Space-borne global astrometric surveys: the hunt for extrasolar planets

The proposed global astrometry mission GAIA, recently recommended within the context of ESA's Horizon 2000 Plus long-term scientific programme, appears capable of surveying the solar neighbourhood within similar to 200 pc for the astrometric signatures of planets around stars down to the magnitude limit of V=17 mag, which includes late M dwarfs at 100 pc. Realistic end-to-end simulations of the GAIA global astrometric measurements have yielded the first quantitative estimates of the sensitivity to planetary perturbations and of the ability to measure their orbital parameters. Single Jupiter-mass planets around normal solar-type stars appear detectable out to 150 pc (V less than or equal to 12 mag) with probabilities greater than or equal to 50 per cent for orbital periods between similar to 2.5 and similar to 8 yr, and their orbital parameters are measurable with better than 30 per cent accuracy to about 100 pc. Jupiter-like objects (same mass and period as our giant planet) are found with similar probabilities out to 100 pc. These first experiments indicate that the GAIA results would constitute an important addition to those that will come from the other ongoing and planned planet-search programmes. These data combined would provide a formidable testing ground on which to confront theories of planetary formation and evolution.