“The system exists against the odds,” said Brian Powell, a data scientist at NASA’s High Energy Astrophysics Science Archive Research Center about the source of starlight that was mysteriously brightening and dimming some 1,900 light-years away. The source, named TIC 168789840, is a system of three pairs of binary stars: three different stellar couplets revolving around three different centers of mass, but with the trio remaining gravitationally bound to one another and circling the galactic center as a single star system.
“Just the fact that it exists blows my mind,” said first author, Powell. “I’d love to just be in a spaceship, park next to this thing and see it in person.”
“The size of each of the stars would depend on your viewing position in the system. So, if you were closer to the quadruple, you would see four sun-like stars, the size of which depends on your position and the phases of the orbits. The two stars in the outer binary would then appear as bright, but very small objects,” Powell told The Daily Galaxy.
Eclipses in the Lightcurves
The breadth of observation of TESS encompasses nearly the entire sky, allowing for the identification of many candidate multiple star systems through the analysis of eclipses in the lightcurves (continuous time series measurements of the stars’ brightness). A collaboration between the NASA Goddard Space Flight Center and the MIT Kavli Institute, in conjunction with expert visual surveyors, has found well over 100 triple and quadruple star system candidates.
Most Systems are Quadruples
The large majority of the TESS discovered candidate triple and quadruple star systems are quadruples, followed by triples since it began searching the galaxy for exoplanets in 2018. But the source of starlight that was mysteriously brightening and dimming some 1,900 light-years away,” reports Robin George Andrews for the New York Times, “may top all those discoveries for its science fiction-like grandeur.”
“Though quadruple systems are much more rare than triple systems,” reports NASA, “the large outer orbit of the third star in a hierarchical triple, necessary for stability, substantially reduces the probability that the eclipse or occultation of the third star will be visually noticed in a TESS light curve. Beyond quadruple stars, the probability of systems with more stars being identified via photometry alone is remote, as the formation of sextuple systems is likely quite rare. This low probability is compounded by the requirement that each binary must be oriented in such a manner that they are all eclipsing.”
A Unique System
Although several of other six-star systems have been discovered, reports Andrews about NASA’s TESS discovery, this is the first in which the stars within each of those three pairings pass in front of and behind each other, eclipsing the other member of its stellar ballet, at least from the TESS space telescope’s view.
“These are the types of signals that algorithms really struggle with,” said lead author Veselin Kostov, a NASA Postdoctoral Fellow at Goddard Space Flight Center working. “The human eye is extremely good at finding patterns in data, especially non-periodic patterns like those we see in transits from these systems.”
Although exoplanets within the star system have yet to be confirmed, only one of the pairs could have any planets. Two of the system’s binaries orbit extremely close to one another, forming their own quadruple subsystem. Any planets there would likely be ejected or engulfed by one of the four stars. The third binary is farther out, orbiting the other two once every 2,000 years or so, making it a possible exoplanetary haven.
The Last Word –Its Origin is a Mystery
“Any conjecture about the origin of this system is highly speculative. A possible scenario is that a young binary captured a third star in a wide orbit, while strong accretion formed seed companions to each of the three stars which grew and migrated inward while the orbits evolved,” Powell wrote in an email to The Daily Galaxy.
“The origin of this whirling six-star system will remain a puzzle until we find others like it,” concludes Andrews.
In 2019, TESS discovered TOI 1338, its first circumbinary planet, a world orbiting two stars, 1,300 light-years away in the constellation Pictor. The two stars orbit each other every 15 days. One is about 10% more massive than our Sun, while the other is cooler, dimmer and only one-third the Sun’s mass. TOI 1338 b, the only known planet in the system. It’s around 6.9 times larger than Earth, or between the sizes of Neptune and Saturn. The planet orbits in almost exactly the same plane as the stars, so it experiences regular stellar eclipses.
Image Credit: NASA/MIT/TESS shows the spacecraft’s 13-sector mosaic of the southern sky, recorded over the course of a year. One object shown in the mosaic is a long, bright edge of our Milky Way galaxy.
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