In a groundbreaking discovery published in the Monthly Notices of the Royal Astronomical Society (MNRAS), astronomers have identified an extremely rare hierarchical quadruple star system consisting of a pair of cold brown dwarfs orbiting a pair of young red dwarf stars, located 82 light years from Earth in the constellation Antlia

An artist's concept depicts the UPM J1040?3551 system against the backdrop of the Milky Way as observed by Gaia. On the left, UPM J1040?3551 Aa & Ab appears 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 Credits: Jiaxin Zhong, Zenghua Zhang.

A Unique Stellar System

The system, named UPM J1040?3551 AabBab, was identified by an international research team led by Professor Zenghua Zhang at Nanjing University through common angular velocity measured by the Gaia astrometric satellite of ESA and the Wide-field Infrared Survey Explorer (WISE) of NASA, followed by comprehensive spectroscopic observations and analysis. In this system, Aab refers to the brighter stellar pair Aa and Ab, while Bab refers to the fainter substellar pair Ba and Bb.

"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," explains Professor Zhang. "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 a hundred thousand years."

The two pairs are separated by an impressive 1656 astronomical units (au), where 1 au equals the Earth-Sun distance. The brighter pair, UPM J1040?3551 Aab, consists of two nearly equal-mass red dwarf stars, which appear orange in colour when observed in visible wavelengths. With a visual magnitude of 14.6, this pair is approximately 100,000 times fainter than Polaris (the North Star) in visible wavelengths. In fact, no red dwarf star is bright enough to be seen with the naked eye—not even Proxima Centauri, our closest stellar neighbor at 4.2 light-years away. To make UPM J1040?3551 Aab visible without optical aid, this binary pair would need to be brought to within 1.5 light-years of Earth, placing it closer than any star in our current cosmic neighborhood. The fainter pair, UPM J1040?3551 Bab, comprises two much cooler brown dwarfs emit virtually no visible light and appear roughly 1,000 times dimmer than the Aab pair when observed in near-infrared wavelengths, where they are most easily detected.

The close binary nature of UPM J1040?3551 Aab was initially suspected due to its wobbling photocentre during Gaia's observations and confirmed by its unusual brightness—approximately 0.7 magnitude brighter than a single star with the same temperature at the same distance, as the combined light from the nearly equal-mass pair effectively doubles the output. Similarly, UPM J1040?3551 Bab was identified as another close binary through its abnormally bright infrared measurements compared to typical brown dwarfs of its spectral type. Spectral fitting analysis strongly supported this conclusion, with binary templates providing a significantly better match than single-object templates.

Cold and Mysterious Objects

Dr. Felipe Navarete of the Brazilian National Astrophysics Laboratory led the critical spectroscopic observations that helped characterize the system components. Using the Goodman spectrograph on the Southern Astrophysical Research (SOAR) Telescope at Cerro Tololo Inter-American Observatory in Chile, a Program of NSF NOIRLab, Dr. Navarete obtained optical spectra of the brighter pair, while also capturing near-infrared spectra of the fainter pair with SOAR's TripleSpec instrument. "These observations were challenging due to the faintness of the brown dwarfs," notes Dr. Navarete. "But the capabilities of SOAR allowed us to collect the crucial spectroscopic data needed to understand the nature of these objects."

Their 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% that of the Sun. The fainter pair contains more exotic objects: two T-type brown dwarfs with temperatures of 820 Kelvin (550°C) and 690 Kelvin (420°C), respectively. Brown dwarfs are small and dense low-mass objects, with the brown dwarfs in this system having sizes similar to the planet Jupiter but masses estimated to be 10-30 times greater. Indeed, at the low end of this 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," notes Dr. MariCruz Gálvez-Ortiz of the Center for Astrobiology in Spain, a co-author of the research paper. "The discovery provides a unique cosmic laboratory for studying these mysterious objects."

Solving the Age-Mass Degeneracy Problem

Brown dwarfs occupy the mass range between stars and planets—too massive to be considered planets but not massive enough to sustain hydrogen fusion like stars. Unlike stars, brown dwarfs continuously cool throughout their lifetime, which changes their observable properties such as temperature, luminosity, and spectral features.

This cooling process creates a fundamental challenge in brown dwarf research known as the "age-mass degeneracy problem." An isolated brown dwarf with a certain temperature could be a younger, less massive object or an older, more massive one—astronomers cannot distinguish between these possibilities without additional information.

"Brown dwarfs with wide stellar companions whose ages can be determined independently are invaluable at breaking this degeneracy as age benchmarks," explains Professor Hugh Jones of the University of Hertfordshire, a co-author of the research paper. "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."

Future Research Opportunities

The team believes the brown dwarf pair (UPM J1040?3551 Bab) could potentially be resolved with high-resolution imaging techniques in the future, enabling precise measurements of their orbital motion and dynamical masses.

"This system offers a dual benefit for brown dwarf science," said Professor Adam Burgasser of the University of California San Diego, a co-author of the research paper. "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."

The discovery of the UPM J1040?3551 system represents a significant advancement in our understanding of these elusive objects and the diverse formation paths for stellar systems in the neighborhood of the Sun.

The Royal Astronomical Society (RAS) made a press release on this discovery at the same time of the publiction of the research paper.

Research paper:

https://doi.org/10.1093/mnras/staf895