In 1995, researchers from the California Institute of Technology (Caltech) at the Palomar Observatory made the initial observation of what appeared to be a brown dwarf orbiting the red dwarf star Gliese 229, located approximately 19 light-years from Earth.
This object, designated Gliese 229 B, has since perplexed astronomers due to its unexpectedly low luminosity relative to its mass. Despite having a mass approximately 70 times that of Jupiter, its brightness was notably lower than predicted. Recently, an international team of astronomers, led by Caltech, resolved this enigma by determining that Gliese 229 B is actually a pair of closely orbiting brown dwarfs.
The study was spearheaded by Jerry W. Xuan, a graduate student in Caltech’s Department of Astronomy, under the guidance of Dimitri Mawet, the David Morrisroe Professor of Astronomy. They collaborated with a global team of experts from various prestigious institutions, including the National Research Council of Canada Herzberg, the European Southern Observatory (ESO), the European Space Agency (ESA), the Laboratory of Space Studies and Instrumentation in Astrophysics (LESIA), the Center for Interdisciplinary Exploration and Research in Astrophysics (CIERA), the Max Planck Institutes for Astronomy (MPIA) and Extraterrestrial Physics (MPE), and NASA’s Jet Propulsion Laboratory (JPL).
Published in Nature, this study was supported by NASA and the Heising-Simons Foundation. Notably, several of the original discoverers of Gliese 229 B in 1995 contributed to this latest research, including Rebecca Oppenheimer, who was a Caltech graduate student at the time (now an astrophysicist at the American Museum of Natural History), Shri Kulkarni, the George Ellery Hale Professor of Astronomy and Planetary Science, and Keith Matthews, a specialist in instrumentation at Caltech.
At the time, their observations revealed the presence of methane in the atmosphere of Gliese 229 B, a characteristic commonly associated with gas giants but not typical of stars. This discovery marked the first confirmed detection of a brown dwarf, a class of cool, star-like objects that bridge the gap between gas giants and stars. These findings were significant, as they provided empirical confirmation of the existence of brown dwarfs, a theoretical class of objects that had been hypothesized approximately 30 years earlier.
“Seeing the first object smaller than a star orbiting another sun was exhilarating,” said Oppenheimer in a Caltech news release, “It started a cottage industry of people seeking oddballs like it back then, but it remained an enigma for decades.”
“Gliese 229 B was considered the poster-child brown dwarf,” added Xuan. “And now we know we were wrong all along about the nature of the object. It’s not one but two. We just weren’t able to probe separations this close until now.”
Since the discovery of Gliese 229 B nearly 30 years ago, astronomers have conducted hundreds of observations, yet its unusually low brightness remained a persistent puzzle. While scientists speculated that Gliese 229 B might consist of two closely orbiting brown dwarfs, the pair would need to be extremely close to avoid detection for almost three decades.
To test this hypothesis, the research team utilized the GRAVITY interferometer on the European Southern Observatory’s (ESO) Very Large Telescope in Chile to spatially resolve the two objects. They further employed the CRyogenic high-resolution InfraRed Echelle Spectrograph (CRIRES+) to identify their distinct spectral signatures and measure their Doppler shifts. The results confirmed that Gliese 229 B comprises two brown dwarfs, Gliese 229 Ba and Gliese 229 Bb, with masses of approximately 38 and 34 times that of Jupiter, respectively. These two objects orbit one another with a period of 12 days and are separated by a distance roughly 16 times the Earth-Moon distance.
The observed brightness levels are consistent with the expected luminosity for two small brown dwarfs of this mass range, finally explaining the object’s dimness.
“This discovery that Gliese 229 B 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 Mawet, a senior research scientist at NASA JPL. The discovery of this duo raises new questions about how tight-knit brown dwarfs form and suggests similar binaries may be out there and waiting to be found.
Some theoretical models propose that brown dwarf pairs may form within a star’s protoplanetary disk when the disk fragments into two protostellar cores that later become gravitationally bound following a close encounter. This same mechanism might also account for the formation of closely orbiting exoplanet binaries, though further observational evidence is needed to validate these ideas. As Rebecca Oppenheimer remarked, this discovery represents a highly exciting development in the study of brown dwarfs and binary systems, potentially offering new insights into the processes governing their formation and evolution.
“These two worlds whipping around each other are actually smaller in radius than Jupiter,” she 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.”
In the future, Xuan and his colleagues plan to identify additional brown dwarf binaries using both current and next-generation observational instruments. These include the Keck Planet Imager and Characterizer (KPIC) and the Keck Observatory’s High-resolution Infrared SPectrograph for Exoplanet Characterization (HISPEC). The KPIC was developed by a team led by Dimitri Mawet, while HISPEC is currently under construction at Caltech and other institutions, also under Mawet’s leadership.
In parallel, an independent study published in The Astrophysical Journal Letters, led by Sam Whitebook, a Caltech graduate student, and Tim Brandt, an associate astronomer at the Space Telescope Science Institute, reached similar conclusions. Their research also determined that Gliese 229 B consists of two closely orbiting brown dwarfs. These converging findings from separate teams further reinforce the significance of this discovery in advancing our understanding of brown dwarf systems.
