In 1995, astronomers made headlines with the first confirmed sighting of a brown dwarf—a celestial entity that falls in between a star and a planet in terms of size. However, recent research has revealed that the initial discovery wasn’t as straightforward as it seemed.
New studies using telescopes in Chile and Hawaii have shown that what was once thought to be a lone brown dwarf is actually a system of two brown dwarfs orbiting a small star at an incredibly close distance. These findings have redefined the object previously named Gliese 229B into two distinct entities: Gliese 229Ba, which boasts a mass 38 times that of Jupiter (the largest planet in our solar system), and Gliese 229Bb, slightly lighter at 34 times Jupiter’s mass.
Both brown dwarfs are located in the constellation Lepus, a mere 19 light-years away from Earth—a relatively short distance on the cosmic scale, with one light-year equal to about 5.9 trillion miles (or 9.5 trillion kilometers).
Binary brown dwarfs are quite rare in the universe. These two are gravitationally bound in a binary system, revolving around one another every 12 days at a distance that is just 16 times farther apart than the Earth is from the moon. This scenario is uncommon—only one other similar pair of brown dwarfs is known.
Brown dwarfs occupy a unique place in the cosmos. They aren’t classified as either stars or planets but instead represent a middle ground. Essentially, these celestial objects are “failed stars” that fell short of reaching the mass needed to trigger nuclear fusion in their cores, similar to stars. At the same time, they’re more massive than even the largest planets.
“A brown dwarf occupies the space between planets and stars,” explained Sam Whitebook, a graduate student at Caltech and the lead author of one of the studies, which appeared in the Astrophysical Journal Letters. “They are defined as objects capable of burning a heavier form of hydrogen, known as deuterium, but not the more prevalent form.”
Whitebook further elaborated, “In practical terms, this means their mass ranges from about 13 to 81 times that of Jupiter. Lacking the ability to fuse hydrogen, they can’t ignite the fusion processes that fuel most stars, which leads them to emit only a faint glow as they cool over time.”
The year 1995 was pivotal for astronomers, not only because of the brown dwarf discovery but also due to the announcement of the first exoplanet outside our solar system. Prior to Gliese 229B, the existence of brown dwarfs was merely theoretical. Observations of Gliese 229B raised questions, particularly when its mass was calculated to be around 71 times that of Jupiter.
“This was puzzling because an object of that size should have been much brighter,” commented Jerry Xuan, a Caltech astronomer and co-author of one of the studies published in Nature. “Models suggest that entities with masses exceeding 70 Jupiter masses undergo hydrogen fusion and become stars, which clearly wasn’t happening in this case.”
The latest observations confirmed the presence of two brown dwarfs, which orbit a common red dwarf star that has about 60% of the mass of our Sun. Even though both brown dwarfs have greater mass than Jupiter, they are actually smaller in diameter because of their higher density.
“We still don’t fully understand the formation of different brown dwarfs, nor do we clearly define the transition between a giant planet and a brown dwarf. The lines are somewhat blurred,” Xuan remarked. “This discovery also highlights that brown dwarfs can have unexpected configurations, illustrating the complexity and unpredictability of stellar formation. We should remain open to new discoveries.”