"Look, that star is so bright!"

Have you ever discovered something bright in the sky and wonder whether that is an airplane, a satellite or a... UFO? Recently, a group of astronomers from Sydney faced a similar problem. They found four "Odd Radio Circles (ORCs)" using radio telescopes. ORCs are roughly circular with bright edges and they are emitting radio waves. ORC's characteristics cannot be explained by known celestial objects. What do you think an ORC is? Let's become a detective to infer the identity of ORCs with a "boldly hypothesise, carefully verify" ScientificMindset.

Possibility 1, Imaging Artefacts
When calibration errors happen in a telescope, a bright ring may form around sources. However, ORCs are observed using two separate, independent telescopes. Well... ORCs are probably not imaging artefacts.

Possibility 2, SupernovaRemnant
When the lives of some stars come to an end they blow themselves up by throwing part of their mass into space called supernova explosions and forming circular remnants. But according to our understanding of supernova remnants, there is only a 0.0021% chance that a supernova remnant would appear in the locations of ORCs. This suggests that ORCs are very unlikely to be supernova remnants.

Possibility 3, RingGalaxy
Some galaxies such as the Cartwheel Galaxy in Sculptor look like a wheel. But those galaxies always emit visible light. That is a huge difference between ring galaxies and ORCs as ORCs only emits radio wave.

Possibility 4, ......
......
Possibility 1000, ......

"When you have eliminated the impossible, whatever remains, however improbable, must be the truth." stated by Sherlock Holmes. If you would like to know more about ORCs, please check our latest issue of HKSpM Newsletter (July – September 2021).