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.

Astrometry

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.

Science Goals

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: 

• Astrometry will confirm the existence of Jupiter sized planets from radial-velocity measurements, and will extend spectroscopic surveys to the large data base of nearby M-dwarfs, complementing ground-based observations
• Gaia will measure the inclinations of the orbital planes
• The Gaia measurements will provide estimates of the actual planet masses, thus contributing to models establishing whether or not dynamical interactions would permit an Earth-like planet to form and survive in the habitable zone of any given star