Gliese 229B is Actually Compact Pair of Brown Dwarfs, Astronomers Find

Gliese 229B was the first known brown dwarf, discovered in 1995. In new research, astronomers observed Gliese 229 B with the GRAVITY interferometer and, separately, the CRIRES+ spectrograph at ESO’s Very Large Telescope. Both sets of observations independently resolved Gliese 229B into two components, Gliese 229 Ba and Bb. They orbit each other every 12.1 days with a semimajor axis of 0.042 astronomical units (AU) and have masses of 38.1 and 34.4 Jupiter masses, respectively.

Gliese 229B was first discovered in 1995 but it was too dim for its mass.

While astronomers had measured its mass to be about 70 Jupiter masses, a brown dwarf of that heft should shine more brightly than what telescopes had observed.

“Scientists suspected Gliese 229B might be twins, but to evade notice by astronomers for 30 years, the two brown dwarfs would have to be very close to each other,” said Caltech graduate student Jerry Xuan.

“Seeing the first object smaller than a star orbiting another sun was exhilarating,” said Dr. Rebecca Oppenheimer, an astrophysicist 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.”

In the new research, the astronomers found that Gliese 229B is actually a pair of tight-knit brown dwarfs, weighing about 38 and 34 times the mass of Jupiter, that whip around each other every 12 days.

The observed brightness levels of the pair match what is expected for two small, dim brown dwarfs in this mass range.

“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 Professor Dimitri Mawet, an astronomer at Caltech and NASA’s Jet Propulsion Laboratory.

To resolve Gliese 229B into two objects, the astronomers used two different instruments, both based at ESO’s Very Large Telescope.

They used the GRAVITY instrument, an interferometer that combines light from four different telescopes, to spatially resolve the body into two, and they used the CRIRES+ instrument to detect distinct spectral signatures from the two objects.

The observations showed that the brown dwarf duo, now called Gliese 229Ba and Gliese 229Bb, orbit each other every 12.1 days with a separation only 16 times larger than the distance between Earth and the Moon.

Together, the pair orbit the M-dwarf star Gliese 229A once every 250 years.

“These two worlds whipping around each other are actually smaller in radius than Jupiter,” Dr. Oppenheimer said.

“They’d look quite strange in our night sky if we had something like them in our own Solar System.”

“This is the most exciting and fascinating discovery in substellar astrophysics in decades.”

The discovery is reported in a paper in the journal Nature.

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J.W. Xuan et al. The cool brown dwarf Gliese 229B is a close binary. Nature, published online October 16, 2024; doi: 10.1038/s41586-024-08064-x