A small, icy world not far from Neptune has a ring around Saturn. What is puzzling is that the ring is at a distance where simple gravitational calculations suggest there should be none.
“It’s very strange,” said Bruno Morgado, a professor at the Federal University of Rio de Janeiro in Brazil. Morgado is lead author of a paper published Wednesday in the journal Nature that describes the ring that surrounds Quaoar, a planetary body about 700 miles in diameter that orbits the Sun at a distance of about 4 billion miles. revolves around
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 one-third the diameter of Earth’s Moon. It is likely large enough to qualify as a dwarf planet, having been pulled into a round shape by its gravity. But no one can say for sure, as images taken by the most powerful telescopes have revealed Quaoar only as a fuzzy blob. The Blob also has a moon, Wewot (son of Quaoar in Tongva belief).
Quaoar orbits the Sun in the Kuiper Belt, a region of frozen debris beyond Neptune that also includes Pluto.
The ring is not visible in telescope images. Rather, astronomers detected it indirectly, when distant stars passed behind the quasar, blocking the starlight. From 2018 to 2021, Quaoar passed in front of four stars, and astronomers on Earth were able to observe the shadow of the eclipse, also known as a stellar occultation.
However, he observed some dimming of the starlight before and after the star blinked. It points to the unexplained chunks of 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,” Morgado said.)
The ring appears to be uneven. In some places, it seems very thin, a few miles wide, while in other parts, it can be more like a hundred miles wide. If the ring particles collected, a moon about 3 miles wide would form, Morgado said.
“I’m impressed by the thoroughness of the analysis they’ve done,” 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 that asteroids and other small bodies were too small to have companions like the moons and rings. But in the last few decades, they discovered many asteroids and moons around Kuiper belt objects. Next they looked at rings – essentially moons that failed to coalesce – around smaller objects.
In 2013, astronomers discovered some 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 too close to their world.
In 1848, Édouard Roche, a French astronomer, calculated what is now known as the Roche limit. Orbiting material closer than this distance will be torn apart by the tidal forces exerted by the parent body. Thus, a ring within the Roche limit will remain a ring, while a ring of debris outside the Roche limit will usually merge into a moon.
The rings around the Solar System’s giant planets—Jupiter, Saturn, Uranus, and Neptune—generally fit within the constraints of the Roche limit. On a distant small world, the rings of Chariklo actually extend slightly beyond Roche’s range. The cordon around Haumea is within range.
Then there’s the Quaoar Ring.
At 2,500 miles, it is well beyond the Roche limit, which scientists calculated to be 1,100 miles. Morgado said that at that distance, according to the physics underlying Roche’s calculations, the particles should have accumulated into a moon in 10 to 20 years.
“It really shouldn’t be there,” he said. “We must revisit this limit and better understand how satellites form.”
One possible explanation for Quaoar’s distant ring is the presence of Vevot. The Moon may have created a gravitational perturbation that prevented ring particles from entering the other Moon. At ultracold temperatures in the outer Solar System, icy particles are also bouncier and less likely to clump together when they collide.
Michael E. Brown, an astronomer at the California Institute of Technology who co-discovered the quasar in 2002, said the discovery of the ring surprised him.
“If the data were not so convincing, I would insist that they were not real,” he said.
French said the discovery demonstrated how much remains to be learned about rings and that there is likely to be much more to be discovered around smaller bodies in the outer solar system.
“The fact that we found rings around three of them means that rings around things are actually very common,” French said.
The rings around small Solar System objects billions of miles away may seem puzzling, but the clumping – or non-clumping – of particles is key to understanding the Solar System’s beginnings.
“You might think that a small ring around a small object in a distant solar system doesn’t have broad applicability,” French said. “But really this process, how particles grow, is actually the early stage of planet formation.”