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