Astrometric view of companions in the inner dust cavities of protoplanetary discs

¹ European Southern Observatory, Karl-Schwarzschild-Str. 2, 85748 Garching bei München, Germany
² School of Physics and Astronomy, University of Leeds, Leeds LS2 9JT, UK
³ Dipartimento di Fisica, Università degli Studi di Milano, Via Celoria 16, 20133 Milano, Italy
⁴ Institute of Astronomy, University of Cambridge, Cambridge, UK
⁵ Max Planck Institute for Extraterrestrial Physics, Giessenbachstrasse 1, 85748 Garching, Germany
⁶ Max-Planck-Institut für Astronomie (MPIA), Königstuhl 17, 69117 Heidelberg, Germany
⁷ Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
⁸ INAF – Istituto di Radioastronomia, Via Gobetti 101, 40129, Bologna, Italy
⁹ Astronomy Unit, School of Physics and Astronomy, Queen Mary University of London, London E1 4NS, UK
¹⁰ Millennium Nucleus on Young Exoplanets and their Moons (YEMS), Chile
¹¹ Departamento de Física, Universidad de Santiago de Chile, Av. Víctor Jara 3659, Santiago, Chile
¹² Center for Interdisciplinary Research in Astrophysics and Space Exploration (CIRAS), Universidad de Santiago de Chile, Chile
¹³ Centro de Astrobiología (CAB), CSIC-INTA, ESAC Campus, Camino bajo del Castillo s/n, 28692 Madrid, Spain
¹⁴ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
¹⁵ University Observatory, Faculty of Physics, Ludwig-Maximilians-Universität München, Munich, Germany
¹⁶ Departament de Física Quàntica i Astrofísica (FQA), Universitat de Barcelona (UB), Martí i Franquès, 1, 08028 Barcelona, Spain
¹⁷ Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, MA 02139, USA

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

Received: 2 September 2025
Accepted: 26 November 2025

Abstract

Context. Protoplanetary discs with inner dust cavities (often referred to as ‘transition discs’) are potential signposts of planet formation. However, few companions have been identified within these cavities, and the role of companions in shaping them remains unclear.

Aims. We used Gaia astrometry to search for planetary and stellar companions in a sample of 98 transition discs, assessing the occurrence rate of such companions and their potential influence on cavity formation.

Methods. For the 98 young stellar objects (YSOs) with inner dust cavities, we computed Gaia proper motion anomalies, which together with the renormalised unit weight error (RUWE), identify companions with mass ratios q ≳0.01 at ∼0.1−30 au. We assessed the impact of disc gravity, accretion, disc-scattered light, dippers, starspots, jets, and outflows on the measured proper motion anomalies, concluding that these effects are unlikely to affect our analyses and that astrometric techniques such as the one of this work can be robustly applied to YSOs.

Results. Significant proper motion anomalies are found in 31 transition discs (32% of the sample), indicative of companions. We recovered 85% of the known companions within our sensitivity range. Assuming that the astrometry of each system is dominated by a single companion, we modelled the semi-major axis and mass required to reproduce the observed astrometric signals. Most inferred companions have M>30 M_J, placing many within or near the stellar mass regime. Seven sources host companions compatible with a planetary mass (M<13 M_J, HD 100453, J04343128+1722201, J16102955-3922144, MHO6, MP Mus, PDS 70, and Sz 76). For the non-detections, we provide the companion masses and semi-major axes that can be excluded in future searches. About half (53%) of detected companions cannot be reconciled with having carved the observed dust cavities.

Conclusions. We have gathered evidence of the presence of companions in a large sample of transition discs. However, we find that the population of transition discs cannot be fully described as a circumbinary population. Transition discs host as many companions within our sensitivity range as do randomly sampled groups of YSOs and main-sequence stars. If dust cavities are shaped by companions, such companions must reside at larger orbital separations than those of the companions detected here, and we predict them to be of planetary mass.