Astrometric Reconnaissance of Exoplanetary Systems (ARES)

I. Methodology validation with HST point-source images of Proxima Centauri

¹ INAF – Osservatorio Astronomico di Padova, Vicolo dell’Osservatorio 5, Padova 35122, Italy
² Department of Astronomy & Astrophysics, University of California, San Diego, La Jolla, California 92093, USA

^★ Corresponding author: This email address is being protected from spambots. You need JavaScript enabled to view it.

Received: 17 October 2025
Accepted: 28 November 2025

Abstract

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