The GAPS programme at TNG

LXX. HD128717 B/Gaia-6 B: A long-period eccentric low-mass brown dwarf from astrometry and radial velocities^★

¹ INAF – Osservatorio Astrofisico di Torino, Via Osservatorio 20, 10025 Pino Torinese, Italy
² INAF – Osservatorio Astronomico di Padova, vicolo dell’Osservatorio 5, 35122 Padova, Italy
³ INAF – Osservatorio Astronomico di Roma, Via Frascati 33, 00078 Monte Porzio Catone, Italy
⁴ INAF – Osservatorio Astrofisico di Catania, Via S. Sofia 78, 95123 Catania, Italy
⁵ INAF – Osservatorio Astronomico di Palermo, piazza del Parlamento 1, 90134 Palermo, Italy
⁶ Department of Astronomy and Steward Observatory, The University of Arizona, 933 North Cherry Ave, Tucson, AZ 85721, USA
⁷ Fundación Galileo Galilei – INAF, Ramble José Ana Fernandez Pérez 7, 38712 Breña Baja, TF, Spain
⁸ School of Earth and Space Exploration, Arizona State University, 781 E Terrace Mall, Tempe, AZ 85287, USA
⁹ Department of Physics, University of Turin, Via Pietro Giuria 1, 10125 Torino, Italy
¹⁰ INAF – Osservatorio Astronomico di Brera, Via E. Bianchi 46, 23807 Merate, Italy
¹¹ Dipartimento di Fisica, Università di Roma “Tor Vergata”, Via della Ricerca Scientifica 1, 00133 Roma, Italy
¹² Max Planck Institute for Astronomy, Königstuhl 17, 69117 Heidelberg, Germany
¹³ Dipartimento di Fisica e Astronomia, Università degli Studi di Padova, Vicolo dell’Osservatorio 3, 35122 Padova, Italy
¹⁴ Centro di Ateneo di Studi e Attività Spaziali “G. Colombo” – Università degli Studi di Padova, Via Venezia 15, 35131 Padova, Italy

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

Received: 25 September 2025
Accepted: 2 December 2025

Abstract

Context. The transition regime between giant planets (GPs) and brown dwarfs (BDs) is still an open subject of study in exoplanetary science. A complete understanding of the population of long-period GPs and BDs would be pivotal for improving our knowledge of this topic, but the number of such objects with precisely measured orbital and physical parameters remains small. Moreover, their dynamical influence on smaller companions in inner orbits is still unclear.

Aims. Within the framework of the Global Architecture of Planetary Systems collaboration (GAPS), we aim to confirm and characterise sub-stellar companion candidates from Gaia DR3, and to study the potential presence of additional lower mass planets in their systems.

Methods. We present the results of an intensive high-precision radial velocity (RV) monitoring of HD 128717, which hosts the astrometric candidate Gaia-ASOI-009. We used the HARPS-N spectrograph at Telescopio Nazionale Galileo (TNG) to collect a high- cadence RV time series of the target. We used Markov chain Monte Carlo (MCMC) analyses to refine the Gaia DR3 orbital solution of the companion and, finally, performed a combined model of RV and proper motion anomaly (PMa) to derive the complete 3D orbit of the companion. We also ran a suite of numerical simulations to confirm our results.

Results. We confirmed the sub-stellar nature of Gaia-ASOI-009 (i.e. Gaia-6 B). From the combined RV+PMa fit, we confirm that it is a high-eccentricity low-mass BD with P_B = 9.37_−0.05^+0.06, M_B = 19.8 ± 0.5 M_J, and e_B = 0.85, i_B = 130°. The derived orbital solution differs significantly from the one published in Gaia DR3. Through a series of dedicated simulations, we demonstrated that this discrepancy arises from a degeneracy in the Gaia DR3 astrometric solution. Specifically, the combination of Gaia-6 B long orbital period and high eccentricity, both poorly constrained by the limited time span of DR3, led to an incorrect solution characterised by a shorter period and lower eccentricity. Finally, we find no evidence of other companions in the system (in the inner or the outer regions). Thus, the origin of the high-eccentricity of Gaia-6 B remains unclear.