Disk fraction among free-floating planetary-mass objects in Upper Scorpius

¹ Laboratoire d’Astrophysique de Bordeaux, Univ. de 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 Instituto de Astrofísica de Canarias, Calle Vía Lactea s/n, 38205 La Laguna, Tenerife, Spain
⁴ Université Paris-Saclay, Université Paris Cité, CEA, CNRS, 91191 AIM, Gif-sur-Yvette, France
⁵ Departamento de Inteligencia Artificial, Universidad Nacional de Educación a Distancia (UNED), c/Juan del Rosal 16, 28040 Madrid, Spain
⁶ Centro de Astrobiología (CAB), CSIC-INTA, ESAC Campus, Camino bajo del Castillo s/n, Villanueva de la Cañada, 28692 Madrid, Spain
⁷ Department of Astronomy, Graduate School of Science, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-0033, Japan
⁸ Astrobiology Center, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
⁹ National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan
¹⁰ Departamento de Física Quàntica i Astrofísica (FQA), Univ. de Barcelona (UB), Martí i Franquès, 1, 08028 Barcelona, Spain
¹¹ Institut de Ciències del Cosmos (ICCUB), Univ. de Barcelona (UB), Martí i Franquès, 1, 08028 Barcelona, Spain
¹² Instituto de Astronomia, Geofísica e Ciências Atmosféricas, Universidade de São Paulo, Rua do Matão, 1226, Cidade Universitária, 05508-090 São Paulo, SP, Brazil

^★ Corresponding authors: tommy.rodrigues@u-bordeaux.fr; herve.bouy@u-bordeaux.fr

Received: 7 July 2025
Accepted: 3 October 2025

Abstract

Context. Free-floating planetary-mass objects (FFPs) have been detected through direct imaging in several young, nearby star-forming regions. The properties of circumstellar disks around these objects may provide a valuable probe into their origin but are currently limited by the small sample sizes explored.

Aims. We aim to perform a statistical study of the occurrence of circumstellar disks down to the planetary-mass regime.

Methods. We performed a systematic survey of disks among the population identified in the 5–10 Myr-old Upper Scorpius association, restricted to members outside the younger, embedded Ophiuchus region, and with estimated masses below 105 M_Jup. We took advantage of unWISE photometry to search for mid-infrared excesses in the WISE (W1–W2) color. We implemented a Bayesian outlier detection method, which models the photospheric sequence and computes excess probabilities for each object, enabling a statistically sound estimation of disk fractions.

Results. We explored disk fractions across an unprecedentedly fine mass grid, reaching down to objects as low as ~6 M_Jup assuming 5 Myr or ~8 M_Jup assuming 10 Myr, thus extending the previous lower boundary of disk fraction studies. Depending on the age, our sample includes between 17 and 40 FFPs. We confirm that the disk fraction steadily rises with decreasing mass and exceeds 30% near the substellar-to-planetary mass boundary at ~13 M_Jup. We find hints of a possible flattening in this trend around 25–45 M_Jup, potentially signaling a transition in the dominant formation processes. This shift in trend should be considered with caution and needs to be confirmed with more sensitive observations. Our results are consistent with the gradual dispersal of disks over time, as disk fractions in Upper Scorpius appear systematically lower than those in younger regions.