Extrasolar planet detection methods. Illustration of the two main methods of detecting extrasolar planets: the radial velocity method (top) and transit photometry (bottom). At top, the spectrum of light from a star is shown with its spectral lines. These lines shift depending on whether the star is moving towards or away from the Earth (a phenomenon known as the Doppler effect). The star will move slightly (a gravitational 'wobble') if there are planets orbiting it. At upper left, the spectral lines are blue-shifted as the star moves towards the Earth, and at upper right the spectral lines are red-shifted as the star moves away from the Earth. At centre, when a planet crosses in front of its parent star, as seen from Earth, transit photometry can be used to analyse the dip in the star's light curve (graph at lower centre of brightness versus time). As of 2019, over 4000 extrasolar planets have been discovered by these and other methods.