Astronomers using NASA’s TESS spacecraft have identified the most compact known 3+1-type quadruple star system ever recorded, a configuration so tightly packed that its inner stellar grouping fits within the orbit of Mercury around the sun.
The system, designated TIC 120362137, belongs to a rare subset of quadruple star systems where three stars orbit each other in a close inner grouping while a fourth star orbits the entire triplet from a greater distance. That outer star sits roughly as far from the inner trio as Jupiter is from the sun. The inner three, by contrast, occupy a region smaller than Mercury’s orbital path.
A Discovery Built in Stages
The system’s complexity revealed itself gradually. Initial TESS data showed what appeared to be a straightforward binary, two stars eclipsing each other on a 3.3-Earth-day cycle, producing brightness dips lasting one to two hours.
“We know thousands of such systems, called eclipsing binaries. Therefore, there was nothing interesting or peculiar at that stage,” said Tamás Borkovits, a researcher at the University of Szeged in Hungary and leader of the discovery team.
The picture changed when the team noticed additional brightness dips, lasting one to two days, occurring every 25 to 26 days. That pattern pointed to a third star with an orbital period of around 51 days, elevating the system to a triply eclipsing triple. A fourth body remained hidden until further eclipses appeared, and its presence was confirmed using the Tillinghast Reflector Echelle Spectrograph on the 1.5-meter Tillinghast telescope at Mt. Hopkins in Arizona.
Why the Outer Star’s Orbit Matters
The fourth star is what makes TIC 120362137 a record-holder. Its orbital period around the inner triplet is approximately 1,046 days, the shortest outer-orbit period among all currently known 3+1 quadruple systems.
“TIC 120362137 is currently the most compact known 3+1-type quadruple star system,” Borkovits told Space.com.
That tight configuration carries scientific weight beyond the record itself. Hierarchical star systems, where multiple stars orbit within a relatively confined space, offer researchers a window into how stars form together and how their mutual gravitational relationships hold or evolve over long timescales. TIC 120362137 is compact enough that the team was also able to determine the system’s eventual fate, though specific details about that outcome were not elaborated upon in the available data.
How Rare These Finds Are
Borkovits was direct about the difficulty of finding systems like this one. Detecting an outer fourth star through eclipse monitoring alone could require observation windows spanning several decades. Confirming one through other means, as happened here with spectroscopic follow-up, tends to occur opportunistically.
“The discovery of such systems, however, is very, very difficult,” he said. “Other kinds of detection of a fourth star may happen, but only serendipitously.”
TESS, originally designed to hunt exoplanets by detecting dips in stellar brightness, has proven effective at surfacing complex multi-star arrangements as a byproduct of its continuous photometric monitoring. TIC 120362137 adds to a growing catalog of unusual stellar architectures uncovered through that same method.
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