Radial velocity extrasolar planet detection method, illustration. At top, the spectrum of light (right) from a star (yellow) is shown with its spectral lines. These lines shift depending on whether the star is moving towards (centre) or away (bottom) from the Earth (a phenomenon known as the Doppler effect). The star will move slightly (a gravitational 'wobble') if there is a planet (blue) orbiting it (red arrows). At centre, the spectral lines are blue-shifted as the star moves towards the Earth, and at bottom the spectral lines are red-shifted as the star moves away from the Earth. Together with transit photometry, this is one of the main methods of detecting extrasolar planets. As of 2019, over 4000 extrasolar planets have been discovered by these and other methods.