Astronomers reported the discovery of TIC 120362137, the most compact known quadruple system with a three-plus-one arrangement. The system contains a tightly bound inner group of three stars and a more distant fourth member. Even so, the outer star is separated from the central trio by roughly the distance between Jupiter and the Sun, while the inner subsystem would fit inside Mercury's orbit.
The team first identified a pair of stars eclipsing one another every 3.3 Earth days in observations from NASA's Transiting Exoplanet Survey Satellite, or TESS. Those events produced brightness drops lasting one to two hours and initially looked like an ordinary eclipsing binary. Additional dimming lasting one to two days appeared every 25 to 26 days, revealing a third star with an orbital period of about 51 days.
Further eclipses then suggested a fourth object. Researchers confirmed it with the Tillinghast Reflector Echelle Spectrograph on the 1.5-metre Tillinghast telescope at Mount Hopkins in Arizona. The fourth star completes an orbit in about 1,046 days, the shortest outer period measured among known three-plus-one quadruple systems.
Team leader Tamás Borkovits of the University of Szeged in Hungary said finding such configurations is difficult because eclipses involving a distant fourth member may require decades of observation or a chance alignment. The discovery offers researchers a rare example for studying how closely packed hierarchical systems form and remain stable over long periods.
Measurements indicated that the three inner stars are hotter and more massive than the Sun. The outer member is cooler, less massive and more similar to the Sun. Computer simulations also allowed the team to trace a possible evolution for the system.
In the model, the most massive star in the innermost pair eventually becomes a red giant and merges with its companion. About 276 million years later, that combined object merges with the third inner star after both reach their red-giant phases. The resulting star loses substantial mass and becomes a white dwarf. The outer star undergoes its own evolution and also ends as a white dwarf.
The final pair is predicted to orbit every 44 days. The remnant produced through the two inner mergers would have about 89% of the Sun's mass, while the white dwarf descended from the outer star would have about 29%. The findings were published in Nature Communications, presenting both a record-setting present-day system and a model of its distant end state.
Its nested orbits give astronomers one system in which to test both the immediate stability of four closely associated stars and the merger sequence projected by the simulations.
