QUAOAR HAS A RING THAT SHOULDN’T BE THERE
ASTRONOMERS HAVE found that the 700-milewide (1,100 kilometers) dwarf planet Quaoar, which resides in the outer solar system, sports a strange ring. But according to researchers, this lumpy ring of material is so far from the icy world’s surface that theory says it should have formed into a moon instead.
“At the end of the day, this ring is not where we expect it to be,” Bruno Morgado, an astronomer at the Federal University of Rio de Janeiro in Brazil and lead author of the new study, tells Astronomy.
The paper detailing the discovery of Quaoar’s ring was published Feb. 8 in Nature.
Scientists spotted Quaoar’s unusual ring, which they describe as “clumpy,” using the European Space Agency’s Cheops space telescope, as well as multiple ground-based observatories. The researchers were able to closely study Quaoar because it passed in front of four different background stars between 2018 and 2021, each time revealing clues about its size and immediate surroundings.
Pretty as planetary rings may be, creating them involves a fair bit of gravitational gore. Dense systems like the rings around Saturn form inside what’s called the Roche limit. This is the radius within which a body’s gravity is strong enough to rip apart objects that stray too close, smearing the resulting debris into rings.
Rings shouldn’t exist far beyond an object’s Roche limit, which is determined by the body’s size and density — but Quaoar doesn’t seem to have gotten the memo. Its newly discovered ring orbits twice as far as Quaoar’s Roche limit, meaning the ring should have coalesced into a tiny moonlet within just a few decades.
“Of course, it could be that we are looking at [Quaoar] exactly in this mid-time between the ring becoming a satellite,” says Morgado. But “it’s very unlikely … that we are exactly at the right time to be able to see this happening. So, it’s more probable that the ring is indeed stable.”
At this point, only two other small bodies in the solar system, Haumea and Chariklo, are known to host rings. However, Morgado expects that to change in the future.
“We believe that many more rings are [out] there,” says Morgado. “We need to keep observing, and also to look far, far away from the bodies just to see if there is something there. Because the Roche limit may not be the limit anymore.”