Multiplicity of young brown dwarfs and isolated planetary mass objects in Taurus and Upper Scorpius

¹ Laboratoire d’astrophysique de Bordeaux, Univ. Bordeaux, CNRS, B18N, allée Geoffroy Saint-Hilaire, 33615 Pessac, France
² Institut universitaire de France (IUF), 1 rue Descartes, 75231 Paris Cedex 05, France
³ Univ. Grenoble Alpes, CNRS, IPAG, 38000 Grenoble, France
⁴ Astronomy Department, University of California Berkeley, Berkeley, CA 94720-3411, USA
⁵ Space Telescope Science Institute, 3700 San Martin Dr., Baltimore, MD 21218, USA
⁶ Departamento de Inteligencia Artificial, Universidad Nacional de Educación a Distancia (UNED), c/Juan del Rosal 16, 28040 Madrid, Spain
⁷ Universidad Nacional Autónoma de México, Instituto de Radioastronomía y Astrofísica, Antigua Carretera a Pátzcuaro 8701, Ex-Hda. San José de la Huerta, 58089 Morelia, Michoacán, Mexico
⁸ Centro de Astrobiología (CAB), CSIC-INTA, ESAC Campus, Camino bajo del Castillo s/n, 28692 Villanueva de la Cañada, Madrid, Spain
⁹ Department of Astronomy, Graduate School of Science, The University of Tokyo, Tokyo, Japan
¹⁰ National Astronomical Observatory of Japan, Tokyo, Japan
¹¹ Astrobiology Center, Tokyo, Japan
¹² Université Paris-Saclay, Université Paris Cité, CEA, CNRS, AIM, 91191 Gif-sur-Yvette, France
¹³ Max-Planck-Institut für Astronomie, Königstuhl 17, 69117 Heidelberg, Germany
¹⁴ Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, Rua do Matão, 1226, Cidade Universitária, 05508-090 São Paulo-SP, Brazil
¹⁵ Departament de Física Quàntica i Astrofísica (FQA), Universitat de Barcelona (UB), Martí i Franquès, 1, 08028 Barcelona, Spain
¹⁶ Institut de Ciències del Cosmos (ICCUB), Universitat de Barcelona (UB), Martí i Franquès, 1, 08028 Barcelona, Spain
¹⁷ Institut d’Estudis Espacials de Catalunya (IEEC), Edifici RDIT, Campus UPC, 08860 Castelldefels (Barcelona), Spain

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

Received: 4 April 2025
Accepted: 25 January 2026

Abstract

Context. Free-floating planetary mass objects – worlds that roam interstellar space untethered to a parent star – challenge conventional notions of planetary formation and migration, but also of star and brown dwarf formation.

Aims. We focus on the multiplicity among free-floating planets. By virtue of their low binding energy (compared to other objects that formed in these environments), these low-mass substellar binaries represent the most sensitive probe of the mechanisms at play during the star formation process.

Methods. We use the Hubble Space Telescope and its Wide Field Camera 3 and the Very Large Telescope and its ERIS adaptive optics facility to search for visual companions among a sample of 77 objects, members of the Upper Scorpius and Taurus young nearby associations, with estimated masses in the range between approximately 6–66 M_Jup.

Results. We report the discovery of one companion candidate around a Taurus member with a separation of 111.9±0.4 mas or ∼18 au assuming a distance of 160 pc, with an estimated primary mass in the range between 3–6 M_Jupand a secondary mass between 2.6– 5.2 M_Jup, depending on the assumed age. This corresponds to an overall binary fraction of 1.8_−1.3^+2.6% among low-mass brown dwarfs and free-floating planetary mass objects over the separation range ≥7 au. Despite the limitations of small-number statistics and variations in spatial resolution and sensitivity, our results, combined with previous high-spatial-resolution surveys, suggest a notable difference in the multiplicity properties of objects below ∼30–50 M_Jup between Upper Sco and Taurus. In Taurus, a binary fraction of 5.6_−2.3^+3.2% is found for objects with masses below 30M_Jup, and of 7.8_−2.4^+3.0% for objects with masses below 50M_Jup, while no binary was found among 80 objects over the matching luminosity range in Upper Sco, corresponding to an upper limit of ≤1.2%.

Conclusions. This difference may point to intrinsically distinct formation conditions, with warmer parental molecular clouds originally present in Upper Sco potentially inhibiting fragmentation into the lowest-mass brown dwarfs and free-floating planets compared to cooler environments such as Taurus.