A small icy world far beyond Neptune possesses a ring like the ones around Saturn. Perplexingly, the ring is at a distance where simple gravitational calculations suggest there should be none.
“That’s very strange,” said Bruno Morgado, a professor at the Federal University of Rio de Janeiro in Brazil. Dr. Morgado is the lead author of a paper published in the journal Nature on Wednesday that describes the ring that encircles Quaoar, a planetary body about 700 miles in diameter that orbits the sun at a distance of about four billion miles.
Quaoar (pronounced KWA-wahr, the name of the creator god for the Indigenous Tongva people who live around Los Angeles) is a little less than half the diameter of Pluto and about a third of the diameter of Earth’s moon. It is likely to be big enough to qualify as a dwarf planet, pulled by its gravity into a round shape. But no one can say that for sure, because images taken by even the most powerful telescopes have revealed Quaoar as only an indistinct blob. The blob also has a moon, Weywot (the son of Quaoar in Tongva belief).
Quaoar orbits the sun in the Kuiper belt, a region of frozen debris beyond Neptune that includes Pluto.
The ring is not visible in telescope images. Rather, astronomers found it indirectly, when distant stars happened to pass behind Quaoar, blocking the starlight. From 2018 through 2021, Quaoar passed in front of four stars, and astronomers on Earth were able to observe the shadow of the eclipses, also known as stellar occultations.
However, they also observed some dimming of the starlight before and after the star blinked out. That pointed to a ring obscuring part of the light, an international team of astronomers concluded in Wednesday’s Nature paper. (Another stellar occultation occurred in 2022, not reported in the Nature paper. “We saw the ring again,” Dr. Morgado said.)
The ring appears to be uneven. In some places, it seems to be very thin, a few miles wide, while in other parts, it may be more like a couple of hundred miles wide. The ring particles, if collected, would form a moon about three miles wide, Dr. Morgado said.
“I’m impressed by the thoroughness of the analysis that they did,” said Richard G. French, an emeritus professor of astrophysics at Wellesley College in Massachusetts who has studied planetary rings for decades. He was not involved with the research.
For a long time, astronomers thought asteroids and other small bodies were too small to have companions like moons and rings. But in the past few decades, they discovered moons around many asteroids and Kuiper belt objects. They then spotted rings — essentially moons that failed to coalesce — around smaller objects.
In 2013, astronomers discovered a couple of rings around Chariklo, a body known as a centaur that orbits the sun between Saturn and Uranus. In 2017, a ring was discovered around another Kuiper belt object, Haumea, also from dimming during a stellar occultation. But those rings are fairly close to their worlds.
In 1848, Édouard Roche, a French astronomer, calculated what is now known as the Roche limit. Material orbiting closer than this distance would tend to be pulled apart by tidal forces exerted by the parent body. Thus, a ring within the Roche limit would tend to remain a ring, while a ring of debris outside the Roche limit would usually coalesce into a moon.
The rings around the giant planets of the solar system — Jupiter, Saturn, Uranus and Neptune — generally fit within the constraints of the Roche limit. Among the distant smaller worlds, Chariklo’s rings actually lie a bit beyond the Roche limit. The ring around Haumea is within the limit.
Then there is the Quaoar ring.
At a distance of 2,500 miles, it is way beyond the Roche limit, which the scientists calculated to be 1,100 miles. At that distance, according to the physics underlying Roche’s calculations, the particles should have coalesced into a moon in 10 to 20 years, Dr. Morgado said.
“It really shouldn’t be there,” he said. “We should look at this limit again and better understand how the satellites are formed.”
A potential explanation for Quaoar’s distant ring is the presence of Weywot. The moon may have created gravitational disturbances that prevented the ring particles from accreting into another moon. At the ultracold temperatures in the outer solar system, icy particles are also bouncier and are less likely to stick together when they collide.
Michael E. Brown, an astronomer at the California Institute of Technology who was a co-discoverer of Quaoar in 2002, said the discovery of the ring baffled him.
“If the data weren’t so convincing, I would insist they weren’t real,” he said.
Dr. French said the discovery demonstrated how much remained to be learned about rings and that many more are likely to be discovered around the small bodies in the outer solar system.
“The fact that we’ve found rings around three of them already means that rings around things are really pretty common,” Dr. French said.
Rings around small solar system bodies billions of miles away may seem esoteric, but the clumping — or non-clumping — of the particles is key to understanding the beginnings of the solar system.
“You might think that a small ring around a small object in the distant solar system doesn’t have broad applicability,” Dr. French said. “But actually this process, of how particles accrete, is really the beginning step of planet formation.”