GAIA and Eddington

In October 2000, the GAIA astrometric mission was approved as one of the next two 'cornerstones' of ESA's science programme, with a launch date target of 2010-12. GAIA will provide astrometric measurements with the accuracies needed to produce a stereoscopic and kinematic census of about one billion stars throughout our Galaxy (and into the Local Group), amounting to about 1 per cent of the Galactic stellar population. GAIA's main scientific goal is to clarify the origin and history of our Galaxy, from a quantitative census of the stellar populations. The survey targets astrometric accuracies of 10 muas at 15 mag, with completeness to V = 20 mag. In addition, comprehensive radial velocity measurements, and extensive multi-epoch multi-band photometry will be obtained. The direct distances measurements will be directly relevant to the understanding of the Eddington data on stellar oscillations, for example by determining luminosities, and allowing these to be compared with oscillation-driven models. In the area of extra-solar planet research, GAIA will make at least three distinct contributions: (i) it will yield precise astrophysical parameters for planetary systems detected by other techniques (e.g. radial velocity or transits). (ii) it will detect some 10 000-20 000 massive planetary systems out to of order 200 pc by virtue of its astrometric accuracy; and (iii) it will probably allow detection of several thousand transiting planets from the photometric data.