A planet in orbit around a star causes the star to wobble slightly. If the star and planet are positioned correctly relative to Earth, the wobble shows up as a shift in the color of starlight as the star moves toward us and then away from us. This phenomenon is called the Doppler effect.

The rate of wobble reveals the presence of one or more unseen planets and gives information about their sizes and orbits. The Doppler technique was the first to succeed in tracking down extra-solar planets and, so far, it is the most widely used method.

The Doppler effect works for any kind of wave phenomena, such as light or sound. It is familiar to most people as the “EEEEEEEEyyyyuuuuuu” sound made by a passing car. The sound is at a higher pitch as the car approaches, since the sound waves are being compressed by the car’s motion toward the listener. As the car passes and recedes, its sound waves are stretched, giving them a lower pitch.

The Doppler effect also works for light waves. The back-and-forth wobble of a star, caused by an orbiting planet, makes the star appear slightly bluer as it moves toward the viewer. As the star moves away from the viewer, the star appears slightly redder.