Strange Circles in the Sky

Odd radio circles are one of the weirdest recent space discoveries

- PHIL PLAIT

IT’S RARE THESE DAYS for astronomers to find a new class of object in the heavens. After all, we've been searching the skies for centuries, so all the easy stuff has already been found. Adding new capabilities to our searches does tend to result in new discoveries, however. Looking in different wavelengths of the electromagnetic spectrum, for example, is a good way to uncover novel things because different objects emit light in different ways.

Objects in newly discovered classes also tend to be faint because, again, bright objects will have been spotted already. This fact is why identifying something completely new is unusual. It’s also why such findings can be baffling—by definition, we’ve never seen anything like them before.

In 2019 astronomers stumbled on just such a thing when they found multiple examples of a previously unknown kind of structure. The objects turned up in a pilot survey using the at-the-time newly completed radio telescope called the Australian Square Kilometer Array Pathfinder telescope (ASKAP), a collection of 36 radio dishes, each 12 meters wide, located in Western Australia.

The newfound celestial objects were relatively big and circular—a common shape for astronomical bodies. When something out there looks circular, it’s very likely that we're actually seeing a spherical shell, like a soap bubble. Near the middle of the bubble our line of sight goes through only a small amount of material, but near the edges that path intersects more of it. If the material glows, then it will look like a circle from any viewing direction. Dying stars that are blowing off winds of gas tend to make these kinds of structures, and many examples are known.