Dwarf planet Quaoar has a weirdly big ring of debris encircling it

Quaoar, a dwarf planet in our solar system, has a ring of debris orbiting it that is far further out than we thought the laws of physics allow

By Alex Wilkins

The dwarf planet Quaoar, which sits beyond Neptune in our solar system, appears to have a ring of debris around it that is much further out than was thought possible.

“We have observed a ring that shouldn’t be there,” says Bruno Morgado at the Federal University of Rio de Janeiro in Brazil.

Until now, every ring or orbiting moon observed by astronomers has obeyed a limit put forward by astronomer Édouard Roche in 1848 that relates to its distance from a parent body. If an object is below the Roche limit, its parent body’s gravity rips apart the orbiting object into a collection of smaller chunks which eventually form a ring, like those seen around Saturn. Outside that limit, dust and debris should coalesce to form larger objects, such as moons.

Quaoar, which is 1110 kilometres across and is slightly less dense than our moon, should have only moons beyond a distance of 2.4 times its radius of 555 kilometres, but Morgado and his colleagues measured the ring at 7.2 times Quaoar’s radius. “It’s very, very far outside this limit,” says Morgado.

To spot Quaoar’s wayward ring, the team observed the dwarf planet against the backdrop of various stars between 2018 and 2021, using Earth-based telescopes as well as the European Space Agency’s CHEOPS exoplanet-hunting space telescope. The researchers used changes in the stars’ brightness to calculate the ring’s characteristics.

They found that the ring appears to be mostly made up of water ice, a bit like Saturn’s F-ring. One unusual property of the ring is its irregular shape – some sections are 5 kilometres wide, while others span more than 100 kilometres. Standing on the surface of Quaoar, you should be able to see some of the ring’s wider sections, says Morgado.

It isn’t clear why Quaoar has a ring so far outside its Roche limit, but the researchers think that the low temperatures – the dwarf planet is a frosty -220°C – might play a role in preventing the ring’s contents coalescing.

It is also possible that interactions between the ring’s particles or with Quaoar’s moon, Weywot, could be sustaining the ring. Further observations of Quaoar and more simulations of the system’s dynamics will be needed before a definitive answer can be found, says Morgado.

Whatever the answer is, we might need to modify the Roche limit, which could have implications for other calculations in astrophysics.

“This concept has been used to analyse, for instance, the formation of our moon and the formation of other satellites in the solar system,” says Morgado. “So, if we have seen something that challenges this limit, we need to rethink and better understand why this ring is where it is.”

Carl Murray at Queen Mary University of London is hopeful that this won’t change things too much, because the Roche limit is only a rough guide, but understanding Quaoar’s unusual ring will help refine it, he says.

“The Roche limit has its uses, but in reality there’s no exact radius,” says Murray. “It’ll depend on the physical properties of the material that’s orbiting and, as they’ve shown here, there are other characteristics that need to be taken account of as well.”

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