What methods are used to detect exoplanets?
The radial velocity technique relies on measuring stellar wobbles introduced by an unseen exoplanet orbiting a star. Both the star and planet orbit their common centre of mass, which is offset somewhat from the centre of the star. When searching for this wobble, we can measure the speed at which the star is moving towards or away from us over its orbit thanks to the Doppler effect. Starlight can appear slightly bluer or redder when the star is moving towards or away from us. The effect is very small and not perceptible to the eye, but by precisely measuring the spectrum of a star with spectrographs, we can pick out these radial velocity shifts.
The most prolific discovery technique is the transit method. When a planet’s orbit is aligned with our line of sight to the star, the planet will pass directly in front of the star once per orbit and block some starlight. We find transiting exoplanets by measuring the brightness of a star and looking for telltale periodic dips. The shape of these transit events tell us the size of the exoplanet and the inclination of its orbit, so along with information from radial velocity measurements, we can measure its true mass and density. Dr Benjamin Rackham is a member of the exoplanet-hunting team, Project EDEN