They’re twins! The mystery of the famous brown dwarf solved

Hundreds of articles have been written about the first known brown dwarf, Gliese 229B, since its discovery by Caltech researchers at the Institute’s Palomar Observatory in 1995. But a pressing mystery persists about this orb: it is too weak for its mass.

Brown dwarfs are lighter than stars and heavier than gas giants like Jupiter. And while astronomers had measured Gliese 229B’s mass to be about 70 times that of Jupiter, an object of that size would be expected to shine brighter than telescopes had observed.

Now, an international team of astronomers led by Caltech has finally solved this mystery: the brown dwarf is actually a pair of close-knit brown dwarfs, weighing about 38 and 34 times the mass of Jupiter, that face each other. around each other every 12 days. . The observed brightness levels of the pair match what is expected for two small, dark brown dwarfs in this mass range.

“Gliese 229B was considered the model brown dwarf,” says Jerry W. Xuan, a graduate student working with Dimitri Mawet, the David Morrisroe Professor of Astronomy. “And now we know that we were wrong all along about the nature of the object. It’s not one but two. We just haven’t been able to probe such close separations until now.”

Xuan is the lead author of a study reporting the results in the journal Naturetitled “Cold brown dwarf Gliese 229B is a close binary.”

A separate independent study in Letters from the astrophysical journalled by Caltech graduate student Sam Whitebook and Tim Brandt, an associate astronomer at the Space Telescope Science Institute in Baltimore, also concluded that Gliese 229B is a pair of brown dwarfs.

This discovery raises new questions about how close-knit brown dwarf duos like this form and suggests that similar brown dwarf binaries – or even exoplanet binaries – could be waiting to be discovered. (An exoplanet is a planet that orbits a star other than our sun.)

“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 boundary between stars and giant planets,” explains Mawet , who is also a principal investigator. scientist at JPL, managed by Caltech for NASA.

Gliese 229B was discovered in 1995 by a Caltech team that included Rebecca Oppenheimer, then a Caltech graduate student; Shri Kulkarni, George Ellery Hale Professor of Astronomy and Planetary Sciences; Keith Matthews, instrument specialist at Caltech; and other colleagues.

Astronomers used the Palomar Observatory to discover that Gliese 229B had methane in its atmosphere, a phenomenon typical of gas giants like Jupiter but not stars. The discovery marks the first confirmed detection of a class of cold star-like objects called brown dwarfs – the missing link between planets and stars – that had been theorized about 30 years previously.

“Seeing the first object smaller than a star orbiting another sun was exhilarating,” says Oppenheimer, co-author of the new study and an astrophysicist at the American Museum of Natural History. “It started a cottage industry of people researching oddities like that at the time, but it remained an enigma for decades.”

Indeed, almost 30 years after its discovery and hundreds of observations later, Gliese 229B still leaves astronomers perplexed by its unexpected brightness. Scientists suspected that Gliese 229B might be twins, but “to escape the attention of astronomers for 30 years, the two brown dwarfs would have to be very close to each other,” says Xuan.

To split Gliese 229B into two objects, the team used two different instruments, both based at the European Southern Observatory’s Very Large Telescope in Chile. They used the GRAVITY instrument, an interferometer that combines light from four different telescopes, to spatially resolve the body in half, and they used the CRIRES+ (CRyogenic high-resolution InfraRed Echelle Spectrograph) instrument to detect the distinct spectral signatures of the two objects.

The latter method involved measuring the motion (or Doppler shift) of molecules in the brown dwarfs’ atmosphere, which indicated that one body was moving toward us on Earth and the other was moving away from us – and vice versa as the two were in orbit around each other.

“It’s so nice to see that almost 30 years later there has been a new development,” says Kulkarni, who is not the author of the present article. “Now this binary system stuns again.”

These observations, carried out over five months, showed that the duo of brown dwarfs, now called Gliese 229Ba and Gliese 229Bb, orbit each other every 12 days with a separation only 16 times greater than the distance between the Earth and the Moon . Together, the pair orbit an M dwarf star (a star smaller and redder than our sun) every 250 years.

“These two worlds circling each other actually have a smaller radius than Jupiter. They would look quite strange in our night sky if we had something similar in our own solar system,” explains Oppenheimer. “This is the most exciting and fascinating discovery in substellar astrophysics in decades.”

How this pair of swirling cosmic orbs came to be remains a mystery. Some theories claim that pairs of brown dwarfs could form inside the swirling disks of material that surround a forming star. The disk would fragment into two brown dwarf seeds, which would then be gravitationally linked after a close encounter. It remains to be seen whether these same formation mechanisms are at work to form pairs of planets around other stars.

In the future, the team would like to search for binary brown dwarfs orbiting even closer together with instruments such as the Keck Planet Imager and Characterizer (KPIC), developed by a team led by Mawet at the WM Keck Observatory in Hawaii. , as well as Keck Observatory’s upcoming High-Resolution Infrared Spectrograph for Exoplanet Characterization (HISPEC), which is being built at Caltech and other laboratories by a team led by Mawet.

“The fact that the first known brown dwarf companion is a binary bodes well for continued efforts to find more,” says Xuan.