In a study published in the Monthly Notices of the Royal Astronomical Society (MNRAS), a team of astronomers identified a hierarchical system where a pair of cold brown dwarfs orbits a pair of young red dwarf stars. This system is located 82 light-years from Earth in the constellation Antlia.
An artist's impression of the UPM J1040−3551 system against the backdrop of the Milky Way as observed by Gaia. On the left, UPM J1040−3551 Aa & Ab appear as a distant bright orange dot, with an inset revealing these two M-type stars in orbit. On the right, in the foreground, a pair of cold brown dwarfs – UPM J1040−3551 Ba & Bb – orbit each other for a period of decades while collectively circling UPM J1040−3551 Aab in a vast orbit that takes over 100,000 years to complete. Image Credit: Jiaxin Zhong/Zenghua Zhang
Astronomers have discovered an extremely rare quadruple star system that could significantly advance the understanding of brown dwarfs.
These objects are larger than planets but too small to sustain the fusion needed to become full-fledged stars.
An international research team led by Professor Zenghua Zhang of Nanjing University identified the system, named UPM J1040−3551 AabBab.
The discovery was made using common angular velocity data from the European Space Agency’s Gaia astrometric satellite and NASA’s Wide-field Infrared Survey Explorer (WISE), followed by detailed spectroscopic observations and analysis.
Since this wide binary pair takes over 100,000 years to complete an orbit, its movement cannot be observed within a few years. Researchers instead examined their shared direction and identical angular velocity.
In this system, the brighter stellar pair (Aa and Ab) is referred to as Aab, while the fainter substellar pair (Ba and Bb) is called Bab.
What makes this discovery particularly exciting is the hierarchical nature of the system, which is required for its orbit to remain stable over a long time period. These two pairs of objects are orbiting each other separately for periods of decades, while the pairs are also orbiting a common centre of mass over a period of more than 100,000 years.
Zenghua Zhang, Professor, Nanjing University
The two pairs in the system are separated by 1,656 astronomical units (au). For context, one au is the distance between the Earth and the Sun.
The brighter pair, UPM J1040−3551 Aab, consists of two nearly equal-mass red dwarf stars, which appear orange in visible light.
With a visual magnitude of 14.6, this pair is about 100,000 times fainter than Polaris (the North Star). In fact, no red dwarf is bright enough to be seen with the naked eye, not even Proxima Centauri, the closest stellar neighbor. For this particular pair to be visible without a telescope, it would need to be moved to within 1.5 light-years of Earth.
The fainter pair, UPM J1040−3551 Bab, is composed of two much cooler brown dwarfs. These objects emit virtually no visible light and are most easily detected in near-infrared wavelengths, where they appear roughly 1,000 times dimmer than the Aab pair.
The close binary nature of the brighter pair, UPM J1040−3551 Aab, was initially suspected due to its wobbling photocentre observed by Gaia. This was confirmed by its unusual brightness, approximately 0.7 magnitude brighter than a single star, as the combined light from the two nearly equal-mass stars effectively doubled the output.
Similarly, the fainter pair, UPM J1040−3551 Bab, was identified as a close binary because of its abnormally bright infrared measurements compared to typical brown dwarfs. This conclusion was strongly supported by a spectral fitting analysis, which showed that binary templates provided a significantly better match than single-object templates.
Dr. Felipe Navarete, of the Brazilian National Astrophysics Laboratory, led the crucial spectroscopic observations used to characterize the system's components.
Using the Goodman spectrograph on the Southern Astrophysical Research (SOAR) Telescope in Chile, Navarete obtained optical spectra of the brighter pair. Navarete also captured near-infrared spectra of the fainter pair using SOAR's TripleSpec instrument.
These observations were challenging due to the faintness of the brown dwarfs, but the capabilities of SOAR allowed us to collect the crucial spectroscopic data needed to understand the nature of these objects.
Dr. Felipe Navarete, Brazilian National Astrophysics Laboratory, Rua dos Estados Unidos
The analysis revealed that both components of the brighter pair are M-type red dwarfs with temperatures of approximately 3,200 Kelvin (about 2,900 °C) and masses of about 17% of the Sun's.
The fainter pair are more exotic objects: two T-type brown dwarfs with temperatures of 820 Kelvin (550 °C) and 690 Kelvin (420 °C). These are small and dense, with sizes similar to the planet Jupiter. However, their masses are estimated to be 10-30 times greater than Jupiter's. At the lower end of this mass range, these objects could be considered "planetary mass" objects.
This is the first quadruple system ever discovered with a pair of T-type brown dwarfs orbiting two stars. The discovery provides a unique cosmic laboratory for studying these mysterious objects.
Dr. MariCruz Gálvez-Ortiz, Study Co-Author, Center for Astrobiology in Spain
Unlike stars, which maintain a stable temperature through nuclear fusion, brown dwarfs continuously cool as they age. This process alters their observable properties, including their temperature, luminosity, and spectral features.
This continuous cooling process presents a major challenge in brown dwarf research known as the "age-mass degeneracy problem."
For an isolated brown dwarf, a specific temperature could mean it is either a younger, less massive object or an older, more massive one. Without additional information, astronomers are unable to distinguish between these two possibilities.
Brown dwarfs with wide stellar companions whose ages can be determined independently are invaluable at breaking this degeneracy as age benchmarks, UPM J1040−3551 is particularly valuable because H-alpha emission from the brighter pair indicates the system is relatively young, between 300 million and 2 billion years old.
Hugh Jones, Professor, Study Co-Author, University of Hertfordshire
The team believes that future high-resolution imaging techniques could resolve the brown dwarf pair (UPM J1040−3551 Bab), enabling precise measurements of their orbital motion and dynamical masses.
This system offers a dual benefit for brown dwarf science. It can serve as an age benchmark to calibrate low-temperature atmosphere models, and as a mass benchmark to test evolutionary models if we can resolve the brown dwarf binary and track its orbit.
Adam Burgasser, Study Co-Researcher and Professor, University of California, San Diego
The discovery of the UPM J1040−3551 system is a significant advancement because it enhances the understanding of elusive brown dwarfs and the diverse ways stellar systems form in the cosmic neighborhood.
Journal Reference:
Zhang, Z. H., et al. (2025) Benchmark brown dwarfs – I. A blue M2 + T5 wide binary and a probable young [M4 + M4] + [T7 + T8] hierarchical quadruple. Monthly Notices of the Royal Astronomical Society. doi.org/10.1093/mnras/staf895