Prospects for detecting the astrometric signature of Barnard’s Star b

¹ Department of Geophysics, Raymond and Beverly Sackler Faculty of Exact Sciences, Tel Aviv University, Tel Aviv 6997801, Israel
e-mail: levtalo@tauex.tau.ac.il
² Institut de Ciències de l’Espai (ICE, CSIC), Campus UAB, c/de Can Magrans s/n, 08193 Bellaterra, Barcelona, Spain
³ Institut d’Estudis Espacials de Catalunya (IEEC), 08034 Barcelona, Spain
⁴ School of Physics and Astronomy, Queen Mary, University of London, 327 Mile End Road, London, E1 4NS, UK
⁵ Institut für Astrophysik, Georg-August Universität, Friedrich-Hund-Platz 1, 37077 Göttingen, Germany

Received: 14 November 2018
Accepted: 9 January 2019

Abstract

A low-amplitude periodic signal in the radial velocity (RV) time series of Barnard’s Star was recently attributed to a planetary companion with a minimum mass of ~3.2 M_⊕ at an orbital period of ~233 days. The relatively long orbital period and the proximity of Barnard’s Star to the Sun raises the question whether the true mass of the planet can be constrained by accurate astrometric measurements. By combining the assumption of an isotropic probability distribution of the orbital orientation with the RV-analysis results, we calculated the probability density function of the astrometric signature of the planet. In addition, we reviewed the astrometric capabilities and limitations of current and upcoming astrometric instruments. We conclude that Gaia and the Hubble Space Telescope (HST) are currently the best-suited instruments to perform the astrometric follow-up observations. Taking the optimistic estimate of their single-epoch accuracy to be ~30μas, we find a probability of ~10% to detect the astrometric signature of Barnard’s Star b with ~50 individual-epoch observations. In case of no detection, the implied mass upper limit would be ~8 M_⊕, which would place the planet in the super-Earth mass range. In the next decade, observations with the Wide-Field Infrared Space Telescope (WFIRST) may increase the prospects of measuring the true mass of the planet to ~99%.