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.
