Most stars in space are not solitary, but form as part of a multiple system. Astronomers have now discovered a particularly rare variant of such a system. It is a hierarchical quadruple star system in which a close trio of stars is orbited by a fourth star. To date, only very few such 3+1 systems have been observed. TIC 120362137 stands out because it is particularly compact: the three inner stars are more massive and larger than our sun, but move in an area that would fit into the orbit of Mercury in our solar system. The fourth, outer star orbits at a distance approximately equal to that of Jupiter to the Sun. This makes this quartet the closest quadruple star system of its kind known to date.
Multiple star systems are common in the cosmos: astronomers estimate that around a third of all stars are double or triple stars. Our sun also probably once had a stellar “sister” that was formed together with it in a collapsing gas cloud. However, larger multiple systems with four, five or even six stars can also arise in such star cradles. However, these systems are usually only stable if they are organized hierarchically. They then consist of several pairs that orbit each other. In 2021, for example, astronomers discovered a six-fold star system made up of three pairs. Two pairs orbit each other, the third pair orbits this inner quartet. In triple star systems there is usually an inner pair orbited by a single star. Quadruple systems usually consist of two pairs.
An inner trio with an outer companion
Now astronomers have discovered a quadruple star system that belongs to a different, rarer hierarchical type. “These 3+1 or (2+1) + 1 systems consist of a hierarchical triad in which the fourth star orbits in a significantly wider orbit around the center of mass of the inner trio,” write Tamás Borkovits of the Baja Astronomical Observatory in Hungary and his colleagues. This form of quadruple systems is probably formed in a different way than the two-pair quartets and is therefore less common. However, such systems can provide valuable information about how they arise, as the astronomers explain. With the help of observations with NASA’s TESS (Transiting Exoplanet Survey Satellite) space telescope, they have now discovered such a 3+1 system. The telescope’s light curves initially showed a clear signal of mutual star occultation by an inner pair of stars that orbit each other closely with a period of 3.28 days. “In addition, further shadows were detected, revealing a third star orbiting the center of mass of this pair in 51.3 days,” report Borkovits and his team. This configuration therefore corresponded to a classic hierarchical triple system.
Observations with additional Earth-based telescopes revealed another component of this system, named TIC 120362137: a fourth star that orbits further out of the trio with an orbital period of 1,045 days. This makes it a 3+1 star system. As closer analyzes revealed, the three inner stars of this quartet are more massive and larger than the Sun: They are between 1.3 and 1.75 solar masses and their radii are between one and a half and three times the solar radius. All three are also hotter than the sun and move on a common, flat plane. “Such flatness is likely a relic of the formation process of this system,” the astronomers write. “It is likely that all four stars were originally formed in the same flat disk.” The fourth, sun-like star no longer moves exactly in this plane, but its orbit is probably not inclined more than ten degrees, as Borkovits and his colleagues determined.
The most compact system of its kind
Despite their size and mass, the four stars of TIC 120362137 form an extremely compact ensemble: “Three of the four stars would fit into an area that is comparable to Mercury’s orbit around the Sun,” report the astronomers. “And the fourth, more distant component is closer to the three inner stars than our sun’s Jupiter.” This makes this quadruple star system the most compact of this 3+1 configuration to date. Previously, only two 3+1 systems were known, whose stars are also relatively close together. “TIC 120362137 is not only the most compact among them, but also the only one in which all four stellar components could be directly detected via spectroscopic observations,” emphasize Borkovits and his team. However, they suspect that there must be numerous other, similarly compact quadruple star systems of this type in the cosmos. “However, their discovery is likely to be difficult and may depend on random, favorable features of these systems,” said the astronomers.
In addition, Borkovits and his colleagues investigated how the compact 3+1 system TIC 120362137 could develop further. Accordingly, the most massive star in the quartet, Aa, will be the first to complete its life cycle and expand until it forms a common shell with its partner star Ab. “This binary system will then merge into a single star A’ of around 3.1 solar masses,” report the astronomers. This then continues to develop normally until it too becomes a red giant in around 275 million years and a mass transfer to the third star of the former central trio begins. A merger occurs again. “We assume that neither of these two mergers will lead to the dissolution of the triple and later binary star system,” said the team. Unlike a supernova, such “silent” mergers usually occur quietly and without major explosions. Of the original inner trio, only one massive star with 4.59 solar masses remains, which forms a pair with the fourth, outer star.
These two stars could now survive as a binary star system for several billion years. The massive merged star will become a white dwarf in around 300 million years, while the fourth, smaller star will only become a white dwarf in around nine billion years, as astronomers report. The end result is a pair of orbiting white dwarfs whose complex history will no longer be visible.
Source: Tamás Borkovits (University of Szeged, Hungary) et al., Nature Communications, doi: 10.1038/s41467-026-69223-4
https://www.nature.com/articles/s41467-026-69223-4