Astronomers have identified a rare, tightly bound star system in which an eclipsing binary—two stars that pass in front of each other from our perspective—also eclipses a third star, while a fourth star orbits farther out.
The international group of astronomers that made the discovery say it’s the most compact quadruple star system ever found, as the outermost star, orbiting the inner three, has the shortest period ever recorded. The study, published Tuesday in Nature, provides a closer look at the weird and chaotic world of hierarchical star systems.
Four’s a crowd
Using NASA’s Transiting Exoplanet Survey Satellite (TESS), the team behind the discovery was on the lookout for triple star systems and found one behaving rather strangely. At first, the object’s brightness dimmed for around 1.5 days, indicating that it consists of at least two stars that orbit each other. Then, every 26 days the object would fade again, confirming that there is a third star in the system.
Additional observations showed that a triple star system was not enough to explain the object’s behavior, with the timing variations of the eclipses revealing that an additional fourth star has to be present in the system.
TESS observed the star system, named TIC 120362137, between 2019 and 2024. Astronomers used the data to determine the orbital period of the fourth star, which turned out to be 1,045.5 days long. That is the shortest orbital period for an outer fourth star ever observed in a system of its kind.
The inner three stars are all packed together within an area similar in size to Mercury’s orbit around the Sun, while the fourth star extends farther out in an area comparable to Jupiter’s orbit. The three innermost stars are more massive and hotter than the Sun, while the outermost companion is more similar to our host star.
“Stars are generally formed in groups via the collapse of large molecular clouds containing dust and gas, and they can form various structures such as clusters, loosely bound associations, or binaries, triples, quadruples, and so on, depending mainly on their formation environment and how gravitational interactions with other objects affect this process,” Tibor Mitnyan, a researcher from the University of Szeged in Hungary and co-author of the new paper, told Gizmodo. “However, the formation of compact hierarchical systems is a very actively studied area of stellar astrophysics with a lot of questions and uncertainties.”
A stellar pair
Using the unique dynamic parameters, the team behind the study was also able to model the future evolution of this quadruple star system. In about 300 million years, the inner stars are going to merge into a single white dwarf (an extremely dense core remnant of large stars).
“The more massive white dwarf is formed after two mergers from the three inner stars, while the less massive white dwarf is simply formed from the originally fourth, most distant star,” Mitnyan said. The two remaining white dwarf stars will continue to circle one another, completing one orbit in around 44 days.
“It is also interesting to note that if such a double white dwarf system is found today, the observers would likely have no idea that it might have come from such an exotic compact 3+1 quadruple system with an outer period of about a thousand days,” Mitnyan added.
