Extreme Brown Dwarfs Found Spinning at Their Physical Limits

New research documents the fastest-spinning brown dwarfs on record. The objects are rotating so rapidly that, should they rotate any faster, they’d likely tear themselves apart. The finding could mean that these so-called “failed stars” have a built-in speed limit.

The three brown dwarfs are spinning 10 times faster than Jupiter, completing a single rotation around their axes once every hour. That’s about 30% faster than the fastest spinning brown dwarfs on record, according to the new paper, which is set to appear in an upcoming issue of the Astronomical Journal (a preprint is currently available at the arXiv).

Brown dwarfs are commonly called failed stars, as they are bigger than a planet but lack the sufficient pressure at their cores to trigger nuclear fusion, which is how stars pop into existence.

To measure the rotational speed of these objects, a team of astronomers from Western University in London, Ontario, Canada, used NASA’s Spitzer Space Telescope. This photometric data was then confirmed with follow-up observations gathered by the Gemini North telescope on Maunakea in Hawai‘i and the Carnegie Institution for Science’s Magellan Baade telescope in Chile.

“We used the Spitzer Space Telescope to monitor the overall brightness of brown dwarfs over time,” Megan Tannock, the first author of the paper and a PhD candidate at Western, explained in an email. “Spots on their surfaces—much like Jupiter’s great red spot—cause dimming or brightening as they rotate in and out of view, so we [were able to] determine the rotation periods of these objects based on repeated patterns in the brightness levels.”

Tannock and her colleagues then used the ground-based telescopes to study the dispersed light being emitted from these objects. The team “investigated how the rapid rotation affects the signatures of molecules in the objects’ atmospheres,” resulting in the detection of a strong effect, which served to confirm the short rotation periods, said Tannock.

The numbers are a bit staggering. The three brown dwarfs (designated 2MASS J04070752+1546457, 2MASS J12195156+3128497, and 2MASSJ03480772−6022270) are spinning at 220,000 miles per hour (350,000 km/hr) along their equators. The “relatively weak gravity of the brown dwarfs is barely holding them together,“ explained Sandy Leggett, an astronomer at Gemini North who wasn’t involved in the study, in a NOIRLab press release.

The researchers, despite trying, could not find any brown dwarfs spinning faster than these three, which means the team has likely stumbled upon an apparent maximum limit. As Leggett hinted, any faster and these objects would tear themselves apart. Tannock said this probably doesn’t happen, and that some kind of braking mechanism must exist to cap the rotational speed of brown dwarfs.

Unsurprisingly, these intense speeds are resulting in some dramatic atmospheric effects.

“These high speeds have an effect on the weather and can set the sizes of the storms that occur,” said Tannock. “When an object like a brown dwarf or a gas giant planet rotates very quickly, the vortices that form in the atmospheres tend to be smaller, and when they rotate more slowly, the vortices that form tend to be larger.”

As to why these objects are rotating so rapidly, the researchers speculate that it has something to do with their local environments when they formed.

“Brown dwarfs form in the same way as a star, from the collapse of a giant molecular cloud, and depending on the amount of material and distribution of the material, that sets the initial rotation rate,” explained Tannock. “Brown dwarfs can also get faster as they age: as they age, they cool, and then they contract, and to conserve a quantity called angular momentum, that means that they also have to spin faster.”

So these three brown dwarfs were likely born this way, but given that they formed hundreds of millions of years ago—and possibly even billions of years ago—it’s difficult to say.

To date, astronomers have measured the rotation periods of only 80 brown dwarfs (most complete a full spin around their axis every two to 10 hours), so the sample size is still quite small. Tannock said her team “got lucky” in finding these three speedsters, but she’s confident that astronomers will find more brown dwarfs with equally short rotation periods in the near future. A good goal moving forward will be to determine the relative population of these quick twisters in the Milky Way.