A pair of recently uncovered brown dwarf twins, named Gliese 229ba and Gliese 229bb. Gliese 229b, discovered in 1995, was the first-ever confirmed brown dwarf, but until now astronomers thought they were observing a single body not two. New observations from the European Southern Observatory’s Very Large Telescope in Chile revealed that the orb is two brown dwarfs tightly orbiting around each other every 12 days (as indicated by the orange and blue orbital lines), with a separation only 16 times larger than the distance between earth and the moon. The brown dwarf pair orbit a cool m-dwarf star every 250 years.

(Credit: K. Miller, R. Hurt (Caltech/IPAC))

Brown dwarfs are supposed to be the ‘Goldilocks’ of celestial objects: lighter than stars, but heavier than gas giants like Jupiter, with a “just right” weight somewhere in between.

But something was amiss with the first known brown dwarf, Gliese 229B. Discovered by Caltech researchers at the Institute’s Palomar Observatory in 1994, astronomers noted that although Gliese 229B weighed about 70 times more than Jupiter, it shined much more dimly given its mass.

Although hundreds of papers have been written about Gliese 229B since its discovery, the mystery about the discrepancy between its size and brightness lingered. Now, two teams of astronomers have explained that anomaly: Gliese 229B is actually a pair of tight-knit brown dwarfs, weighing about 38 and 34 times the mass of Jupiter.

A Pair of Brown Dwarfs

The observed brightness levels of the pair match what is expected for two small, dim brown dwarfs in this mass range, according to reports in the journal Nature and The Astrophysical Journal Letters (AJL). The observations, from the European Southern Observatory’s Very Large Telescope in Chile, revealed that the two brown dwarfs tightly orbit each other every 12 days.

“Gliese 229B was considered the poster-child brown dwarf,” Jerry W. Xuan, a Caltech graduate student who is an author on the Nature paper, said in a press release. “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.”

Read More: A Failed Star can Form Brown Dwarf Stars, Which Host Their Own Planetary Systems

High-Resolution Pictures of a Novel Orbit

Rebecca Oppenheimer was a graduate student member of the Caltech team that first discovered Gliese 229B. As an author of the APL report, she’s thrilled anew — because she and her team again witnessed a novel phenomenon — two brown dwarfs closely orbiting each other.

“Seeing the first object smaller than a star orbiting another sun was exhilarating,” Oppenheimer, now an astrophysicist at the American Museum of Natural History, said in a press release. “It started a cottage industry of people seeking oddballs like it back then, but it remained an enigma for decades.”

Both teams captured clearer pictures of what the double brown dwarf was doing through newer, higher-resolution telescopes. How and why the twins formed is still a mystery. Some theories say brown dwarf pairs were seeded from the materials that surround a forming star.

The discovery also leads to questions about if there are more such twins in space. Astronomers will continue to turn ever-more powerful instruments to the skies to answer those questions.

Read More: 7 of the Brightest Stars You Can See with the Naked Eye on Earth

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Before joining Discover Magazine, Paul spent over 20 years as a science journalist, specializing in U.S. life science policy and global scientific career issues. He began his career in newspapers, but switched to scientific magazines. His work has appeared in publications including Science News, Science, Nature, and Scientific American.