‘Impossible’ new ring system discovered at the edge of the solar system

Astronomers have discovered an entirely new ring system within the solar system, and it’s so far removed from its parent dwarf planet that it should be impossible.

The ring surrounds Quaoar, which is about half the size of Pluto and is behind Neptune. It is only the third ring around a minor planet and the seventh ring system in the solar system, with the most famous and well-studied rings surrounding the giant planets Saturn, Jupiter, Neptune and Uranus.

“The six [previously known] Planets with ring systems all have rings fairly close to the planet’s surface. So that really challenges our ring formation theories,” study co-author Vik Dhillon, a professor of physics and astronomy at the University of Sheffield in England, told Live Science. “It used to be thought impossible to have rings that far out. In short, the Ring of Quaoar is a real challenge to explain theoretically.”

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The ring system is located at a distance of seven planetary radii from Quaoar (i.e. seven times Quaoar’s radius), which is twice the theoretical maximum limit for a ring system, known as the Roche limit. For comparison: the main part of Saturn’s rings is only three planetary radii away from the gas giant.

It was once thought that rings beyond the Roche limit could not survive so far from their parent body.

“Rings formed outside the Roche limits are not said to be stable; they should rapidly coalesce into small moons, consuming all the ring material,” Dhillon said. “With this discovery, we have a ring not only outside the Roche limit, but well beyond it.”

The team’s findings were published in the journal Nature on February 8.

How a distant ring grew out of a dwarf planet

Dhillon and the team believe Quaoar’s ring formed similarly to other rings in the solar system: collisions of moons orbiting the mother planet created debris that settled into a ring of rock, ice and dust particles.

These particles can’t reform a small moon if they’re close to the planet and inside the Roche limit because the mother body’s tidal forces are constantly tearing them apart and preventing them from clumping together, the researchers said. But that can’t be the case with Quaoar’s ring.

“We have to find a way to stop this little moon so far out,” Dhillon said. “The particles in the ring are constantly colliding, and if those collisions are elastic, that means the particles can’t stick together into a little moon.” (An elastic collision is one in which two colliding objects bounce apart rather than balling together, like a rubber ball hitting the ground.)

Elastic collisions could be possible if the ring particles have an icy outer coating, Dhillon said — something that’s plausible given Quaoar’s position at the edge of the solar system. However, more data is needed to confirm this idea.

A chance discovery

Researchers discovered the ring system while investigating whether Quaoar has an atmosphere. The team used the high-speed HiPERCAM instrument at the Gran Telescopio Canarias, a telescope in Spain’s Canary Islands that can detect small variations in the light from background stars. The ring became visible as it caused about a 5 to 10 percent falloff in light from a background star both before and after Quaoar’s main body passed the star. This event, known as the occultation, lasted less than a minute.

“The discovery came as a bit of a surprise,” Dhillon said. “We knew there was a way we could find them, but we didn’t really look for them.”

Quaoar’s ring is too small and faint to be seen by direct imaging, even with an instrument as powerful as the Hubble Space Telescope. Dhillon added that barring occultation events, the only way to spot these dwarf planet rings is to send a robotic probe to visit them.

“This discovery shows you the amazing variety of things that reside in our own cosmic backyard,” Dhillon said. “You don’t have to look light years into the distant universe to find the unexpected. There are still many surprises in our own solar system.”

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