The method by which most exoplanets have been discovered involves measuring variations in a star’s velocity, thanks to the Doppler-Fizeau effect. In the same way that a planet is attracted to a star, a star is drawn to a planet. They thus gravitate around the same centre of gravity with the same orbital period. But given the much greater mass of a star, the resulting velocity for the latter is a lot smaller. For example the speed gathered by the sun under the Earth’s influence is hardly 0.4 km/h.
The movements of a star are thus influenced by the presence of a planet orbiting around it, which causes a periodic lag in its position, an oscillation. It is this oscillation which is measured by the method of radial velocities. Depending on this discrepancy it is possible to estimate the orbit of the planet as well as to deduct a minimum value for its mass. This method is all the more effective when the planet has a heightened mass and is close to its star, the reason why the majority of planets discovered up until now are gas giants with relatively short orbital periods. Read also the article An exoplanetary drama: a planet collapses on its star.