When a planet orbits a star, it exerts a gravitational pull over it, inducing a reflex motion of the star with respect to the common centre of mass of the system. The star will thus describe a small elliptical orbit with the same period as that of the planet.
Two methods aim at detecting this star wobble:
Measurements try to detect the periodic variation of the star's radial velocity induced by the presence of a planet. Most of the extrasolar planets presently known have been discovered by this technique. It is most sensitive to massive planets orbiting close to the star; Earth mass planets cannot be detected through radial velocity techniques.
Click on the thumbnail below to explore how the gravity of planets and star interact with each other:
|Dynamical Perturbation of Stars|
Measurements look for the angular change in position of a star due to the pull of an orbiting planet. This technique is most sensitive to high mass planets with large periods orbiting nearby low-mass stars. The great advantage of this method is that it allows the determination of the mass and orbital inclination of the planet. Astrometric measurements are affected by the Earth's atmosphere, so planet hunting by this method will require satellites like Gaia going to space to gather the data.