Strange Circles in the Sky Are Still Baffling Astronomers
ORCs—odd radio circles—are one of the weirdest recent discoveries in the heavens above
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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 already been spotted. This is why finding something completely new is unusual. It’s also why they can be baffling—by definition, we’ve never seen anything like them before.
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In 2019 astronomers stumbled upon just such a thing when they found multiple examples of a previously unknown 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 (or ASKAP) telescope, a collection of 36 radio dishes, each 12 meters wide, located in Western Australia.
The newfound celestial objects were relatively big and circular—a shape that’s common in astronomical bodies. When something out there looks circular, you’re very likely actually seeing a spherical shell, like a soap bubble. Near the middle of the bubble, your line of sight only goes through a small amount of material, but near the edges, that path intersects more. 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.
The very first ORC seen, called ORC 1 (of course), provided a clue to its origin. Searching images from other telescopes, the astronomers found an object emitting visible and infrared light right at the ORC’s center. Later observations indicated it’s an elliptical galaxy about five billion light-years from Earth. If it’s the source of ORC 1, this first-of-its-kind odd celestial circle is a staggering two million light-years across—more than 15 times wider than our own Milky Way galaxy—making it vast indeed. It’s also possible, however, that this galaxy may instead just happen to be coincidentally located near the apparent center of ORC 1 in the sky.
The MeerKAT image of ORC 1, superimposed on optical data from the Dark Energy Survey.
Worse, the other ORCs in the discovery paper are even weirder. ORCs 2 and 3 are roughly the same size in the sky and happen to be so close together that they’re nearly touching, strongly implying they’re related to each other in some way. But while ORC 2 is ring-shaped and bright, ORC 3 is faint and more like a disk, an evenly filled circle. If they’re related, then why are they so different? There are several galaxies apparently near to them, but again, these objects occupy a lot of real estate on the sky, so the odds of coincidentally finding galaxies in proximity to them are good.
Astronomers have found more ORCs in the years since those initial discoveries, and some of them also appear to have a galaxy in their center, which does seem to strengthen the correlation. If true, then those ORCs are also in the million-light-year range in size.
If this connection is real, what physical mechanism is creating them? Ideas abound. One possible driver is a supermassive black hole. As far as we can tell, every big galaxy has one of these monsters at its heart. As matter falls in, it piles up around the point of no return in a huge disk. Strong magnetic fields that spin like a tornado near the center can launch incredibly powerful beams of matter and energy that scream away from the black hole at high speed. A 2024 study published in the Astrophysical Journal shows that these beams can inflate the gas that exists between galaxies, creating structures very much like the ORC observations.
Not all ORCs are created equal, though. In correspondence published in the journal Astronomy and Astrophysics, astronomers reported they had found a diffuse x-ray emission at the location of an ORC they dubbed Cloverleaf, and its spectrum indicated that the high-energy light comes from hot gas typically found in low-mass galaxy groups that are somewhat heftier than our own Local Group (of which the Milky Way and Andromeda galaxies are the biggest members). This would put the ORC about 600 million light-years from Earth.
The structure of the Cloverleaf emission is somewhat irregular, displaying a patchiness that is usually associated with the collision and merger of two galaxy groups. Such a catastrophic event can dump a lot of energy into the gas around the galaxies, again creating an expanding wind that can take on a roughly spherical shape. An ORC is born.
If this interpretation of the Cloverleaf is correct, it indicates there’s more than one way to make an ORC. These weird objects may, in fact, reflect a wide range of physical structures and distinctly differing origins. Another object, first reported in a 2022 paper, bears a resemblance to the original ORCs but lacks a central galaxy or galaxy cluster and appears as a ring in ASKAP images. Its location, though, is suspicious: it’s located just three degrees from the edge of the Large Magellanic Cloud (or LMC), a satellite galaxy of the Milky Way. An association with the LMC would mean this ORC is only 160,000 light-years from us, which would make it only about 150 light-years in diameter! That would imply an entirely different formation history, making this ORC a likely supernova remnant, the expanding debris from a star that exploded long ago. What’s odd is that it’s located outside the LMC, where stars are sparse. Still, sometimes stars get ejected from galaxies; they can get catapulted away if they pass close to a massive black hole, for example, or if they were once part of a binary system and their companion star exploded, flinging them away at high speed.
It looks like the ORC category can encompass several different kinds of objects. That’s not too surprising; as we have discovered over and again in astronomy, a collection of phenomena that all look similar can have radically different causes. Some supernovae are from high-mass stars that explode when their core collapses at the end of their life, while others are from already dead white dwarfs that accumulate enough matter on their surface to cause a catastrophic star-wide thermonuclear explosion. Gamma-ray bursts can be caused by extremely massive stars exploding or from two tiny but superdense neutron stars colliding. The list of coincidental similarities in a class of objects goes on and on.
Remember, the term ORC is descriptive, not explanatory. Some may be from galaxy group collisions, others from exploding stars and still others from supermassive black hole belches. Although astronomers have studied ORCs for several years now, they are still a brand-new class of object, meaning we’re likely to have more theoretical explanations for them than we do actual examples to study in the sky. More observations should help astronomers classify them, and, as always, the hope is to categorize them, explain them and learn how they work.
Phil Plait is a professional astronomer and science communicator in Virginia. His column for Scientific American, The Universe, covers all things space. He writes the . Follow him online.
Source: www.scientificamerican.com