Ghostly circles in the sky can't be explained. And astronomers are excited. - Livescience.com

In September 2019, my colleague Anna Kapinska gave a presentation showing interesting objects she’d found while browsing our new radio astronomical data.

Among them, labeled by Anna as WTF?, was a picture of a ghostly circle of radio emission, hanging out in space like a cosmic smoke-ring.

We dubbed them ORCs, which stands for “odd radio circles”.

But we soon confirmed they are real, using other radio telescopes?

When we look in images taken with optical telescopes at the position of ORCs, we see nothing?

The rings of radio emission are probably caused by clouds of electrons, but why don’t we see anything in visible wavelengths of light.

Could they be supernova remnants, the clouds of debris left behind when a star in our galaxy explodes.

Could they be the rings of radio emission sometimes seen in galaxies undergoing intense bursts of star formation.

Could they be the giant lobes of radio emission we see in radio galaxies, caused by jets of electrons squirting out from the environs of a supermassive black hole.

Not likely, because the ORCs are very distinctly circular, unlike the tangled clouds we see in radio galaxies.

Could they be Einstein rings, in which radio waves from a distant galaxy are being bent into a circle by the gravitational field of a cluster of galaxies.

In our paper about ORCs, which is forthcoming in the Publications of the Astronomical Society of Australia, we run through all the possibilities and conclude these enigmatic blobs don’t look like anything we already know about.

My colleague Bärbel Koribalski notes the search is now on, with telescopes around the world, to find more ORCs and understand their cause.