Methods for discovering exoplanets
The transit and radial velocity methods have enabled researchers to discover the majority of exoplanets to date.
Radial velocity method
A planet and its star always orbit a common centre of mass. This is usually much closer to the star, as the star has a greater mass. As a result, the star only moves very slightly from our point of view. This movement leads to a Doppler shift in the spectrum of the emitted starlight that reaches the Earth. If the star moves towards the Earth, its spectrum is blue-shifted. If it moves away from the Earth, it is red-shifted. By measuring the light spectrum several times in succession, researchers notice such shifts and can surmise that an exoplanet is in the vicinity of the star. The discovery of the first exoplanet, 51 Pegasi b, by Mayor and Queloz was achieved using this method.
Transit method
If a planet moves in front of the star it is orbiting, we on Earth can see that this star shines a little less brightly. Since a planet orbits its star repeatedly, researchers can detect the reduction in brightness at regular intervals and conclude that a planet exists. This method is also used by CHEOPS, the space telescope developed for ESA under the leadership of the University of Bern.
