The PRIMA astrometric planet search: goals and prospects

ESO's PRIMA (Phase-Referenced Imaging and Micro-arcsecond Astrometry) facility at the VLT Interferometer on Cerro Paranal in Chile is expected to be fully operational in only a couple of years from now. With PRIMA/VLTI, it will then be possible to perform relative astrometry with an accuracy of the order of 10 muas over angles of about 10". The main science driver for this astrometric capability is the detection and characterization of extrasolar planets, including (1) the observation of known radial velocity planets and planetary systems to fully constrain their orbital geometry and accurately determine the mass of the planet(s), (2) a search for extrasolar planets around stars which are less suitable for the radial velocity method (for example young and active stars as well as early type stars), and (3) a systematic search around the most nearby stars to detect low mass planets (Uranus or Neptune masses). Preparatory observations of possible target stars, with the aim of identifying nearby suitable astrometric reference stars, have already started with SOFI at the NTT and are described. Furthermore, we compare the goals and prospects of the PRIMA Astrometric Planet Search to other projects aiming at detecting planets astrometrically, mostly from space