English: A star and its companion planet both orbit the centre of mass, which results in a small "wobble" of the star (top left). When viewed edge-on (bottom left), the star moves periodically towards and away from an observer (in the direction out of the screen and into the screen). This radial speed (top right) can be measured using doppler spectroscopy, which is a technique that splits the star's light into a spectrum of different colours. Absorption lines in the spectrum are due to chemicals in the star's atmosphere, and their exact colours are determined by the chemical structure. The radial speed of the star causes a doppler shift of the star's light, which can be detected if the absorption lines are different from their expected value (bottom right).