This is the most used method to search for exoplanets, it works measuring the change of brightness of a star when one of its planets passes in front of it, if this decrease of brightness is periodically, it means that there is a planet between the star and us. The amount of obscurity depends on the distance and the mass from the planet and the star, using Doppler spectroscopy method, you can tell the size and its distance from a light curve.
The radial velocity method for detecting exoplanets is based on detecting variations in the velocity of the central star, due to the change in direction of an exoplanet's gravitational force as it orbits the star. When the star moves towards us, its spectrum is blueshifted while it is redshifted when it moves away from us.
