SIM PlanetQuest will be a powerful tool for discovering planets around nearby stars, through detection of the stellar reflex motion. The astrometric method complements the radial velocity technique which has already yielded many new planets and has the additional benefit of measuring planetary masses rather than mass lower limits. In a frame defined by nearby reference stars, the singlemeasurement precision of SIM will be 1 microarcsecond, enabling searches for planets with masses as small as a few Earth masses around the nearest stars. SIM will be able to fully characterize multiple-planet systems which are now known to exist. It will explore the nature and evolution of planetary systems in their full diversity, including age, by including young (0.5-100 Myr) solar-type stars. The mission will also serve as a precursor for future astrophysics missions using interferometers, demonstrating several needed technologies.

SIM PlanetQuest will make global astrometric measurements by observing an all-sky grid of reference stars, anchored by observations of distant quasars. Relative to this frame, the mission will deliver positions and parallaxes to 4 microarcsecond accuracy on stars as faint as V = 20. This unprecedented precision will allow stellar masses and luminosities to be measured to accuracies better than 1%, which is currently very hard. By observing samples of stars in the Galactic halo, SIM PlanetQuest will probe the gravitational potential of the Galaxy, and trace its past history of interactions with dwarf companions. Finally, SIM PlanetQuest will probe some of the most powerful objects in the universe, through astrometry of the structure of the nuclei of active galaxies.