Astronomers have discovered a bunch of weird unidentified circles in space, visible only in radio light, thanks to images captured by one of the most sensitive observatories on the planet.
The mysterious rings “do not seem to correspond to any known type of object” and so have been simply dubbed Odd Radio Circles, or ORCs, according to a new study, led by Western Sydney University astrophysicist Ray Norris.
“We have discovered, to the best of our knowledge, a new class of radio-astronomical object, consisting of a circular disc, which in some cases is limb-brightened, and sometimes contains a galaxy at its centre,” Norris and his colleagues said in the study, which was recently posted on the preprint server arXiv and has not yet been peer-reviewed.
“None of the known types of radio object seems able to explain it,” the team said.
The team describes four of these inscrutable ORCs, three of which were detected by the Australian Square Kilometre Array Pathfinder (ASKAP) telescope, a network of radio antennae that covers four square kilometers of the Australian Mid West. ASKAP has been scanning the sky in the radio spectrum to create an Evolutionary Map of the Universe that could help scientists better understand the development of stars and galaxies.
Norris and his colleagues noticed three odd blobs in ASKAP’s 2019 observations. Each of the circles measures about one arcminute across, which is roughly equivalent to 3 percent the diameter of the Moon. However, it’s difficult to tell how far away the ORCs are based on these images, and that in turn makes it challenging to estimate the actual size of the objects, at least until more detailed observations are made.
The glowing circles are so bizarre that Norris and his colleagues wondered if they might be an instrumental glitch, especially since radio imagery often contains errors that look like rounded apparitions, according to the study.
But when the team went hunting through archival datasets, they were surprised to discover that a fourth ORC was imaged all the way back in 2013 by India’s Giant Metrewave Radio Telescope, though nobody had made note of it at the time.
By combing through past radio surveys, as well as obtaining new images with the Australia Telescope Compact Array, the researchers were able to collect at least two independent observations of each ORC. The fact that the circles show up across multiple telescope datasets makes instrumental error “a very improbable explanation,” the team said in the study.
So if the ORCs are real celestial objects, what could they be? Norris and his colleagues outline several possible identities for the objects, though none of them are an obvious fit.
The circles might be the fallout of exploded stars, or bubbles blown out by winds in galactic star factories, or “Einstein rings,” which are signatures of warped spacetime created by the gravity of massive objects. They could be the ghosts of highly energetic events that occurred millions of years ago, such as gamma ray bursts, fast radio bursts, or plasma jets emitted by active galactic cores.
“We also acknowledge the possibility that the ORCs may represent more than one phenomenon,” the team noted, adding that they may have been “discovered simultaneously because they match the spatial frequency characteristics of the ASKAP observations, which occupy a part of the observational parameter space which has hitherto been poorly studied.”
Norris and his colleagues plan to continue examining the ORCS to see if they can tease out some of these tantalizing mysteries. One thing’s clear, however: discoveries like this are likely to become more common as radio astronomy matures in the coming years.
Within the next decade or so, ASKAP will join the Square Kilometre Array, a massive intercontinental observatory currently in construction, which will be by far the most sensitive radio instrument on Earth once it’s operational. The discovery of the ORCs is fascinating by itself, but it also foreshadows a new era of astronomy that is already sharpening our view of space.