The dwarf planet Quaoar has a ring too big for her metaphorical fingers. While all other rings in the Solar System lie at or near a mathematically defined distance from their parent bodies, Kvaoar’s ring is much further away.
“For Quaoar, it’s very, very strange that the ring is outside this limit,” says astronomer Bruno Morgado of the Federal University of Rio de Janeiro. The discovery could force a rethinking of the rules governing planetary rings, Morgado and colleagues say in a study published Feb. 8 in Nature .
A quaoar is an icy body about half the size of Pluto located in the Kuiper Belt at the edge of the Solar System . At such a great distance from Earth, it is difficult to get a clear picture of the world.
So Morgado and his colleagues watched Quaoar block light from a distant star, a phenomenon called a stellar eclipse. The time a star blinks in and out of view can reveal details about Quaoar, such as its size and whether it has an atmosphere.
The researchers took data from occultations from 2018 to 2020 observed from around the world, including Namibia, Australia and Grenada, as well as space. There was no indication that Quaoar had an atmosphere. But surprisingly there was a ring. The discovery makes Quaoar only the third dwarf planet or asteroid in the Solar System known to have a ring, after the asteroid Chariklo and the dwarf planet Haumea.
Even more surprising is that “the ring is not where we expect it to be,” says Morgado.
A distant ring
In this illustration, the dwarf planet Haumea and the asteroid Chariklo have rings (white) that are close to the Roche limits (yellow) as far as astronomers understand. Quaoar, in contrast, has a ring that is clearly well above the Roche limit, the imaginary line at which rings are considered unstable.
Rings around three small objects in the solar system
SOURCE: MM HEDMAN
Known rings around other objects are located at or near the so-called Roche limit, an invisible line where the gravitational force of the main body diminishes. Within a limit, this power can tear the moon into pieces, turning it into a ring. Outside, the force of attraction between smaller particles is stronger than the gravity of the main body, and the rings merge into one or more satellites.
“We always consider [ліміт Roche] sorry, says Morgado. “One side is the formation of the moon, the other is a stable ring. And now this border is not a border.”
There are several possible explanations for Quaoar’s distant ring, says Morgado. Perhaps observers caught the ring at just the right moment, just before it turned into a moon. But such a good moment seems unlikely, he notes.
Perhaps Kvaoar’s known satellite, Veivot, or some other unseen satellite contributes to the gravity that somehow keeps the ring stable. Or maybe the ring particles collide in such a way that they don’t clump together and form satellites.
The particles have to be particularly bouncy for this to work, “like a ring of those bouncy balls from toy stores,” says planetary scientist David Jewitt of the University of California, Los Angeles, who was not involved in the new work.
The observations are compelling, says Jewitt, who helped discover the first objects in the Kuiper Belt in the 1990s. But it is not yet possible to know which explanation is correct, in part because there are no theoretical predictions for such distant rings that can be compared to the Kvaoar situation.
As for the Kuiper Belt, that’s fine. “Everything in the Kuiper Belt was essentially discovered, not predicted,” says Jewitt. “This is the opposite of the classical model of science, where people predict things and then confirm or reject them. People discover things out of the blue, and everyone tries to explain it.”
More observations of Quaoar, or more discoveries of seemingly misplaced rings elsewhere in the solar system, could help reveal what’s going on.
“I have no doubt that many people will be working with Quaoar in the near future to try to get that answer,” says Morgado.