The brown dwarf Gliese 229B is a celebrity among celestial objects because it was the first clearly identified brown dwarf – an intermediate form of planet and star. Astronomers have now solved one of the big mysteries of Gliese 229B: why this brown dwarf is far too faint for its large mass of around 71 Jupiter masses. Images taken by the European Southern Observatory’s Very Large Telescope in Chile reveal that this brown dwarf is actually a binary system. In it, two brown dwarfs with 34 and 38 times the mass of Jupiter orbit each other so closely that they could not be separated from each other in previous observations. However, the new findings now raise questions about the formation of such extremely close pairs of brown dwarfs.
They are too small and cold to be stars, but too warm and massive to be planets: brown dwarfs are considered “failed stars” because their mass is not sufficient to ignite hydrogen fusion inside them. As a result, they only glow faintly and are much cooler than normal stars. According to the common definition, the boundary between the planet and the brown dwarf is around 13 Jupiter masses. Astronomers at the Palomar Observatory in California only discovered that brown dwarfs even existed in 1995. At that time, a team led by Rebecca Oppenheimer from the California Institute of Technology (Caltech) demonstrated for the first time that the star Gliese 229, 19 light-years away, was orbited by an object , which is 70 Jupiter masses but has methane in its gas shell – clear evidence that it could not be a star. “It was exciting to find the first celestial body around an alien sun that is smaller than a star,” remembers Oppenheimer. Since then, Gliese 229B has been considered the first clearly identified brown dwarf.
Too faint for its mass
But the brown dwarf Gliese 229B has been puzzling astronomers for just as long. “It is at least two to six times less luminous than it should be given its dynamic mass of 71.4 Jupiter masses,” explain lead author Jerry Xuan from Caltech and his colleagues. In addition, with this mass, the brown dwarf is already moving in an area in which nuclear fusion of hydrogen should actually begin within its interior. But that is apparently not the case. To clarify these contradictions, Xuan and his team have now re-targeted Gliese 229B over five months using two high-resolution instruments from the European Southern Observatory’s Very Large Telescope in Chile. The GRAVITY instrument combines the data from the four telescope units using interferometry and thereby achieves a particularly high spatial resolution. The CRIRES+ spectrograph can make the smallest shifts in the light spectrum of a celestial object visible.
Analyzes of the new observation data revealed that, contrary to previously thought, Gliese 229B is not just one brown dwarf, but consists of two. These two brown dwarfs are around 34 and 38 Jupiter masses in weight and orbit each other very closely. “So we now know that we were mistaken about the nature of this object all along,” says Xuan. “It’s two instead of one. But we haven’t been able to detect this until now because of their small distance.” Oppenheimer adds: “These two worlds together are therefore smaller than the radius of Jupiter. If they were in our solar system, it would look pretty strange in the night sky.” The data also shows that the two brown dwarfs Gliese 229 Ba and Bb orbit each other at a distance of only 0.042 astronomical units, meaning they only need around twelve days to complete one orbit. Their distance corresponds to around 16 times the distance from the Earth to the Moon. As astronomers explain, this makes the two celestial bodies the closest pair of brown dwarfs orbiting a star.
How can such close couples come about?
The dual nature of Gliese 229B now also clarifies the mystery of its lack of brightness: Because the two brown dwarfs are rather cool and small, the luminosity of the binary system is also low. “The discovery that Gliese 229B is a binary system resolves the contradiction between its mass and luminosity and deepens our knowledge of brown dwarfs,” says co-author Dimitri Mawet from Caltech. At the same time, the close couple also raises new questions. How such pairs of dwarfs form in orbit around a star has only been partially understood. Current theories assume that there is a fragmentation of dense gas and dust clouds in the disk of matter around young stars, which then leads to the collapse of two parts of these clouds and the formation of brown dwarfs. Because of their proximity to each other, these newly emerging celestial bodies remain connected by gravity and form a double system.
“This density-limited fragmentation limits the original distances of such objects to more than ten astronomical units,” explain the astronomers. “This means that substantial dynamic and dissipative processes are required to form a very tight binary system of brown dwarfs.” In addition, the orbits of Gliese 229 Ba and Bb are slightly tilted against the rotation axis of their central star – this is also difficult to explain with current models. “Even 30 years after its discovery, Gliese 229B continues to teach us new things about substellar objects,” say Xuan and his colleagues. They suspect that some other unusual massive brown dwarfs are actually close pairs. “This is the most exciting and fascinating discovery in substellar astrophysics in recent decades,” says Oppenheimer.
Source: Jerry Xuan (California Institute of Technology, Pasadena) et al., Nature, doi: 10.1038/s41586-024-08064-x