far more massive than the planet, this center lies deep within the star's interior, causing the star to appear to wobble in place.This motion is detected using the Doppler effect, which describes how wave frequency changes relative to an observer. In 1995, Michel Mayor and Didier Queloz of the University of Geneva used this method to discover 51 Pegasi b, the first exoplanet found orbiting a Sun-like star. They made this discovery using the ELODIE spectrograph at the Haute-Provence Observatory in France. This landmark achievement earned them the Nobel Prize in Physics in 2019.Modern instruments like the High Accuracy Radial velocity Planet Searcher (HARPS) can detect stellar movements as slow as 3.5 kilometers per hour—roughly the speed of a walking person. By analyzing the period and amplitude of these shifts, astronomers can calculate the planet's minimum mass and its distance from the star. Although the Transit Method now discovers more planets, the Doppler method remains crucial for confirming a planet's mass and density.
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FP-0003739 · Feb 18, 2026