In astronomy, an Odd radio circle (ORC) is a very large unexplained astronomical object that, at radio wavelengths, is highly circular and brighter along its edges. As of 27 April 2021, there have been five such objects (and possibly six more) observed. The observed ORCs are bright at radio wavelengths, but are not visible at visible, infrared or X-ray wavelengths. Three of the ORCs contain optical galaxies in their centers, suggesting that the galaxies might have formed these objects.
The ORCs were detected in late 2019 after astronomer Anna Kapinska studied a Pilot Survey of the Evolutionary Map of the Universe (EMU), based on the Australian Square Kilometre Array Pathfinder (ASKAP) radio telescope array. Every detected ORC, about 1 arcminute in diameter, are some distance from the galactic plane, at high galactic latitudes. The possibility of a spherical shock wave, associated with fast radio bursts, gamma-ray bursts, or neutron star mergers, was considered, but, if related, would have to have taken place in the distant past due to the large angular size of the ORCs, according to the researchers. Also according to the astronomers, "Circular features are well-known in radio astronomical images, and usually represent a spherical object such as a supernova remnant, a planetary nebula, a circumstellar shell, or a face-on disc such as a protoplanetary disc or a star-forming galaxy, ... They may also arise from imaging artefact around bright sources caused by calibration errors or inadequate deconvolution. Here we report the discovery of a class of circular feature in radio images that do not seem to correspond to any of these known types of object or artefact, but rather appear to be a new class of astronomical object."