‘It’s twin’ twist: Astronomers solve 29-year-old brown dwarf mystery

A long-standing mystery that puzzled astronomers for decades has been finally solved.

The “first known brown dwarf,” Gliese 229B, always seemed a bit off. Too dim for its mass, it defied expectations.

A team of astronomers, led by Jerry Xuan from Caltech, decided to delve deeper into this enigma. Using advanced telescopes and instruments, they began to unravel the secrets of Gliese 229B.

To their surprise, the astronomers discovered that Gliese 229B wasn’t a single object but a pair of brown dwarfs, orbiting each other closely.

“Gliese 229B was considered the poster-child brown dwarf. 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,” said Xuan, a graduate student working with Dimitri Mawet, the David Morrisroe Professor of Astronomy.

Orbiting each other closely

Astronomers used the Palomar Observatory to discover Gliese 229B in 1995.

This was a significant finding as it marked the first confirmed detection of a brown dwarf, a celestial object that is heavier than gas giants but lighter than stars.

The discovery of methane in Gliese 229B’s atmosphere was a key indicator of its brown dwarf nature.

The Caltech team initially estimated the mass of Gliese 229B to be around 70 times that of Jupiter. However, they were perplexed by the fact that it appeared dimmer than expected for an object of that mass.

These initial findings prompted additional research, and after almost 100 observations, the team confirmed the presence of a pair of tightly bound brown dwarfs.

The two brown dwarfs — now called Gliese 229Ba and Gliese 229Bb — are about 38 and 34 times the mass of Jupiter. They orbit each other every 12 days, which was hidden from view for so long.

The astronomers note that the pair’s observed brightness levels are consistent with what is expected for two low-mass, dim brown dwarfs.

Extensive observations

To confirm that Gliese 229B has a companion, the team used two instruments at the European Southern Observatory’s Very Large Telescope based in Chile.

The GRAVITY instrument spatially resolved the object into two separate bodies, while the CRIRES+ instrument detected spectral signatures from each body.

The CRIRES+ instrument measured the movement of molecules in the brown dwarfs’ atmospheres, confirming their orbital motion.

The brown dwarfs are separated by approximately 16 times the distance between Earth and the Moon. As a pair, they orbit an M-dwarf star (smaller, red star than the Sun) every 250 years.

“These two worlds whipping around each other are actually smaller in radius than Jupiter. 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,” said Rebecca Oppenheimer, who was part of the 1995 Caltech team behind the discovery.

The formation of the brown dwarf pair is still unknown. One theory suggests they formed within a swirling disk of material around a forming star. The disk might have fragmented into “two brown dwarf seeds,” which later became gravitationally bound. Whether this process also forms planet pairs is still unclear.

The team plans to use instruments like the Keck Planet Imager and Characterizer (KPIC) and High-resolution Infrared SPectrograph for Exoplanet Characterization (HISPEC) to search for more closely orbiting brown dwarf binaries.

These instruments will help them study these objects in more detail and learn more about their formation and properties.

“The fact that the first known brown dwarf companion is a binary bodes well for ongoing efforts to find more,” concluded Xuan.

The findings were published in the journal Nature.