There Are Giant Rings in Deep Space — and Nobody Knows What Made Them

The Note in the Margin

The discovery looks almost ordinary in the photograph. A blurry blue circle drifts across a black field, threaded with the brighter dots of background galaxies. There is no caption, no annotation, just the image and three letters scrawled in the corner: W·T·F.

The astronomer who wrote them was Anna Kapinska, then at the National Radio Astronomy Observatory. She had been working her way through preliminary data from the Australian Square Kilometre Array Pathfinder telescope — known as ASKAP — when the circle stopped her. She had no idea what she was looking at. She also had not yet realized what she had done. She had just discovered an entirely new kind of object in the universe.

Five years and five confirmed examples later, astronomers have a working name for them — Odd Radio Circles, or ORCs — but the question Kapinska wrote in the margin of that first image is, at the time of writing, still the most accurate summary of where the field stands.

A New Telescope, A New Kind of Mystery

To understand why ORCs were missed for as long as they were, you have to understand the telescope that finally found them.

ASKAP is operated by Australia's Commonwealth Scientific and Industrial Research Organisation. It is built from 36 individual radio dishes, each twelve meters across, working as a single instrument. Its defining feature is something called a phased array feed: a receiver design that lets each of those dishes look at thirty square degrees of sky simultaneously — an area roughly equivalent to one hundred and fifty full moons placed side by side. That is a field of view roughly an order of magnitude greater than older radio telescopes could manage in a single pointing.

The flagship project running on ASKAP, the Evolutionary Map of the Universe — abbreviated EMU and named after the Australian aboriginal "Emu in the Sky" constellation — is intended to multiply the number of known radio sources in the universe by a factor of about thirty. Where the entire history of radio astronomy had cataloged roughly two and a half million radio sources, EMU alone is expected to add another seventy million.

The project's principal investigator, Ray Norris of CSIRO and Western Sydney University, predicted from the start that an instrument this powerful, looking at this much sky, would find things nobody had thought to look for. He turned out to be right almost immediately.

What ORCs Aren't

The first instinct, when you see a perfectly circular halo of radio waves in deep space, is to assume you are looking at something familiar. The most common circular radio source we already know about is the supernova remnant — the expanding shockwave of a stellar explosion. Supernova remnants are radio-bright. They are often roughly circular. They were the natural first guess.

The natural first guess turned out to be wrong, on multiple grounds. Supernova remnants are concentrated near the plane of our own galaxy, where most stellar explosions occur. ORCs are found at high galactic latitudes, well off the plane. Supernova remnants are typically a few dozen light-years across at most. ORCs are hundreds of thousands of light-years across — large enough to swallow entire galaxies. And the ORCs that have been studied most carefully appear to have galaxies sitting at or near their centers, which is not a feature of supernova remnants.

The second instinct was that the circles might be artifacts of the telescope itself, perhaps an optical illusion in the data processing. Independent observations using completely different instruments — particularly the South African MeerKAT array, with its 64 receptors — confirmed that the rings were physically real. They were not artifacts.

What ORCs are not, then, is straightforward. What they are is the harder question.

Hundreds of thousands of light-years across, drifting at the edges of the universe, with no obvious cause and no comparable object on record. Astronomers had to invent a new category to file them under.

ORC 1 in High Resolution

In March 2022, Norris and his collaborators published the first detailed follow-up study of ORC 1 — the original — in the Monthly Notices of the Royal Astronomical Society. They had targeted the object with MeerKAT for ten hours of integration time, producing a much deeper and sharper image than ASKAP's discovery scan.

The MeerKAT image revealed structure that had been invisible before. ORC 1 is not a uniform smoke ring. It contains internal arcs, brighter knots, and a complex filamentary geometry — features that any working hypothesis about its origin needs to explain. The image also confirmed an elliptical galaxy sitting near the geometric center of the ring, at a distance estimated at roughly five billion light-years from Earth.

If that central galaxy is the source of the ring rather than an unrelated foreground or background coincidence, then ORC 1 is enormous. The team's analysis put its diameter at approximately one million light-years, with later estimates extending up to twice that figure. By way of comparison, our entire Milky Way galaxy is roughly one hundred thousand light-years across. ORC 1 is at least ten times larger.

With this image in hand, Norris's team laid out three plausible scenarios for how an object on this scale could form, and modeled each one against the observations.

The Three Hypotheses

The first hypothesis is the merger of two supermassive black holes inside the central galaxy. When two such black holes spiral together, the energy released can drive a spherical shockwave outward through the galaxy and into the surrounding medium. That shock would accelerate electrons throughout a large volume of intergalactic space, producing a roughly spherical bubble of radio emission. From a sufficient distance, the brightened edge of that bubble would look — to a radio telescope — like a ring.

The second hypothesis is that we are looking down the barrel of a relativistic jet from an active galactic nucleus, or AGN. AGNs are the brightly radiating cores of galaxies powered by accretion onto a central supermassive black hole. They often launch twin jets of plasma in opposite directions. If you happened to be observing such a system end-on, with one jet pointed almost directly at you, the geometric appearance of the lobes superimposed on each other could create an ORC-like signature. ORC 1's central galaxy does show evidence of being a radio-loud AGN, which makes this scenario at least dimensionally plausible.

The third hypothesis is what the team called a starburst termination shock. In this scenario, the central galaxy underwent a brief but extremely intense burst of star formation in its past — millions of stars born in a window of only a few tens of millions of years. The combined stellar winds and supernova outflows from that burst would expand outward into the surrounding intergalactic medium for billions of years, eventually piling up into a spherical termination shock that lights up at radio wavelengths. The Norris team's mathematical model showed that this scenario could, in principle, produce a structure with the size and shape of ORC 1.

The 2022 paper concluded that all three hypotheses remained consistent with the data, and that none of them could yet be eliminated. Four years later, that is still essentially true.

The Cloverleaf Complication

The fourth ORC to be discovered, called ORC 4 or the Cloverleaf because of its irregular four-lobed shape, has become the most thoroughly studied of the bunch and the source of two newer competing explanations.

In 2024, two separate research teams turned different telescopes on the Cloverleaf and arrived at different conclusions. A team led by Alison Coil, an astronomer and astrophysicist at the University of California, San Diego, used the W. M. Keck Observatory in Hawaii to detect a vast region of fluorescent oxygen-2 emission stretching across more than 130,000 light-years of the Cloverleaf's structure. They argued that the geometry, age, and energetics of this oxygen nebula were best explained by an ancient starburst termination shock — supporting one of the original Norris hypotheses, now applied to a different ORC.

That same year, a team led by Esra Bulbul at the Max Planck Institute for Extraterrestrial Physics, working with Xiaoyuan Zhang and others, detected diffuse X-ray emission within the Cloverleaf using the European Space Agency's XMM-Newton telescope. Their interpretation was different: the X-rays implied a recent merger between two groups of galaxies, and the Cloverleaf was the radio fossil of that merger's shock front. Their model accounts well for the X-ray data — but, awkwardly, similar galaxy group mergers do not generally produce comparable radio rings, leaving the question of what makes the Cloverleaf special.

It is at this point that some astronomers have begun to suggest the Cloverleaf may not be a true ORC at all, but a related-but-distinct kind of object that simply got swept into the same descriptive umbrella. The category we call ORCs may turn out to contain more than one thing.

The simplest possibility is that ORCs all have a single explanation. The next-simplest possibility is that they don't.

What We Don't Know — And How We'll Find Out

As of 2026, five ORCs are confirmed. Several more are candidates, identified in archival data and awaiting follow-up. The continuing EMU survey is expected to push the catalog much higher, possibly into the dozens or hundreds, as ASKAP works through its share of the southern sky.

Two large surveys may, between them, cover the full range of possibilities. One is purely scientific: deeper observations with MeerKAT and ASKAP, and eventually the much larger Square Kilometre Array now under construction in Australia and South Africa, which will see fainter and more distant ORCs than current instruments can detect. The other is an unusual hybrid: a citizen-science project called the Radio Galaxy Zoo: EMU, where any volunteer with a browser can help classify radio sources in the EMU data and flag candidate ORCs for the professional team. As of 2025, the project had over two thousand volunteers and was processing tens of thousands of sources.

Whichever explanation eventually wins out — the merger of two supermassive black holes, an end-on jet from an AGN, an ancient starburst's expanding shockwave, or something nobody has thought of yet — the underlying lesson of ORCs is straightforward and important: when you build an instrument with enough field of view and enough sensitivity, you will find things you did not expect. The universe has more categories of object than the categories we currently have for it.

An astronomer wrote three letters on an image she did not understand, and accidentally named an entirely new branch of cosmic objects. The right reaction to a properly designed survey of the sky is, sometimes, exactly that.