The spectral energy distribution of YSES 1 b and its circumplanetary disc^★

¹ SRON – Space Research Organisation Netherlands, Niels Bohrweg 4, 2333 CA Leiden, The Netherlands
² Leiden Observatory, Leiden University, PO Box 9513, 2300 RA Leiden, The Netherlands
³ Steward Observatory, The University of Arizona, 933 North Cherry Avenue, Tucson, AZ, USA
⁴ Subaru Telescope, National Observatory of Japan, NINS, 650 N. A’ohoku Place, Hilo, HI, USA
⁵ Wyant College of Optical Sciences, The University of Arizona, 1630 E University Boulevard, Tucson, AZ, USA
⁶ Astrobiology Center, National Institutes of Natural Sciences, 2-21-1 Osawa, Mitaka, Tokyo, Japan
⁷ Northrop Grumman Corporation, 600 South Hicks Road, Rolling Meadows, IL, USA
⁸ Center for Computational Astrophysics, Flatiron Institute, 162 5th Avenue, New York, NY, USA
⁹ Draper Laboratory, 555 Technology Square, Cambridge, MA, USA
¹⁰ Department of Astronomy, University of Michigan, 323 West Hall, 1085 S University Ave, Ann Arbor, MI 48109, USA
¹¹ Starfire Optical Range, Kirtland Air Force Base, Albuquerque, NM, USA

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

Received: 16 January 2026
Accepted: 24 April 2026

Abstract

Context. Direct imaging enables the characterisation of substellar companions on wide orbits. These objects provide a testbed for our formation theories; therefore, it is important to obtain accurate physical parameters for them. One of these objects is YSES 1 b.

Aims. Our objective is to improve the spectral energy distribution (SED) modelling of YSES 1 b and determine the bulk and atmospheric parameters.

Methods. We obtained observations in the r′, i′, and z′ bands using MagAO-X on the 6.5 metre Magellan Clay telescope at Las Campanas Observatory. We combined this data with archival VLT/SPHERE and VLT/NACO data and used a forward-modelling approach to estimate the physical parameters. We tested models both without and with a circumplanetary disc (CPD) model. We represented the CPD by including a dust extinction model and a blackbody radiation component. Using the derived bolometric luminosity, we estimated the mass of YSES 1 by fitting evolutionary models.

Results. Including the CPD model provides a significantly better fit to the photometric data, yielding an object that is considerably warmer (2854₋₉₄⁺¹¹⁰ K vs 1727₋₁₂₇⁺¹⁷² K) and smaller (1.58_−0.07^+0.06 R_J vs 3.0_−0.7^+0.2 R_J) than previous estimates. The newly determined radius suggests that the addition of dust extinction could resolve the large radius anomaly identified previously. Depending on the age of the system, the estimated mass increases from 14 ± 3 M_J (17 Myr) to either 25.7_−3.6^+4.1 (17 Myr) or 41.6_−3.4^+3.6 M_J (27 Myr).

Conclusions. Dust extinction and blackbody radiation from a CPD can substantially change the estimated physical parameters of an object. For YSES 1 b, this moves it into the brown dwarf regime.