Extrasolar planet detection using the radial velocity method. This is one of the main ways that astronomers can find planets around distant stars. It relies on the Doppler effect altering the light from the star in a regular way. In this diagram, the orbit of the planet is shown as a dotted line. The planet and the star orbit their common centre of mass, meaning the star varies in position, shown by the inner dotted circle. The effect has been greatly exaggerated here. At the bottom is the spectrum of light from the star. When the star is moving towards Earth, the Doppler effect shifts the spectrum towards the blue end, and when it is moving away it is redshifted. Studying the period of this cycle reveals the period of the planet磗 orbit, and also its mass.brRestrictions: