Two articles, one published in the journal “Nature” and one in “The Astrophysical Journal Letters”, report the results of two independent studies on Gliese 229 B, the first brown dwarf whose existence was confirmed, which conclude that it’s actually a pair of close brown dwarfs. A team led by Caltech researcher Jerry Xuan used the GRAVITY and CRIRES+ instruments mounted on ESO’s VLT in Chile while a team led by Sam Whitebook, also from Caltech, and Tim Brandt of the Space Telescope Science Institute in Baltimore, used the NIRSPEC instrument installed at the Keck Observatory in Hawaii. There were already suspicions about the nature of Gliese 229 B due to some inconsistencies detected in its characteristics but only now was it possible to find the evidence.
The first evidence of the existence of a brown dwarf came in 1994 but hypotheses regarding the existence of objects halfway between planets and stars were offered decades earlier. According to estimates from observations conducted in the 1990s, Gliese 229 B is slightly smaller than Jupiter but 70 times more massive. Its detected gravitational bond with the red dwarf Gliese 229 A, more than six billion kilometers away, showed that it already had a companion.
The problem already detected at the time following the interest aroused by Gliese 229 B and the subsequent studies conducted, concerned its brightness. This brown dwarf seemed too dim for the mass that was estimated. One of the possible solutions to this inconsistency was that it was a pair of close brown dwarfs that the instruments of the time couldn’t resolve. Now the situation has changed.
The new studies confirm the hypothesis of the two brown dwarfs. According to the new estimates, they have masses that are 38 and 34 times Jupiter’s and orbit each other in 12 Earth days at a distance that is about 16 times that of the Earth from the Moon. This pair of brown dwarfs orbits the red dwarf Gliese 229 A about every 250 Earth years.
The two brown dwarfs were cataloged as Gliese 229 Ba and Gliese 229 Bb. Their discovery represents a step forward, but researchers want to understand how they formed. They could be the result of the division of a circumstellar disk into two smaller disks, too small to form “real” stars. In that situation, two brown dwarfs could have formed that remained close together due to their gravitational bond.
Professor Dimitri Mawet of Caltech, one of the authors of the research published in “Nature”, noted that the evidence for the binary nature of Gliese 229 B doesn’t resolve the inconsistency between their estimated total mass and their observed brightness, but it deepens our knowledge of brown dwarfs, which are between stars and giant planets.
Studying such dim objects isn’t easy even though the Gliese 229 B pair is about 19 light-years from Earth and therefore very close from an astronomical point of view. Not accidentally, it took thirty years to find evidence of its binary nature. Even more powerful and sensitive instruments will come into service in the coming years offering the possibility of studying the Gliese 229 B pair even better and also of discovering other close brown dwarf binary systems.
