"Gliese 229B was considered the poster-child brown dwarf," said Jerry W. Xuan, a Caltech graduate student who led the study under the guidance of Dimitri Mawet, a professor of astronomy. "And now we know we were wrong all along about the nature of the object. It's not one but two. We just weren't able to probe separations this close until now." The study, published in *Nature*, was corroborated by a separate investigation led by Caltech's Sam Whitebook and Tim Brandt from the Space Telescope Science Institute, which also concluded that Gliese 229B consists of two brown dwarfs.

The discovery raises intriguing questions about the formation of such closely bound brown dwarf pairs and hints that other similar binary systems, possibly including exoplanet pairs, may still be undiscovered. Brown dwarfs, which are more massive than gas giants like Jupiter but lighter than stars, occupy a unique position in the celestial hierarchy. This newfound binary nature of Gliese 229B not only explains its dimness but also enriches scientists' understanding of brown dwarfs.

Gliese 229B was initially identified by Caltech researchers at the Palomar Observatory, including Rebecca Oppenheimer, then a graduate student, alongside astronomers Shri Kulkarni and Keith Matthews. The team discovered methane in its atmosphere, a trait characteristic of gas giants but not stars, marking the first confirmed detection of brown dwarfs - celestial objects that had been theorized for decades as a bridge between planets and stars.

"Seeing the first object smaller than a star orbiting another sun was exhilarating," said Oppenheimer, who co-authored the recent study and is now at the American Museum of Natural History. "It started a cottage industry of people seeking oddballs like it back then, but it remained an enigma for decades."

Even with nearly 30 years of observations, the dimness of Gliese 229B remained unexplained. Scientists had speculated it could be a binary, but proving this required resolving two objects at an exceptionally close distance. The breakthrough came with observations using the European Southern Observatory's Very Large Telescope in Chile, employing two different instruments: GRAVITY, which combined light from four telescopes to spatially distinguish the two brown dwarfs, and CRIRES+, which detected distinct spectral signatures and the motion of atmospheric molecules indicative of a binary system.

Keith Matthews, one of the original discoverers, noted, "It is so nice to see that almost 30 years later, there has been a new development. Now this binary system stuns again." The recent data showed that the two brown dwarfs, designated Gliese 229Ba and Gliese 229Bb, orbit each other at a distance only 16 times that between Earth and the Moon, while jointly orbiting an M-dwarf star every 250 years.

"These two worlds whipping around each other are actually smaller in radius than Jupiter. They'd look quite strange in our night sky if we had something like them in our own solar system," added Oppenheimer. "This is the most exciting and fascinating discovery in substellar astrophysics in decades."

The formation of this brown dwarf duo remains a topic for further study, with some theories suggesting that such pairs could form from fragments in the disks of material around a young star. Whether similar mechanisms apply to exoplanet pairs is yet to be determined. Moving forward, the researchers plan to search for more closely bound brown dwarf binaries using advanced instruments like the Keck Planet Imager and Characterizer (KPIC) and the upcoming HISPEC instrument at the Keck Observatory.

"The fact that the first known brown dwarf companion is a binary bodes well for ongoing efforts to find more," Xuan remarked.

Research Report:The cool brown dwarf Gliese 229 B is a close binary

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