New ring discovered around small, icy world in our solar system

But this newly discovered ring is unlike the telltale halos of ice and dust that orbit planets such as Saturn and Jupiter in a key aspect—it is unusually far from its host body, according to a study published Wednesday in the journal Nature.

“It is a factor of two further out than what was previously recognized as the limit for how far a ring system can exist around a parent body," said Vikram Dhillon, a professor of astrophysics at the University of Sheffield in the U.K. and co-author of the new study. He added that the finding seriously challenges existing theories about ring formation in our corner of the cosmos.

An international team of nearly 60 researchers from more than a dozen countries used telescopes on Earth and in space to help confirm the existence of the disk, orbiting roughly 2,500 miles away from its parent body’s center.

Finding a ring that is so far out “either implies that there is something really unusual about this system, or that rings can be found in a broader range of conditions than we previously thought," said Matthew Hedman, a University of Idaho physicist who wasn’t involved in the research but wrote an editorial accompanying the study.

The new ring system encircles a small, celestial body named Quaoar, located in what astronomers call the Kuiper belt, a ring of icy objects—including dwarf planets such as Pluto—around the sun beyond Neptune’s orbit. This isn’t the first time a ring system has been found around a Kuiper belt object—scientists announced the existence of one around the dwarf planet Haumea in 2017.

“Quaoar is like a cousin of Pluto. It is a little bit further away from us and from our sun and is around half Pluto’s size," said Bruno Morgado, an astronomer at the Federal University of Rio de Janeiro in Brazil and lead author of the new research. It was discovered using a telescope at the Palomar Observatory in southern California in 2002.

Quaoar—pronounced “kwa-whar" and named after the creator god of the Tongva people indigenous to the area around Los Angeles—is on average 4 billion miles away from the sun. At such vast distances, any ring would be too small and faint to see directly. So the researchers instead studied the 680-mile-wide Quaoar at times when it passed in front of bright stars—observing how the object blocked that background star’s light.

“How long the light blinks out tells us how big Quaoar is," Dr. Dhillon said. The study’s authors examined data from four different events like this involving Quaoar between 2018 and 2021, including observations from Namibia, Australia, the Canary Islands and from space using the European Space Agency’s CHEOPS telescope.

Dr. Dhillon led the project that built a sensitive high-speed camera mounted on the world’s largest optical telescope on La Palma in the Canary Islands, which observed Quaoar in June 2019. This observation was key to resolving subtle and rapid changes in how much starlight the distant world blotted out, he said. The camera helped reveal small, symmetrical dips in light before and after Quaoar passed in front of a star.

“If you see that kind of symmetric signature, that indicates the presence of a ring," said Matthew Tiscareno, an astrodynamicist at the SETI Institute in Mountain View, Calif., who wasn’t involved in the study. “This is a time-honored technique; the rings of Uranus and Neptune were discovered this way back in the 1970s."

The researchers determined that Quaoar’s ring system—made of water ice and rock debris—orbits far enough from the icy world that Dr. Morgado said he knew the team was seeing “something very, very strange and unexpected."

All previous observed ring systems, including those around the four outermost planets, fall within what astronomers call the Roche limit. If an orbiting object goes inside this limit, Dr. Morgado said, it will break up due to a planet or host body’s gravitational forces and become a ring. Conversely, if a ring goes beyond this critical distance, debris in the ring isn’t disrupted by the host’s gravity and can combine to become an orbiting object, like a moonlet. Yet Quaoar’s rings exist far beyond this Roche limit, which had “previously been considered kind of sacrosanct," Dr. Dhillon said.

“Many of the individual observations wouldn’t be convincing by themselves, but taken together, it is clear what is being seen: a complicated ring system," said Henry Throop, a National Aeronautics and Space Administration astronomer in the planetary science division at the agency’s headquarters in Washington, D.C. Dr. Throop, who wasn’t involved in the research, called the finding “a really exciting new result."

After modeling Quaoar’s ring system and running simulations, the study’s authors offered several ideas to explain the unique ring. One suggestion is that Quaoar’s ring is debris from a moonlet that recently got broken up by a disruptive impact, and that debris hasn’t come back together yet. But Dr. Morgado said that the coalescing process would take 10 to 20 years at most.

“At the end of the day, it is very unlikely—if you consider the billions of yearslong history of the solar system—that we are in the right place at the right time to be observing this kind of thing," he added.

Another scenario is that the icy particles in the ring have a hard, frosty coating—making the particles more likely to bounce off each other than stick together after colliding, Dr. Dhillon said. Gravitational interactions between Quaoar, its moon Weywot, and objects in the ring system could also be keeping the particles moving at very high speeds, which would prevent them from combining into larger objects. Gravitational forces from an undiscovered moon could be a factor as well, he added.

Dr. Tiscareno said more observations of Quaoar’s ring and more simulations of the system should help identify the mechanisms responsible for its existence.

The new finding ultimately suggests there are more ring systems awaiting discovery around similar-sized celestial bodies in our solar system, according to Dr. Morgado. He said he thinks new and better instruments, such as NASA’s James Webb Space Telescope, could help make that happen.