This artwork highlights a pair of recently uncovered brown dwarf twins, named Gliese 229Ba and Gliese 229Bb. Gliese 229B, discovered in 1995, was the first-ever confirmed brown dwarf, but until now astronomers thought they were observing a single body not two.There are thought to be billions of brown dwarfs in the Milky Way galaxy. Most go undetected because these objects, which are more massive than planets but lighter than stars, are so dim.
In the mid-1990s, scientists observed one for the first time, starting a cascade of thousands of brown dwarf discoveries in the decades since. Now, astronomers have returned their focus to that original discovery, a brown dwarf called Gliese 229B, and solved a long-standing mystery: why does this brown dwarf shine so faintly despite having a significant mass—70 times that of Jupiter?
The answer, which is detailed today in the journal Nature, is that this brown dwarf is actually two objects, orbiting very closely around each other.
“When we first found it, the only object we could compare it to was Jupiter, but it was also quite different from Jupiter,” said Rebecca Oppenheimer, a curator in the Museum’s Department of Astrophysics, who was a co-discoverer of Gliese 229B in 1995 and is an author on the new study. “Over the years it became more and more enigmatic, and the system it is in, more and more interesting. I think this is the whackiest alien solar system known at this point.”
[Two round objects spiral around each other in a corkscrew shape in space. The path of one orb is labelled in orange, and the other path is labelled in blue.
Text appears: “Two brown dwarfs, Gliese 229 Ba and Gliese 229 Bb, whip tightly around each other every 12 days, separated by only 16 times the distance between Earth and the Moon. The dancing duo orbit a cool M-dwarf star every 250 years.
The orbs continue to spiral around each other with bright red star in the background.]
In 1994, Oppenheimer was a grad student at the California Institute of Technology, working with astronomer Shrinivas Kulkarni, when she and a team of researchers at the Palomar Observatory tested out a new coronograph—an instrument that blocks starlight in order to allow observations of dim objects orbiting nearby. One night, they found a faint smudge next to the star Gliese 229, about 18 light years away. They looked for the object the following year with an infrared camera at Palomar and found it again, moving along with its host star.
These observations also revealed that the object contained methane in its atmosphere, which is commonly seen in planets like Jupiter but not in stars. Together, these findings were the first confirmed detection of brown dwarfs, objects that had been predicted 30 years prior.
Since then, Gliese 229B has puzzled astronomers with its unexpected dimness. Scientists suspected that this “single” brown dwarf might be twins, but even after hundreds of observations, astronomers were not able to show it.
“Gliese 229B is the poster-child brown dwarf,” says Jerry W. Xuan, a graduate student at the California Institute of Technology and the lead author of the new study. “And now we know we were wrong all along about the nature of the object. It’s not one but two. We weren’t able to probe separations this close until now.”
Using two specialized instruments at the European Southern Observatory’s Very Large Telescope in Chile, the researchers found that Gliese 229B is a pair of tight-knit brown dwarfs, weighing about 38 and 34 times the mass of Jupiter. Now called Gliese 229Ba and Gliese 229Bb, the brown dwarfs are extremely close to each other, astronomically speaking—just 16 times larger than the distance between Earth and the Moon. The pair race around each other every 12 days and orbit their 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,” Oppenheimer said.
The discovery leads to new questions about how tight-knit brown dwarf duos like this one form and suggests that similar systems are likely out there.
“This discovery that Gliese 229B is binary not only resolves the recent tension observed between its mass and luminosity but also significantly deepens our understanding of brown dwarfs, which straddle the line between stars and giant planets,” said study coauthor Dimitri Mawet, an astronomer at Caltech and NASA’s Jet Propulsion Laboratory.