A tiny binary system is not only small but also ancient, billions of years older than other such systems, which raises questions about its origins.

Astronomers say they’ve found the tiniest binary star system yet — a pair of ultracool dwarf stars in the constellation Taurus that complete a mutual orbit every 20.5 hours. First spotted in archival spectral data, the new system is so small that it would fit comfortably between Earth and the Sun, the research team announced at the 241st Meeting of the American Astronomical Society.

Illustration of binary shows their current and past diameters
The binary star system LP 413-53AB has a mutual orbit so tight that when the members were younger and larger (their past outlines are drawn as dashed lines), they would have been practically on top of each other.
Adam Burgasser / University of California San Diego

Chih-Chun “Dino” Hsu (Northwestern University) first identified the system, dubbed LP 413-53AB, while re-analyzing archival spectral data. Hsu initially thought he was looking at just one star but eventually worked out that it was probably two — just orbiting very, very closely.

“The challenge is we're looking at very low mass, very faint sources,” says Adam Burgasser (University of California, San Diego), who worked with Hsu to make the discovery. “Looking at the archives, there's actually 20 years of data that have already been observed. And so at some level, we're kind of letting other folks in the past do a lot of our work for us.”

Follow-up observations with the Keck Observatory on Mauna Kea, Hawai‘i, confirmed Hsu’s initial finding. The star system is the lowest-mass binary ever found.

The system’s primary star is a red dwarf, and its smaller companion might be an even smaller brown dwarf. Sometimes called “failed stars,” brown dwarfs fall in between giant planets and hydrogen-fusing stars. But while they lack the gravitational heft for sustained fusion reactions in their cores, they still glow — albeit faintly and mostly at infrared wavelengths.

Comparison diagram to show system size
For context, this diagram compares the LP 413-53AB system to other nearby systems, including TRAPPIST-1 and some of its planets, as well as Jupiter and its largest moons.
Adam Burgasser / University of California San Diego

Scientists have detected three such ultracool binary systems with short orbital periods before, but LP 413-53AB stands out from even this small crowd.

Estimated to be few billion years old, the new system is much older than the previously discovered systems, which max out at 40 million years old. LP 413-53AB’s tiny twins also revolve around each other three times faster than the previous discoveries thanks to their incredibly tight orbit. The distance separating the two is around 1% of the distance between the Sun and Earth.

Ultracool dwarfs shrink as they age, which makes the newly discovered system even stranger. “At the beginning of their formation, if they stayed in this location, [the stars] would actually be roughly in contact,” says Hsu.

It’s possible that the pair simply drifted closer as they evolved, or that they were thrown together in the chaos of ejecting a now-lost triplet.

"The real science will come in as we start to figure out ways to see how common this type of object is,” said Burgasser. “That will allow us to answer bigger questions about how these low-mass stars form, how the planetary systems around these systems form, and whether they are common or rare.”

If close binaries like this one are common, that could have implications for the search for habitable planets around other dwarf stars, such as TRAPPIST-1, Hsu adds. “If you're in such a close binary,” he says, “then even if there is an exoplanet, that’s going to make it uninhabitable.”


Image of Alan


January 14, 2023 at 11:02 pm

Where in Taurus is this located and what is its magnitude ?

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Monica Young

January 17, 2023 at 11:24 am

From Dr. Hsu:
"This M9 dwarf system has the coordinate of (RA, Dec) = 03h 50m 57.40s +18d 18m 06.7s from Gaia and its optical Gaia magnitude of 17.2. It was observed in near-infrared K band of 11.8, which is much brighter."

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Alan MacRobert

January 15, 2023 at 10:23 am

What spectral types are they? L? T?

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