16653

Proxima Centauri is the closest star to the Sun and the closest known exo-planetary system to Earth. It is therefore a key benchmark for detailed investigations of multi-planet system architecture comma particularly for low mass stars. Recent ground-based radial velocity campaigns indicate the presence of at least one --and possibly a second-- exo-planet orbiting Proxima Centauri. To test the multi-planet configuration of this system comma we propose to use HST in spatial-scanning mode to obtain the most precise relative astrometry for Proxima Centauri to date (virgul20 micro-arc-seconds). The proposed sequence of six (6) epochs of observations over three (3) years will unambiguously detect the astrometric perturbation of Proxima Centauri induced by the hypothesized planet c comma and constrain that planet.s orbital geometry and mass.