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Dwarf planet Quaoar hosts a ring that shouldn’t be there

Researchers say the dwarf planet’s strange and “clumpy” ring should be a moon instead.
Quaoar, seen here in this artist’s concept, is an icy dwarf planet like Pluto. But with a diameter of just 700 miles (1,100 kilometers), it is only about half the diameter of Pluto. Astronomers recently discovered an unexpected ring around Quaoar, making it the third small body in the solar system known to host a ring.

Astronomers discovered that dwarf planet Quaoar, which resides in the outer solar system, sports a strange ring around it. And according to researchers, this lumpy ring of material is so far from the icy world’s surface that theory says it should have formed 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.

Scientists spotted Quaoar’s unusual ring, which they describe as “clumpy,” using 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.

A paper detailing the discovery of Quaoar’s ring was published Feb. 8 in Nature.

Weird ring

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 much farther out than it has any right to. In fact, researchers say, the ring exists twice as far out as Quaoar’s Roche limit, meaning the ring should have coalesced into a tiny moonlet within just a few decades.

Quaoar’s ring, seen in this artist’s impression, is located twice as far as the dwarf planet’s Roche limit, which is the theoretical limit beyond which rings should not form.
ESA/Paris Observatory

“Of course, it could be that we are looking at 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.”

Stellar Occultation

Astronomers spotted the new ring by training their telescopes on faraway stars right as Quaoar passed in front of them. Such events are called stellar occultations.

Occultations are a bit like eclipses, except that instead of the Sun, the Moon, and Earth aligning, a more distant object (like a dwarf planet or asteroid) passes between Earth and a distant background star.

“When an object passes in front of the star, you see a dip [in brightness] when it enters the field of view of the star — it basically blocks the starlight,” Laurence O'Rourke, an astronomer at the European Space Agency who was not involved in the new study, told Astronomy. “Then, when it's finished, it's on the other side and the starlight is back to normal.

Watching how a star flickers during an occultation tells astronomers about the size and shape of the object crossing in front of it. And because any extra material hanging around a dwarf planet also blocks starlight, occultations can reveal atmospheres, moons, and rings around small bodies, too.

Much like Quaoar, the small solar system object Chariklo (center) passed in front of a background star on Oct. 18, 2022. The James Webb Space Telescope captured 63 individual observations of the event, called a stellar occultation. The brief dip of starlight that occurs just before the star passes behind Chariklo is due to the object's rings blocking some of the light.
NASA/ESA/CSA/Nicolás Morales (IAA/CSIC)

But catching an occultation can be tricky. To see one, astronomers need to be “at the right place at the right time,” says Morgado. This is especially difficult for a space telescope because it’s hard to predict exactly where the orbiting observatory will be when an occultation occurs.

“What's unique with this is the fact that, for the first time, they've used a satellite in space to perform a stellar occultation,” says O'Rourke. It’s only possible to “know the ring exists because of the quality of Cheops, because [the telescope] is outside the atmosphere. So, there are no special effects from the atmosphere which might affect the signal.”

Data from the Cheops space telescope also has a very high signal-to-noise ratio, which made it possible for the astronomers to be confident they were really seeing an out-of-place ring around Quaoar. The team also analyzed occultation data collected by astronomers around the world, including observations made by citizen scientists.

Pushing the Limit

The new study is part of a bigger project called Lucky Star, which uses occultations to study dwarf planets in the outer solar system like Quaoar.

These distant objects preserve primordial material from the system’s earliest days. Their orbits also provide clues about how the worlds’ orbits evolved over time, offering scientists a glimpse into the distant past. And, as Quaoar’s rings show, dwarf planets can also teach researchers a lot about how ring systems and moons form.

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.”



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