Six stars, six eclipses: ‘The fact that it exists blows my mind’

From star-destroying black holes to exploding comets, NASA’s Transiting Exoplanet Survey Satellite, or TESS, has spotted its share of surprises 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 may top all those discoveries for its science fiction-like grandeur.

The source, named TIC 168789840, is a system of six stars. That alone makes it a rarity, but what makes this sextuplet even more remarkable is that they consist 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. Although a handful of other six-star systems have been discovered, this one is unique: It 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 dance troupe, at least from our space telescope’s line of sight.

In other words, scientists have found a sextuply eclipsing sextuple star system. The discovery, posted online this month, has been accepted for publication in The Astronomical Journal.

Exoplanets within the star cluster have not yet been confirmed, but if you lived on a world within, the night sky would be something special, said Tamás Borkovits, an astronomer at the Baja Astronomical Observatory in Hungary and co-author. Any inhabitants of these worlds, “could see two suns, just like Luke Skywalker on Tatooine,” Borkovits said, as well as four other very bright stars dancing around the sky.

But 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.

Exotic stellar collections like this don’t just look cool. They refine and challenge our understanding of how multiple star systems form, said Patricia Cruz, an astrophysicist at the Center of Astrobiology in Madrid who was not involved with the work.

The depth and duration of TIC 168789840’s eclipses let astronomers determine the dimensions, masses and relative temperatures of its stars — vital information that can be plugged into models of star formation. But even with those clues, the origin of this whirling six-star system will remain a puzzle until we find others like it.

“The system exists against the odds,” said Brian Powell, a data scientist at NASA’s High Energy Astrophysics Science Archive Research Center in Greenbelt, Maryland, and the study’s lead author.

NASA’s TESS satellite looks for exoplanets by searching for temporary dips in a star’s light, caused by a planet orbiting in front of it from our perspective. But, Cruz said, scientists originally used the same light-blocking principle with other telescopes to spy stars obscuring other stars.

Using this concept, Powell, working with Veselin Kostov, an astrophysicist at the SETI Institute, designed a neural network that could identify eclipsing binary stars using TESS data.

The neural network studied an archive of nearly 80 million records of light-intensity changes, way more than humans alone could handle. “What machine learning can do is take this intractable data set and turn it into something a human can work with,” Powell said. It found a surfeit of multiple star systems, including the superlative TIC 168789840 last March.

Late last year the data was turned over to “hawk-eyed and very enthusiastic” professional and amateur stargazers all over the world, Borkovits said. Their efforts confirmed that TIC 168789840 was a sextuple system and helped clarify its stars’ characteristics, orbital dimensions and paths.

Andrei Tokovinin, an astronomer at the Cerro Tololo Inter-American Observatory in La Serena, Chile, and a co-author of the study, suggests one explanation for how the system came to be: Three stars formed within an expansive gas cloud, all orbiting each other in a triple-star system. Later, they encountered a dense clump of gas from the same cloud. That encounter led to disks forming around the original trio of stars, eventually giving each of them smaller companions.

Trying to unravel its origins is a worthwhile endeavor. But for Powell, “working with literally the most interesting data in the universe” to simply find this strange sextuplet is reward enough.

“Just the fact that it exists blows my mind,” he said. “I’d love to just be in a spaceship, park next to this thing and see it in person.”

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