AI-generated image of what the system could look like. Image not to scale.

Astronomers have just identified a rare cosmic “huddle” of stars. We’re talking three stars, each more massive and hotter than our Sun, packed into a space smaller than the orbit of Mercury. To top it off, there’s a fourth Sun-like star circling that entire trio closer than Jupiter sits to our own Sun.

Meet TIC 120362137, the most compact 3+1 quadruple star system ever discovered.

A Tight-Knit Family

Most stellar systems aren’t as neat and ordered as our own. Researchers estimate that up to 85% of all stellar systems have more than one star, yet quadruple star systems remain very rare. Astronomers have identified roughly 101 potential quadruple candidates in recent TESS data, but confirming such systems is challenging.

These systems typically come in two flavors. The common one is the 2+2 structure, with two inner and two outer stars; the 3+1 configuration is exceptionally rare. Just two other compact examples were documented before the discovery of TIC 120362137.

To confirm this system, researchers started with the Transiting Exoplanet Survey Satellite (TESS), a NASA space-borne telescope designed to monitor the brightness of millions of stars across nearly the entire sky. TESS searches for periodic dips in brightness that appear when something (like a planet) passes in front of a star.

In this case, TESS revealed nine such dips in the brightness of a binary star system.

This supplemental data, along with rhythmic shifts in the binary’s timing (known as Eclipse Timing Variations or ETV), proved a third star was orbiting the pair every 51.3 days. But some irregularities in their orbits suggested this still wasn’t the whole picture.

The discovery of the fourth star was done through an algorithm called QUADCOR, which isolates the distinct spectral fingerprints of all four stars simultaneously. By detecting the individual signals, astronomers transformed a messy signal into the most precisely measured and compact 3+1 quadruple system ever documented.

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Meet the Stars

The team didn’t just rely on TESS. They organized a global effort, using telescopes in Hungary, Arizona, the Czech Republic, and Slovakia. They gathered 73 spectra from the Fred L. Whipple Observatory in Arizona alone. By combining all this data, they calculated the masses and sizes of the stars with incredible precision—often within 1%.

The three “core” stars are incredibly close. In our solar system, Mercury takes 88 days to orbit the Sun. In TIC 120362137, three separate stars (all larger and hotter than our Sun) fit within a space smaller than that. It is a cosmic mosh pit.

The primary star, Aa, is the heavyweight. It is 1.75 times the mass of our Sun. Its partner, Ab, is 1.36 times the Sun’s mass. The third wheel, Star B, sits right in the middle at 1.48 solar masses.

Then there is the loner. A fourth star, Star C, circles this entire chaotic trio. It takes 1,046 days to complete one lap. While that sounds like a long time, it is still closer to its central trio than Jupiter is to our Sun. Star C is the most familiar of the bunch. It is roughly the same mass and temperature as our own Sun.

Tamás Borkovits and colleagues focused only on star detection. We don’t know if there are any planets in the system. Even if there were any planets, it would be extremely unlikely for life to find a way to exist on them. Three of the stars (Aa, Ab, and B) are significantly hotter and more luminous than our Sun.

Because these stars eclipse one another frequently (the inner pair every 3.28 days and the third star every 51.3 days), any planet would experience “extra eclipses” and dramatic fluctuations in light and heat.

Why This System Is So Interesting

These systems are very rare because the configuration is difficult to stabilize gravitationally. Without a tight balancing act, these systems would eventually destabilize and eject members.

But they are also uniquely valuable as cosmic laboratories because they provide a “stress test” for our theories on star formation and can eventually merge into exotic objects, such as the pair of white dwarfs predicted for TIC 120362137. Furthermore, by observing how these stars “tug” on each other today, researchers can better predict the formation of exotic objects like neutron stars or black hole X-ray binaries, offering a preview of the high-energy events that shape the evolution of our galaxy.

Ultimately, this dance is unlikely to remain stable for too long (in a cosmic sense). TIC 120362137 is perfectly balanced for now, but the future is set to be much more dramatic.

Stars don’t stay the same forever. Eventually, they run out of fuel and swell up. In this system, the heavyweight primary star, Aa, will be the first to go. It will expand until it fills its “Roche lobe”—the region where its gravity can no longer hold onto its outer layers. When that happens, things get messy. Material will start spilling from star to star. Orbits will shift and new stars might even merge.

The team ran simulations to see how the story ends. They predict this frantic four-way dance will likely end in a quiet pair of white dwarfs.

This bizarre system is a reminder that perhaps, our own Sun, sitting by itself, is actually the weird one. Most stars in the galaxy have a partner. Some have two. And a very special few, like those in TIC 120362137, have a family so close they can almost touch.

Journal Reference: Discovery of the most compact 3+1-type quadruple star system TIC 120362137. DOI 10.1038/s41467-026-69223-4