Astrometric Reconnaissance of Exoplanetary Systems (ARES)

We present the first results of the Astrometric Reconnaissance of Exoplanetary Systems (ARES) project, aimed at validating and characterizing candidate exoplanets around the nearest systems using multi-epoch Hubble Space Telescope (HST) data. In this first paper, we focus on Proxima Centauri, leveraging archival and recent HST observations in point-source imaging mode. We refined the geometric-distortion calibration of the HST detector used and developed a robust methodology to derive high-precision astrometric parameters by combining HST measurements with the Gaia DR3 catalog. We determined Proxima’s position, proper motion, and parallax with uncertainties at the ~0.4-mas, 50-µas yr −1 , and 0.2-mas levels, respectively. This allowed us to achieve consistent results with Gaia measurements within ~1 σ . We further investigated the presence of the candidate exoplanet Proxima c by analyzing the proper-motion anomaly derived from combining long-term HST-based and short-term Gaia astrometry. Based on the assumption of a circular, face-on orbit, we obtained an estimated mass of mc = 3.4 −3.4 +5.2 M ⊕ , which is broadly consistent with radial-velocity constraints, but still limited by our current uncertainties. These results establish the foundation for the next phase of ARES, which will exploit HST spatial-scanning observations to achieve astrometric precisions of a few tens of µ as, while also enabling a direct search for astrometric signatures of low-mass companions.