Stars in the universe rarely exist in isolation. Many are born and move together as binary, triple, or even larger multi-star systems. A recent study has revealed an especially remarkable celestial object: a tightly bound four-star system.
This system has been named TIC 120362137. Research shows it is one of the most compact “3+1 quadruple star systems” currently known, so tightly packed that it could fit entirely within the orbit of Mercury around the Sun. For astronomers, discovering it is like finding a precisely operating “four-star clock” in the cosmos.
The hierarchical dance of four stars
TIC 120362137 is not simply four stars randomly grouped together; it is a hierarchical multi-star system, meaning its structure is composed of layered orbital arrangements.
At the core is a binary pair that eclipses each other, with an orbital period of about 3.28 days. A third star orbits this binary with a period of approximately 51.3 days. The fourth star resides in an outer orbit, taking roughly 1,046 days (almost three years) to orbit the entire triple system.
Astronomers call this a “3+1 quadruple system,” where three stars form a compact inner subsystem and a fourth orbits farther out. While multi-star systems are not uncommon in the universe, a system like TIC 120362137—with clear hierarchical structure and extremely tight spacing—is very rare.
Success
You are now signed up for our newsletter
Success
Check your email to complete sign up
A star system smaller than Mercury’s orbit
The most surprising feature of this system is its spatial scale. The inner three-star subsystem is so compact that its overall size is smaller than Mercury’s orbit around the Sun. In other words, if we scaled it to the size of the Solar System, the three stars orbit each other within an extremely small region.
Even more remarkable, these three stars are all more massive and hotter than the Sun, while the outer fourth star is similar to the Sun. That multiple stars can coexist and remain dynamically stable in such a confined space is itself a fascinating case in celestial mechanics.
TESS satellite detects mysterious ‘multiple eclipses’
The discovery of this system is linked to NASA’s Transiting Exoplanet Survey Satellite (TESS). Between 2019 and 2024, TESS repeatedly observed this star and astronomers noticed a clear signal in its light curve: every 3.28 days, the brightness dipped, indicating eclipses between the binary stars.
Even more unusual, additional eclipses lasting 1–2 days were detected with a period of roughly 51.3 days, meaning the third star also participates in eclipses, forming a rare three-body eclipse event. This observation was a key clue confirming the presence of the third star.
Spectral analysis reveals the fourth star
Further research showed that three stars alone could not fully explain the observed orbital variations. Using high-resolution spectroscopy from ground-based telescopes, the team applied a technique called QUADCOR to separate the signals from different stars in the spectrum.
The analysis revealed faint spectral lines from a fourth star, allowing researchers to directly measure the radial velocities of all four stars and reconstruct the system’s orbital architecture. TIC 120362137 thus became one of the few 3+1 quadruple systems where all four stars can be directly observed spectroscopically.
A rare family in the cosmos
Tightly bound four-star systems like TIC 120362137 are extremely rare in astronomical databases.
The closest known examples are HIP 41431 and TIC 114936199, but these systems differ in structure or mass distribution. In HIP 41431, the fourth star has not been directly observed spectroscopically, and in TIC 114936199, the outer star has an orbital period of about 2,100 days.
Compared to these, TIC 120362137 is more compact and allows direct measurement of all four stars, making it one of the most complete cases studied to date.
Future fate: two white dwarfs
The research team modeled the system’s long-term evolution. Results indicate that as the stars age and exhaust their fuel, the system will likely evolve into a pair of white dwarfs. This outcome means that this tightly orbiting quadruple system will transform into a dramatically different cosmic scene billions of years from now.
The precise mechanics of the cosmos
For astronomers, systems like TIC 120362137 are especially valuable. The gravitational interactions among multiple stars create complex dynamics, effectively serving as natural “astrophysical laboratories” for studying multi-star formation, orbital stability over long timescales, and the effects of strong gravitational interactions.
In the vast universe, stars are often imagined as solitary points of light. Systems like TIC 120362137 demonstrate that stars can, in some places, exist not in isolation but in intricate, elegant orbital choreography—a cosmic spectacle unfolding over billions of years.This report is based on content published in Nature Communications in Nature.
