NASA TESS Spacecraft Uncovers Triply Eclipsing Triple-Star System So Coplanar Its Three Stars Lie Within a Third of a Degree of a Single Plane

Astronomers have discovered an extraordinary triple star system in which two Sun-like stars dance around each other every 4.75 days while a giant tertiary star ten times the size of our Sun loops around the pair every 412 days — and all three orbit in a plane so flat it is tilted less than one-third of a degree from perfect coplanarity, making it the most precisely aligned triple-star architecture ever found. The system, designated TIC 295741342 and located roughly 700 light-years from Earth in the constellation Cygnus, was identified in data from NASA's Transiting Exoplanet Survey Satellite by a team of astronomers from the Hungarian Konkoly Observatory and the University of Tasmania, who published their findings May 19 on the arXiv preprint server.

What makes the discovery scientifically valuable is that, because the orbital planes of all three stars are essentially edge-on from Earth's perspective, every star passes in front of every other star from our line of sight — a phenomenon astronomers call a "triply eclipsing" triple system, of which only a handful have ever been catalogued in three decades of all-sky surveys. The eclipses produce a distinctive light curve in which the system's brightness drops at three different cadences corresponding to the binary's 4.75-day period, the giant's 412-day period, and the rare combined geometry when all three line up. "It is the cleanest dynamical laboratory you could ask for," said lead author Dr. Tamás Borkovits of the Baja Astronomical Observatory in Hungary. "From those eclipses alone we can measure the masses, radii, and orbital architecture of all three stars to better than two percent — a level of precision normally reserved for our own solar system."

The sun-like inner binary consists of two stars of roughly 1.05 and 0.92 solar masses, each shining at temperatures within a few hundred kelvin of our own Sun, locked in an extraordinarily tight orbit only six million miles across — closer than Mercury is to the Sun. The giant tertiary is a red giant at the end of its main-sequence life, with a radius now 9.7 times that of the Sun and an estimated age of about three billion years. Computer models suggest that within the next 100,000 years — an astronomical blink — the giant will swell past its Roche limit and begin transferring mass onto the inner binary in a runaway process that could either spin the binary up into a contact configuration or, in a more dramatic outcome, eject the smaller of the two suns from the system entirely. "This is a star system that is going to die spectacularly, and we are seeing it just in time," Borkovits said.

The near-perfect coplanarity is itself a major mystery. Standard models of triple-star formation predict that distant tertiary companions should form with significant orbital tilts relative to inner binaries — often 30 degrees or more — because they are thought to be captured later or to form in separate fragments of a collapsing molecular cloud. The TIC 295741342 system instead suggests that all three stars formed essentially simultaneously from the same flattened protostellar disk and have preserved that alignment for billions of years. "If this turns out to be common, we will have to revise the theory of how multiple-star systems are born," said co-author Dr. George Zhou of the University of Southern Queensland. "The dynamics that keep three stars in such a flat plane for three billion years are not trivial."

The discovery also lifts the lid on a population of triple systems that has been hiding in plain sight. The TESS spacecraft has surveyed more than 200,000 of the brightest stars in the sky over its seven-year mission and has now identified eight confirmed triply eclipsing triples, of which TIC 295741342 has the longest outer period yet measured. Borkovits' team is already mining new TESS sectors for additional candidates and intends to follow up the brightest with the European Space Agency's CHEOPS satellite for precise photometry and with ground-based spectroscopy from the Magellan Telescopes in Chile. "We thought we knew how stars are born in groups," Zhou said. "This system is telling us we were wrong about the details, and the next ten years are going to teach us a great deal."