An international group of astronomers has discovered that the well-known brown dwarf, Gliese 229B, is not a single entity but rather a pair of closely orbiting brown dwarfs, with masses roughly 38 and 34 times that of Jupiter, completing a revolution around each other every 12 days.
Since its discovery in 1995 by researchers at Caltech’s Palomar Observatory, Gliese 229B has been the subject of numerous studies. However, a significant enigma remained regarding its dimness relative to its mass. Brown dwarfs fall between stars and gas giants like Jupiter in terms of weight, and astronomers had estimated Gliese 229B’s mass to be about 70 times that of Jupiter. Yet, something was off, as such a massive object should exhibit greater brightness than what had been observed.
Recently, a team of astronomers led by Caltech has unveiled the solution: Gliese 229B is actually two closely linked brown dwarfs. Their characteristics match those of smaller, fainter brown dwarfs within this mass range, causing them to orbit each other every 12 days.
“For a long time, Gliese 229B was seen as the quintessential brown dwarf,” stated Jerry W. Xuan, a graduate student working with Dimitri Mawet, the David Morrisroe Professor of Astronomy. “Now we realize our previous understanding was incorrect; it consists of not one but two brown dwarfs. We simply weren’t able to observe such closely situated objects until now.” Xuan is the primary author of a new study published in the journal Nature, and a parallel study in The Astrophysical Journal Letters, led by Caltech graduate student Sam Whitebook and Tim Brandt from the Space Telescope Science Institute in Baltimore, reached the same conclusion.
This discovery raises fresh questions about the formation of tightly orbiting brown dwarf pairs and hints that more such brown dwarf binaries—or even pairs of exoplanets—could be waiting to be discovered. (Exoplanets are planets that orbit stars other than our Sun.)
“Identifying Gliese 229B as a binary not only clarifies the discrepancies between its mass and brightness but also enhances our understanding of brown dwarfs, which sit on the borderline between stars and gigantic planets,” explained Mawet, who is also a senior research scientist at JPL, managed by Caltech for NASA.
Gliese 229B was first identified in 1995 by a Caltech team including Rebecca Oppenheimer, then a graduate student; Shri Kulkarni, the George Ellery Hale Professor of Astronomy and Planetary Science; Keith Matthews, an instrument specialist at Caltech; along with others. They discovered that Gliese 229B’s atmosphere contained methane, a feature seen in gas giants like Jupiter but not found in stars, marking the first confirmed detection of brown dwarfs—thought to be the missing link between planets and stars for about three decades.
“Seeing the initial object smaller than a star orbiting another sun was thrilling,” recalled Oppenheimer, co-author of the current study and an astrophysicist with the American Museum of Natural History. “It sparked a wave of interest in peculiar objects like it, but it remained a puzzle for many years.”
Even after nearly 30 years of various observations, Gliese 229B continued to mystify scientists with its unexpected dimness. Researchers speculated that Gliese 229B might consist of two objects, but “to evade detection by scientists for 30 years, both brown dwarfs would need to be extremely close together,” commented Xuan.
To differentiate Gliese 229B into two separate entities, the team utilized two instruments at the European Southern Observatory’s Very Large Telescope in Chile. They employed the GRAVITY instrument, an interferometer that merges light from four telescopes to clearly delineate the objects, and the CRIRES+ (CRyogenic high-resolution InfraRed Echelle Spectrograph) instrument to capture different spectral signatures from both brown dwarfs. This technique involved measuring the motion of molecules within their atmospheres, demonstrating that one brown dwarf was moving towards Earth while the other was moving away, in sync with their orbit.
“It’s fantastic to see this new development nearly 30 years later,” remarked Kulkarni, not a co-author of this paper. “This binary system continues to amaze.”
Data collected over five months revealed that the brown dwarf pair, now designated Gliese 229Ba and Gliese 229Bb, orbit one another every 12 days, maintaining a distance of only 16 times that of the Earth-Moon gap. Together, they orbit a smaller, redder M-dwarf star every 250 years.
“These two celestial bodies, which revolve around each other, have a smaller radius than Jupiter. If something similar existed in our solar system, it would look quite unusual against our night sky,” noted Oppenheimer. “This is one of the most thrilling and significant discoveries in substellar astrophysics in decades.”
The origins of this swirling pair remain unclear. Some theories propose that brown dwarf pairs may form within the spinning disks of material surrounding a young star, which could break into fragments that later become gravitationally bound following close encounters. It remains to be seen whether similar processes help form pairs of planets around other stars.
Moving forward, the team aims to investigate even more closely orbiting brown dwarf pairs using tools like the Keck Planet Imager and Characterizer (KPIC), developed by Mawet’s team at the W. M. Keck Observatory in Hawaii, as well as the upcoming High-resolution Infrared Spectrograph for Exoplanet Characterization (HISPEC), currently being built at Caltech and other facilities under Mawet’s lead.
“The fact that the first known brown dwarf companion is a binary offers good prospects for our ongoing search for more,” says Xuan.
The research detailed in the paper titled “The cool brown dwarf Gliese 229B is a close binary,” was funded by NASA and the Heising-Simons Foundation. Other contributors from Caltech include Yapeng Zhang, a 51 Pegasi b Postdoctoral Scholar Research Associate in Astronomy; Aniket Sanghi, a graduate student; Konstantin Batygin, Professor of Planetary Science; and Heather Knutson, Professor of Planetary Science.
