Zenghua Zhang, an astronomer at Nanjing University in China, and his colleagues were combing through catalogs of stars in search of cold brown dwarfs, interstellar objects that fall somewhere between planets and stars.
They found something odd, and rare, in the Milky Way.
First, the astronomers identified what they believed was a lone brown dwarf orbiting a bright single star. Further investigation revealed that the brown dwarf was actually two. After submitting a paper about the discovery, Dr. Zhang said, they realized that the bright companion was a pair of stars, too.
“I like to call this a double-double,” said Adam Burgasser, an astrophysicist who leads the Cool Star Lab at the University of California, San Diego, and was involved in the discovery.
A study describing the quadruple star system — a brown dwarf dancing around another, locked in an orbit with two brighter stars also circling each other — was published by the Monthly Notices of Royal Astronomical Society this summer.
The discovery will help scientists untangle the properties of brown dwarfs. These objects form like stars but have too little mass to consistently fuse hydrogen, a process that heats a star and makes it shine. Brown dwarfs have atmospheres similar to gas giant planets, like Jupiter or Saturn.
But because they are cold and faint, brown dwarfs can be difficult to study. Astronomers typically search for brown dwarfs orbiting companion stars, which often burn brighter and are easier to measure.
According to Dr. Burgasser, these binaries of brown dwarfs and brighter stars often formed out of the same material, at the same place and time. Measuring the brighter stars, then, can be useful for estimating the properties of the fainter brown dwarfs, like their age, temperature and composition.
Such systems become benchmarks for understanding lone brown dwarfs across the galaxy, and the formation and evolution of the least massive stars. Finding two brown dwarfs around two brighter stars “helps twice as much,” Dr. Burgasser said. “It becomes kind of like a super benchmark.”
To hunt for brown dwarfs orbiting companion stars, Dr. Zhang and his team scanned stellar databases made with data from two retired space missions, NASA’s Wide-field Infrared Survey Explorer and the European Space Agency’s Gaia telescope.
They found a brown dwarf and a brighter stellar companion about 82 light-years from Earth, in an orbit more than 1,600 times as wide as the distance between our planet and the sun. But both the brown dwarf and the star were brighter than expected. Measurements of their spectra, taken with the Southern Astrophysical Research telescope in Chile, helped the astronomers confirm that both were actually pairs.
The brighter pair consists of two red dwarfs, the most common type of star in the Milky Way, both about 17 percent the mass of the sun. Their combined brightness is 100,000 times as faint as the North Star in visible light, Dr. Zhang said.
The brown dwarfs, by contrast, emit almost no visible light. They are both about the size of Jupiter and have detectable methane in their atmosphere. According to Dr. Burgasser, there are most likely billions of brown dwarfs like this in our galaxy — though only about 30 have been found orbiting brighter companions.
Future observations, possibly with NASA’s James Webb Space Telescope, will help the team take sharper images of the two brown dwarfs and nail down their exact masses, which can then be used as a benchmark for similar objects elsewhere in the galaxy.
Upcoming star surveys, including with the Euclid space telescope and the ground-based Vera C. Rubin Observatory, will allow astronomers to probe ever deeper for more faint brown dwarfs across the Milky Way.
Quadruple star systems are not unheard of — astronomers have discovered arrangements with as many as seven stellar objects. That they exist indicates that such systems are able to survive the processes involved in early stellar formation. Stars that form too close together, by contrast, may gravitationally knock one another away and scatter at hyperfast speeds through the Milky Way.
“These are the kinds of clues that we’re seeing, the output of multiple star formation,” Dr. Burgasser said. “But we’re still interested in what got it there in the first place.”
Katrina Miller is a science reporter for The Times based in Chicago. She earned a Ph.D. in physics from the University of Chicago.
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