The sheer number of stars to be investigated for planets by Gaia constitutes the most significant contribution that the mission will provide to the science of extra-solar planets. Within 200 parsec (~650 light-years) of the Sun, and limiting counts to bright solar-type main-sequence stars, about 3 × 105 objects are predicted to exist. These are stars brighter than 13th magnitude and with spectral types earlier than K5: young stars in the process of fusing hydrogen into helium in their cores.
Based on current estimates of the distribution of planets and their orbits, Gaia will be capable of discovering thousands of extra-solar planets around these stars, using both astrometric and transit methods to detect them.
Using Gaia's microarcsecond-precision to measure stellar positions, planetary systems are discovered through the wobble they introduce in the parent star's position. This astrometry method is suitable for detecting long-period planets, but requires precise measurements and long time spans.
Photometry of Planetary Transits
For those systems with the proper alignment of the orbit relative to the Earth, transits of an extra-solar planet across the disc of its parent star can be observed. Gaia will see many of such events during its mission, and for stars brighter than ~16th magnitude will be able to make detailed observations. Its millimagnitude accuracy will allow the easy detection of Jupiter-size planets around Sun-like stars down to 14th magnitude. For known planets around bright stars, Gaia photometry may yield significant additional information.
The results derived from Gaia will help to improve our understanding of planetary orbital parameters and the distribution of exoplanets over their mass. They provide important data to constrain theoretical models of the formation, migration, and dynamical evolution of planetary systems.
Among the goals of Gaia's search for exoplanets are: