In the last few years, astronomers have spotted a handful of gigantic and almost perfectly circular radio objects out in the distant universe. Though no one has an explanation for these mysterious entities yet, a team has recently added another one to their catalog, potentially moving them closer to solving this head-scratcher.
The enigma began shortly after the Australian Square Kilometre Array Pathﬁnder (ASKAP), a bank of 36 colossal dishes in Western Australia that scans the heavens in the radio part of the electromagnetic spectrum, began producing maps of the entire night sky in 2019.
ASKAP scientists were mainly looking for bright sources that could indicate the presence of black holes or huge galaxies glowing in radio waves. But some in the team are always on the hunt "for whatever is weird, whatever is new, and whatever looks like nothing else," Bärbel Koribalski, a galactic astronomer at Commonwealth Scientific and Industrial Research Organisation (CSIRO) and Western Sydney University in Australia, told Live Science.
Related: The 12 strangest objects in the universe
In the data, group member Anna D. Kapińska of the National Radio Astronomy Observatory in Socorro, New Mexico, spotted four bright radio circles, Koribalski recalled, though initially the rest of the researchers dismissed them as a more familiar phenomenon.
But when telescopes tried to look at the objects in other wavelengths, such as the optical light our eyes use to see, they turned up empty, leading the team to dub them odd radio circles (ORCs).
Even stranger, each of the ORCs had a galaxy perched almost exactly in its center, like a bullseye. The astronomers were able to determine that the entities were each several billion light-years away and potentially as big as a few million light-years in diameter.
No one had seen anything like these before, and in a paper published last year, the team offered 11 potential explanations as to what they could be, including imaging glitches, warps in space-time known as Einstein rings, or a new type of remnant from a supernova explosion.
The researchers have since scanned the skies again with ASKAP and found one more ORC to add to their collection, an entity about 1 million light-years across located about 3 billion light-years away. They posted their findings on April 27 to the preprint database arXiv, and they have been accepted for publication in the Monthly Notices of the Royal Astronomical Society.
The team has now narrowed their ideas down to three potential explanations, Koribalski said. The first is that perhaps there are additional galaxies forming a cluster near the object and bending bright material into a ring-like structure. These might simply be too faint to be picked up by current telescopes.
Another possibility is that the central supermassive black hole of these galaxies is consuming gas and dust, producing humongous, cone-shape jets of particles and energy. Astronomers have often spotted such phenomena in the universe, though generally the jets align in such a way with Earth that observatories see them as moving out of the sides of the galaxy.
Perhaps in the case of the ORCs, the jets are simply pointing directly towards our planet, Koribalski suggested, so that we are in essence looking down the barrel of a long tube, creating a circular, two-dimensional image around a central galaxy.
"The other explanation is more exciting," she said. "This could be something completely new."
It's possible that some unknown but highly energetic event took place in the middle of these galaxies, creating a blast wave that traveled out as a sphere and resulted in a ring structure. Koribalski isn't yet sure what type of event could leave such a signature, though perhaps it's a previously unknown product of crashing black holes such as the kind seen in gravitational waves at the Large Interferometer Gravitational Wave Observatory (LIGO) in the United States.
But Harish Vedantham, an astronomer at the Netherlands Institute for Radio Astronomy who was not associated with the work, favors the simpler idea — that the ORCs are a manifestation of a well-known phenomenon, and are bright jets shooting from a galaxy at a rarely seen angle.
Vedantham is guided in this by the principle of Occam's razor, which prefers mundane explanations over strange, new ones. "You can construct an exotic scenario," he told Live Science. "But the simplest answer is almost always correct."
In a similar vein, the possibility that an ORC is an invisible galactic cluster isn't appealing to him because "it's kind of hard to hide a cluster," he said. The objects are far away, but they are not that far, so at least a few additional galaxies should be noticeable, he added.
Both Vedantham and Koribalski agree that more telescope observations in other wavelengths should help scientists get a better idea of what's going on. New data will be forthcoming in the next six months or so, hopefully adding additional ORCs to their catalog, Koribalski said.
In the meantime, she is somewhat enjoying the mystery. "You become a detective. You look at all the clues and weigh them up against each other," she said. "Sometimes the universe just comes up with weird and wonderful shapes."
Originally published on Live Science.
Editor's Note: This story was updated to note that the new research has been accepted for publication in the Monthly Notices of the Royal Astronomical Society.
Live Science newsletter
Stay up to date on the latest science news by signing up for our Essentials newsletter.
Adam Mann is a freelance journalist with over a decade of experience, specializing in astronomy and physics stories. He has a bachelor's degree in astrophysics from UC Berkeley. His work has appeared in the New Yorker, New York Times, National Geographic, Wall Street Journal, Wired, Nature, Science, and many other places. He lives in Oakland, California, where he enjoys riding his bike.
By Harry Baker
By Rebecca Sohn