A star with an orbiting planet does not sit still. The planet pulls on the star, causing it to wobble slightly. That wobble changes the star's motion toward and away from Earth, which shifts the wavelengths of its spectral lines. When the star moves toward us, its light shifts blue. When it moves away, it shifts red. Measuring that shift reveals the planet.

This is the radial velocity method, and it was the first technique to detect an exoplanet around a sun-like star. In 1995, Michel Mayor and Didier Queloz used it to find 51 Pegasi b, a hot Jupiter orbiting its star in four days. The discovery launched the field of exoplanet science.

The method works best for massive planets close to their stars, because they induce the largest wobbles. A Jupiter-mass planet might cause its star to move at 12 meters per second. Earth causes the Sun to move at only 9 centimeters per second. Detecting an Earth twin requires instruments sensitive to that tiny shift, which is why the method is still pushing the limits of precision.

Radial velocity complements the transit method. Transits tell us a planet's size. Radial velocity tells us its mass. Together they give density, which hints at composition. The combination is how astronomers determine whether a planet is rocky, gaseous, or something in between.