Quaoar, a small world beyond Neptune has a ring like those around Saturn. But calculations suggest there shouldn’t be.
A small icy world far beyond Neptune has a ring like those around Saturn. Curiously, the ring is at a distance where simple gravitational calculations suggest there shouldn’t be.
“It’s very strange,” said Bruno Morgado, a professor at the Federal University of Rio de Janeiro in Brazil. Morgado is the 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. .
Quaoar (pronounced KWA-wahr, the name of the creator god of the indigenous Tongva people who live around Los Angeles) is just under half the diameter of Pluto and about a third the diameter of Earth’s moon. It is likely to be large enough to qualify as a dwarf planet, pulled by its gravity into a round shape. But no one can say for sure, because images taken by the most powerful telescopes revealed Quaoar as just an indistinct blob. The blob also has a moon, Weywot (the son of Quaoar in Tongva belief).
Quaoar orbits the sun in the Kuiper Belt, a region of frozen debris beyond Neptune that includes Pluto.
The ring is not visible in telescope images. Instead, astronomers found it indirectly, when distant stars passed behind Quaoar, blocking starlight. From 2018 to 2021, Quaoar passed four stars and astronomers on Earth were able to observe the shadow of eclipses, also called stellar occultations.
However, they also observed some attenuation of starlight before and after the star died out. This indicated a ring obscuring some of the light, an international team of astronomers concluded in Wednesday’s Nature journal. (Another stellar occultation occurred in 2022, not reported in the journal Nature. “We saw the ring again,” Morgado said.)
The ring looks uneven. In some places it appears to be very thin, a few kilometers wide, while in other parts it may be a few hundred kilometers wider. The ring particles, if collected, would form a moon about 3 miles wide, Morgado said.
“I’m impressed with the rigor of the analysis they’ve done,” said Richard G. French, professor emeritus of astrophysics at Wellesley College in Massachusetts, who has studied planetary rings for decades. He did not participate in the research.
For a long time, astronomers thought asteroids and other small bodies were too small to have companions like moons and rings. But over the past few decades, they have discovered moons around many asteroids and Kuiper Belt Objects. They then spotted rings — essentially moons that didn’t merge — around smaller objects.
In 2013, astronomers discovered a few 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 obscured during a stellar occultation. But these rings are quite close to their worlds.
In 1848, Édouard Roche, a French astronomer, calculated what is now called the Roche limit. Matter orbiting closer than this distance would tend to be pulled apart by tidal forces exerted by the parent body. Thus, a ring inside the Roche boundary would tend to remain a ring, while a debris ring outside the Roche boundary would generally merge into a moon.
The rings around the giant planets of the solar system – Jupiter, Saturn, Uranus and Neptune – generally fall within the constraints of the Roche limit. Of the small, remote worlds, the rings of Chariklo actually lie a little beyond the limit of Roche. The ring around Haumea is within the limit.
Then there is the Quaoar ring.
At a distance of 2,500 miles, that’s well beyond the Roche limit, which scientists calculated at 1,100 miles. At that distance, according to the physics underlying Roche’s calculations, the particles should have merged into a moon in 10 to 20 years, Morgado said.
“It really shouldn’t be there,” he said. “We should revisit this boundary and better understand how satellites form.”
A potential explanation for Quaoar’s distant ring is the presence of Weywot. The moon may have created gravitational disturbances that prevented ring particles from accreting into another moon. At the ultra-cold temperatures of the outer solar system, ice particles are also more bouncy and are less likely to stick together when they collide.
Michael E. Brown, a California Institute of Technology astronomer who was co-discoverer of Quaoar in 2002, said the discovery of the ring left him baffled.
“If the data weren’t so compelling, I would insist that it’s not real,” he said.
French said the discovery demonstrates how much remains to be learned about the rings and that many more are likely to be discovered around small bodies in the outer solar system.
“The fact that we found rings around three of them already means that rings around things are really quite common,” French said.
Rings around small solar system bodies billions of miles away might seem esoteric, but the clustering — or non-clustering — of particles is key to understanding the early solar system.
“You might think that a small ring around a small object in the distant solar system doesn’t have wide applicability,” French said. “But in fact, this process of particle accumulation is really the start of planet formation.”
This article originally appeared in The New York Times.
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