Scientists Discovered an 'Impossible' Ring Around Dwarf Planet

 


Astronomers have discovered an entirely new ring system within the solar system, and it's located at such a great distance from its dwarf planet parent that it should be impossible. 

“That’s very strange,” said Bruno Morgado, a professor at the Federal University of Rio de Janeiro in Brazil. Dr. Morgado is the lead author of a paper published in the journal Nature on Wednesday that describes the ring that encircles Quaoar, a planetary body about 700 miles in diameter that orbits the sun at a distance of about four billion miles.

Quaoar (pronounced KWA-wahr, the name of the creator god for the Indigenous Tongva people who live around Los Angeles) is a little less than half the diameter of Pluto and about a third of the diameter of Earth’s moon. It is likely to be big enough to qualify as a dwarf planet, pulled by its gravity into a round shape. 

But no one can say that for sure, because images taken by even the most powerful telescopes have revealed Quaoar as only an indistinct blob. The blob also has a moon, Weywot (the son of Quaoar in Tongva belief).

"The six [previously known] planets with ring systems all have rings which are quite close to the surface of the planet. So this 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 was previously thought to be impossible to have rings that far out, so in a nutshell, the ring of Quaoar is a real challenge to explain theoretically."

The ring system is located at a distance of seven planetary radii away from Quaoar (that is, seven times Quaoar's radius), which is twice as far out as the theoretical maximum limit for a ring system, known as the Roche limit. For comparison, the main part of Saturn's rings sits at just three planetary radii from the gas giant. 

Previously, it was thought that rings past the Roche limit wouldn't be able to survive this far out from their parent body. 

"Rings that are formed outside Roche limits aren't meant to be stable; they should rapidly accrete into moonlets, using up all the ring material," Dhillon said. "With this discovery, we have a ring not just outside the Roche limit, but way beyond it."

How a dwarf planet grew a distant ring

An illustration showing the location of the dwarf planet Quaoar's rings, compared to the Roche Limit, which was thought to be the maximum distance at which planetary rings could form. (Image credit: Paris Observatory)


Dhillon and the team think Quaoar's ring formed similarly to other solar system rings: Collisions of moonlets orbiting the parent planet created debris that settled into a ring made of rock, ice and dust particles. 

These particles can't reform a moonlet if they are close to the planet and within the Roche limit, because tidal forces from the parent body constantly rip them apart and prevent them from clumping, according to the researchers. But that can't be the case with Quaoar's ring.

"We've got to find some way of stopping that moonlet forming that far out," Dhillon said. "The particles in the ring are colliding all the time, and if these collisions are elastic, it means the particles can't stick together to form a moonlet." (An elastic collision is one in which two colliding objects bounce away from each other rather than clumping together, like a rubber ball hitting a floor.)

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

A chance discovery

The researchers discovered the ring system while  investigating whether Quaoar has an atmosphere. The team used the high-speed HiPERCAM instrument on the Gran Telescopio Canarias), a telescope in Spain's Canary Islands that can spot small variations in light from background stars. The ring became visible when it caused a roughly 5% to 10% dip in light from a background star, both before and after  the main body of Quaoar passed in front of the star. This event, known as an occultation, lasted less than a minute.

"The discovery came as a bit of a surprise," Dhillon said. "We knew there was a possibility we might find them, but we weren't really looking for them."

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

"This discovery shows you the amazing diversity of things that are in our own cosmic backyard," Dhillon said. "You don't have to look light-years away into the distant universe to find the unexpected. Surprises are still aplenty in our own solar system."