2026A&A...706A.112L

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 ∼1σ. 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^+5.2M_⊕, 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 µas, while also enabling a direct search for astrometric signatures of low-mass companions.