Washington, October 19, 2024 – Nearly 30 years after the groundbreaking discovery of the first-ever brown dwarf, astronomers have found that what was initially thought to be a single celestial object is actually two brown dwarfs in a rare binary system. This unexpected revelation comes from two recent studies, utilizing advanced telescopes in Chile and Hawaii, offering a new perspective on the mysterious objects known as brown dwarfs.
Back in 1995, astronomers announced the first confirmed sighting of a brown dwarf, a body that falls between a planet and a star in size but lacks the mass needed to ignite nuclear fusion like a star. This discovery was groundbreaking at the time, as the existence of brown dwarfs had only been hypothesized until then. The object, named Gliese 229B, was seen as the final piece in solving a celestial puzzle. However, new research has now revealed that Gliese 229B is actually two distinct brown dwarfs orbiting each other in a gravitational lock, adding a new layer of complexity to our understanding of these objects.
The two brown dwarfs, now named Gliese 229Ba and Gliese 229Bb, are located approximately 19 light-years away from our solar system in the constellation Lepus. In astronomical terms, this is relatively close, making them one of the closest known binary brown dwarf systems to Earth. The two objects are exceptionally massive Gliese 229Ba has a mass 38 times that of Jupiter, while Gliese 229Bb is 34 times Jupiter’s mass. Despite their enormous mass, these brown dwarfs are smaller in diameter than Jupiter, as their density is much greater.
What makes this system even more fascinating is their tight orbit around each other. The brown dwarfs orbit every 12 days at a distance only 16 times the separation between the Earth and the moon. This is an extremely close orbit for such massive objects, and only one other known pair of brown dwarfs in the universe orbits each other as closely as this.
Brown dwarfs are often referred to as “failed stars” because they form like stars from collapsing clouds of gas but do not accumulate enough mass to initiate hydrogen fusion in their cores, which is the process that powers stars like our sun. However, they are much larger than planets, sitting in a unique position between the two. The boundary between a brown dwarf and a massive planet is still not entirely understood, and this discovery adds to the complexity of the formation and classification of these objects.
Astronomers originally measured the mass of Gliese 229B to be around 71 times that of Jupiter, leading to questions about its true nature. “This didn’t make sense, because an object of that mass should have been much brighter than Gliese 229B,” explained Jerry Xuan, an astronomer at Caltech and lead author of one of the studies published in Nature. “In fact, models suggest that an object with that mass would fuse hydrogen and become a star, which clearly wasn’t happening here.”
The new findings suggest that the confusion arose because astronomers were actually observing two brown dwarfs, not one. The duo orbits around a small red dwarf star, a type of star that is much cooler and smaller than our sun. The red dwarf, Gliese 229A, has about 60% of the mass of our sun and serves as the gravitational anchor for the two brown dwarfs.
Brown dwarfs like Gliese 229Ba and Gliese 229Bb can burn deuterium, a heavy form of hydrogen, during their early stages of formation but lack the mass required to sustain hydrogen fusion like regular stars. This means they gradually cool and fade over time, glowing faintly as they age.
Sam Whitebook, a Caltech graduate student and lead author of one of the studies published in the Astrophysical Journal Letters, described the discovery as “an exciting surprise.” Whitebook noted, “We still don’t really know how different brown dwarfs form, and the boundary between a giant planet and a brown dwarf remains fuzzy.”
The discovery of this binary system provides critical insights into the formation processes of celestial bodies. “Brown dwarfs can come in weird configurations that we were not expecting,” Xuan said, emphasizing how unpredictable and complex the star-formation process can be.
This discovery, along with the first confirmed exoplanet—also detected in 1995 marks a major moment in astronomical history. While brown dwarfs remain an enigma, these findings are crucial in advancing our understanding of how stars and planets evolve and interact in our universe. The complexity of these objects continues to challenge existing models, and astronomers anticipate more surprises in the future as they continue to observe and study these cosmic phenomena.
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