Illustration of the Doppler effect for detection of planets around nearby stars. A planet does not orbit a star while the latter stays fixed in space. Instead, both star and planet are in motion. They each orbit their common centre of mass (red dot). This is the point between them - closer to the star by virtue of its greater mass - where they would balanced if joined with a stick. The star moves in a much smaller orbit than the planet, a motion astronomers often refer to as a 'wobble'. While the star's motion is directed away from Earth (left panel) its light is shifted towards the red end of the spectrum. When the star approaches us, the light is blueshifted. This periodic Doppler shifting can be detected using sensitive equipment, and used to infer the presence of a planet even if it cannot be seen directly.
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