Shakespeare once wrote, “there are more things in heaven and earth, Horatio, than are dreamt of in your philosophy.” Certainly, scientists have been finding that out for centuries; every time we think we have a handle on how the universe works, some new discovery throws us a curveball. A recent discovery by scientists at the University of Sheffield in England has tossed a new one into the field of planetary science. The dwarf planet Quaoar has rings.
Quaoar is one of the dwarf planets that lie out past Pluto, in the Kuiper Belt. At approximately 1,110 kilometers across, Quaoar is roughly half of Pluto’s size, and its distance from the sun gives it an average temperature of 44 Kelvin, or -229 degrees Celsius. It has a small moon, named Weywot, orbiting Quaoar at roughly 14,500 km.
Quaoar isn’t the only dwarf planet known to have rings; the dwarf planets Chariklo and Haumea also have them. But the unusual thing about Quaoar’s rings – the thing that has thrown a curveball to scientists – are where those rings are.
Every ring system we know of – the rings that circle the gas giants of Jupiter, Saturn, Uranus, and Neptune, as well as Chariklo and Haumea – follow a specific rule in their formation and survival. All of these rings are formed inside what is known as Roche’s Limit. Roche’s Limit, for those unfamiliar, is a distance from a planet inside which tidal forces will tear apart any large (say more than a kilometer across) object. A ring is formed when a moon drifts inside Roche’s Limit, is torn apart by the tides, and the material scatters itself around the moon’s former orbit. Because this material remains inside Roche’s Limit, it can never clump back together to form a new moon; the tides keep it all riled up.
Roche’s Limit is defined as approximately 2.5 times the planet’s radius (the actual equation takes into account planetary density and a few other factors, but 2.5*R will work for this article). For Earth, this translates to 15,927 km. For Quaoar, this works out to 1,387 km. So: you would think that Quaoar’s rings would be sitting just inside this radius, right?
Wrong! The astronomers found Quaoar’s rings orbiting at 4,148 km – over 7 times Quaoar’s planetary radius, and almost three times further than Roche’s Limit for Quaoar. By rights, those rings shouldn’t be there.
It’s early days for hypotheses on how Quaoar’s rings formed and why they still survive, but we know a few things already. Specifically: the rings are clumpy; that is, they have parts that are thin and parts that are thick, rather than being uniform like the other ring systems we know. In that, they bear a resemblance to Saturn’s outermost ring (the ‘F’ ring); there, small “shepherd moons” keep Saturn’s F ring in place. It’s possible Quaoar also has tiny shepherd moons skimming the edges of its ring; using gravity to “herd” the ring matter within its narrow band, yet also keep that matter from clumping together to form a new moon or two.
It’s also possible the rings are composed of certain ices, that, at the temperatures present around Quaoar, will be just elastic enough that the icy particles will bounce off of each other rather than glom together.
I expect astronomers will be giving Quaoar and it’s strange ring more attention in years to come, searching for more clues on what happened there and why. While no space probes are targeting Quaoar right now, some missions to the outer solar system are still in the planning stages, and Quaoar might become one of their destinations. However, it will be some years before such a probe can be launched, and at least thirteen years before it can reach Quaoar, due to the vast distance involved. I’d likely be in my seventies before we learn the answer.
But one way or another, Quaoar is forcing scientists to rethink how ring systems form. Our universe continues to surprise us. Indeed, it’s those surprises that are one of the big reasons why we keep exploring.