In the same way as the radial-velocity method, precise astrometry takes advantage of the binary nature of a star+planet system, and allows for the detection of the wobble of the central star position on the sky induced by the perturbing effect of the planet. Its sensitivity range is, however, different to that for radial velocities, being more sensitive to planets at larger separations, typically beyond 1 AU (depending on the star distance from the Sun, and the span and precision of the measurements). Interestingly, because the method is much less sensitive to stellar variability, astrometric measurements may be conducted on stars for which radial velocity surveys have difficulty in getting precise measurements (e.g. young stars or massive stars), enlarging the parameter space of planet-search surveys. On the ground, astrometric searches for planets need accuracies that are not routinely reached for bright stars yet. For fainter targets, however, very encouraging results have been obtained with narrow field astrometry on large aperture telescopes that uses a large number of reference stars present in the field of view.
The Geneva exoplanet group is developing scientific activities using astrometry in the framework of several projects:
- The development of the PRIMA DDL instrument for the VLT, with the aim of reaching 10 micro-arcsec astrometry for differential interferometric measurements.
- The PALTA project with FORS/VLT aiming at the detection of planets orbiting brown dwarfs primaries. A precision of the order of 100 micro-arcsec is obtained by correcting the distortions of the field of view using reference stars.
- The GAIA precise astrometric measurements allowing the detection of most of the giant planets with separation between 1 and 7 AU from virtually all bright stars of the solar neighbourhood.
- Direct planet imaging with SPHERE/VLT, following the evolution of the orbits of planets in the outer regions of the systems.