Astrometry concerns the measurement of positions and motions of solar system bodies, stars within the Galaxy and, in principle, galaxies and clusters within the Universe. Traditionally, an important goal has been to determine stellar parallaxes and proper motions.
In the present context, repeated high-accuracy astrometry aims to determine the transverse component of the displacement of the host star due the gravitational perturbation of an orbiting planet. This dynamical manifestation of their gravitational influence in the plane of the sky is closely related to radial velocity measurements, which are sensitive to the corresponding photocentre displacement along the line-of-sight.
Current best accuracies, of around 1 mas achieved with Hipparcos and the HST–Fine Guidance Sensors, only just touch the regime where displacements of star positions due to orbiting planets can be detected.
The panorama will change with Gaia, due for launch in 2012, which will measure all billion stars to V = 20 mag, at accuracies of ∼ 20 – 25μas at 15 mag. This should lead to several thousand star–planet systems discovered astrometrically. Absolute orbits, planet masses without the sin i ambiguity, and information on coplanarity will become available. Developments in interferometry, notably with VLTI–PRIMA, or possibly from space as proposed for SIM, also hold promise.
A related manifestation of the dynamical displacement of the primary is the modulation of timing signals of rotating, pulsating, or eclipsing systems.