Jewelry of the astronomical kind: Researchers have identified a ring around the planetoid Quaoar at the edge of our solar system. Amazingly, it orbits the celestial body at a much greater distance than is typical for other known ring systems. According to previous assumptions, the material there should actually clump together to form a moon. This now raises questions about how the ring around Quaoar might have formed.
Saturn is the most famous example: its rings are so large and bright that they can even be seen through small telescopes. But our solar system has even more rings to offer: Powerful telescopes show that the planets Jupiter, Uranus and Neptune are also surrounded by such structures made of chunks of ice and other materials. Rings have also been spotted on smaller celestial bodies in the outer solar system: Chariklo and Haumea. An international team of astronomers has now added another to these well-known ring bearers: the celestial body Quaoar, which orbits the sun together with its moon Weywot outside of Neptune’s orbit. With a diameter of around 1110 kilometers, Quaoar is considered a so-called dwarf planet candidate.
Telltale fluctuations in brightness
The scientists did not discover the ring directly, because even the most powerful telescopes cannot optically capture its indistinct structures. Instead, they turned to stellar occultation, using telescopes to record the variations in brightness of stars as Quaoar passed them. Among other things, a high-speed camera was used with which the Gran Telescopio Canarias (GTC) on La Palma is equipped. The data showed two subtle starlight eclipsing events in the passage of Quaoar, which followed each other in quick succession. The team’s calculations showed that these fluctuations in brightness are due to a ring orbiting the celestial body at a distance seven times its radius.
This is a surprisingly large gap. For comparison: the main rings around Saturn are within three planetary radii. As the researchers explain, the ring of the Quaoar lies well outside the so-called “Roche limit”. It is believed to be the furthest distance believed to allow a ring to survive indefinitely. According to this, the gravitational effects of a celestial body only prevent ring material from accumulating in this area. Further out, however, the clumps form together into moons. “It was an unexpected discovery in our solar system and the features were doubly unexpected,” says co-author Vik Dhillon of the University of Sheffield.
As the researchers explain, the rings known so far are close to or within the Roche limit. But in the case of Quaoar, the structure is 4100 kilometers from its center, while its Roche limit is 1780 kilometers. According to the current theory, the mutual attraction forces of the ring chunks should lead to larger accumulations there. The existence of the newly discovered structure around Quaoar is therefore puzzling, the astronomers say.
In their study, they also discuss possible explanations. It could be the debris of a collision, one might think. But according to the researchers, this seems implausible. Because it would only take a few decades for the material to accumulate again. The probability that we are looking exactly at this short phase therefore seems rather low. Another explanation would be that the ring material is particularly elastic, which means that the particles tend to fly apart in collisions rather than sticking together. Another possibility is that the ring particles are subject to external gravitational perturbations that keep their collision velocities high enough to prevent aggregation. In fact, it seems possible
“Future observations of Quaoar’s ring should now determine the specific mechanisms responsible for its existence,” writes Matthew Hedman of the University of Idaho Moscow in a comment on the study. In conclusion, Dhillon says: “Everyone learns about Saturn’s magnificent rings as a child, and we hope this new discovery will provide further insight into the formation of such structures,” said the astronomer.
Source: University of Sheffield, Article: Nature, doi: 10.1038/s41586-022-05629-6