Letter to the Editor

Astrometric detection of a Neptune-mass candidate planet in the nearest M-dwarf binary system GJ65 with VLTI/GRAVITY

¹ Max Planck Institute for Extraterrestrial Physics, Giessenbachstraße 1, 85748 Garching, Germany
² LESIA, Observatoire de Paris, Université PSL, CNRS, Sorbonne Université, Université de Paris, 5 Place Jules Janssen, 92195 Meudon, France
³ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁴ European Southern Observatory, Karl-Schwarzschild-Straße 2, 85748 Garching, Germany
⁵ Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
⁶ 1st Institute of Physics, University of Cologne, Zülpicher Straße 77, 50937 Cologne, Germany
⁷ CENTRA – Centro de Astrofísica e Gravitação, IST, Universidade de Lisboa, 1049-001 Lisboa, Portugal
⁸ Universidade de Lisboa – Faculdade de Ciências, Campo Grande, 1749-016 Lisboa, Portugal
⁹ European Southern Observatory, Casilla 19001, Santiago 19, Chile
¹⁰ Faculdade de Engenharia, Universidade do Porto, Rua Dr. Roberto Frias, 4200-465 Porto, Portugal
¹¹ Hamburger Sternwarte, Universität Hamburg, Gojenbergsweg 112, 21029 Hamburg, Germany
¹² Departments of Physics & Astronomy, Le Conte Hall, University of California, Berkeley, CA 94720, USA
¹³ Max Planck Institute for Astrophysics, Karl-Schwarzschild-Straße 1, 85748 Garching, Germany
¹⁴ Max Planck Institute for Radio Astronomy, Auf dem Hügel 69, 53121 Bonn, Germany
¹⁵ ORIGINS Excellence Cluster, Boltzmannstraße 2, 85748 Garching, Germany
¹⁶ Department of Physics, Technical University of Munich, 85748 Garching, Germany
¹⁷ Dublin Institute for Advanced Studies, 31 Fitzwilliam Place, D02 XF86 Dublin, Ireland
¹⁸ School of Physics, University College Dublin, Belfield, Dublin 4, Ireland
¹⁹ Institute of Astronomy, University of Cambridge, Madingley Rd., Cambridge CB3 0HA, UK

Received: 8 February 2024
Accepted: 20 March 2024

Abstract

The detection of low-mass planets orbiting the nearest stars is a central stake of exoplanetary science, as they can be directly characterized much more easily than their distant counterparts. Here, we present the results of our long-term astrometric observations of the nearest binary M-dwarf Gliese 65 AB (GJ65), located at a distance of only 2.67 pc. We monitored the relative astrometry of the two components from 2016 to 2023 with the VLTI/GRAVITY interferometric instrument. We derived highly accurate orbital parameters for the stellar system, along with the dynamical masses of the two red dwarfs. The GRAVITY measurements exhibit a mean accuracy per epoch of 50−60 ms in 1.5 h of observing time using the 1.8 m Auxiliary Telescopes. The residuals of the two-body orbital fit enable us to search for the presence of companions orbiting one of the two stars (S-type orbit) through the reflex motion they imprint on the differential A–B astrometry. We detected a Neptune-mass candidate companion with an orbital period of p = 156 ± 1 d and a mass of m_p = 36 ± 7 M_⊕. The best-fit orbit is within the dynamical stability region of the stellar pair. It has a low eccentricity, e = 0.1 − 0.3, and the planetary orbit plane has a moderate-to-high inclination of i > 30° with respect to the stellar pair, with further observations required to confirm these values. These observations demonstrate the capability of interferometric astrometry to reach microarcsecond accuracy in the narrow-angle regime for planet detection by reflex motion from the ground. This capability offers new perspectives and potential synergies with Gaia in the pursuit of low-mass exoplanets in the solar neighborhood.