Quaoar Ring
(Nanowerk News) In 1610, Saturn’s rings were observed for the first time when Galileo Galilei pointed his telescope at the massive gas giant, although his instruments were not powerful enough to tell the true nature of this extraordinary structure. In 1655, Dutch astronomer Christiaan Huygens was the first to accurately describe Saturn’s rings as a disk of particles surrounding the planet. In subsequent centuries, rings were also found around Jupiter, Uranus and Neptune. Until recently, it was not known that rings were not exclusive to the larger bodies of the Solar System.
In 2013, a two-ring system was discovered around the Centaur Chariklo object. Later, a ring around the dwarf planet Haumea was discovered in 2017. Using data from stellar occultations between 2018 and 2021, the same team found a ring around the Trans-Neptunian dwarf planet candidate Quaoar, albeit at a great distance. The ring discoveries were published in Nature in February 2023 (“A dense ring of trans-Neptunian object Quaoar beyond its Roche limit”).
Now, the team has announced that Quaoar’s ring system is more complex than previously thought, after discovering a second ring that orbits closer to Quaoar than the first, but is still out of the question. The existence of these rings challenges previous ideas about where a ring system could exist relative to the object it orbits.
“Rings are structures that attract people’s attention, especially Saturn’s majestic rings,” said Chrystian Luciano Pereira, a PhD student at Observatório Nacional, Brazil and lead author of the paper. “Our work shows that small objects have rings that are even stranger than those observed on giant planets. In addition, our work includes the participation of citizen astronomers, who help make these unexpected astronomical discoveries.”
On August 9, 2022, the team observed a stellar occultation — when an object in the Solar System passes in front of a star and temporarily blocks its light — to better understand the first Quaoar ring (Q1R) discovered months earlier.
The results of this observational campaign are published in a journal Astronomy & Astrophysics (“Two rings (50,000) Quaoar”).
With the power of high-resolution imaging from the ‘Alopeke instrument on Gemini North, half of the Gemini International Observatory, operated by NOIRLab NSF, the team was able to detect tiny variations in starlight as it passes behind the thin and tenuous Quaoar ring. system. During these observations, the team was surprised when they accidentally discovered a second ring (Q2R) orbiting between Quaoar and Q1R.
Unlike the observed rings around Chariklo, Haumea, and the four giant planets, the Quaoar ring is located in a region far beyond the Roche limit. According to a theory proposed by the French astronomer Édouard Albert Roche in 1848, anything orbiting inside the boundary would disintegrate to form rings of particles, while outside the boundary such particle rings would rapidly clump together into dense satellites. For Quaoar, the Roche limit is estimated to be 1,780 kilometers from the center of the body.
Q1R orbits Quaoar at a distance of 4,060 kilometers and Q2R orbits at a distance of 2,520 kilometers. However, even outside the Roche limit, the two rings remain as a stream of particles rather than coalescing into a solid. How they managed to maintain this structure is still uncertain, although it is thought that the relationship between the rotational velocity of the Quaoar and the orbital velocity of the rings may be an important factor, as has been proposed for the rings around Chariklo and Haumea.
Another unusual property of the Quaoar ring is the variability of Q1R width and opacity. Observations of Q1R during stellar occultation events reveal two distinct ring regions. In one area, the particle stream is a narrow, confined structure about 5 kilometers wide and opaque, meaning it is quite dense. In other areas, the flow is wider, with an average width of 90 kilometers, and has a thinner dispersion of particles that is less than 1% opaque as the densest region.
“This type of structure has not been detected around small bodies in the Solar System,” Pereira said. One explanation for this confinement is that Q1R was affected by the presence of Weywot, a small moon orbiting Quaoar. The Q2R, on the other hand, has a consistent width of around 10 kilometers throughout its structure.
The variability of Q1R’s width and opacity, why Q2R doesn’t exhibit the same behavior, and how both retain their particle structure all add to the curiosity of the Quaoar ring. It’s possible that small undiscovered “shepherd” satellites surround the rings and hold them in place.
Future work on determining the exact shape of the Quaoar, as well as new observations of its rings, will be important for a better understanding of dynamical systems and the role that resonance plays in ring maintenance and confinement. But regardless of what forces are at play here, the existence of Q1R and Q2R suggests that the classic notion of the Roche limit may need to be revised for small planetary bodies.
“Roche’s theory is strong enough to explain how a satellite is disturbed to form rings when it gets too close to the central body,” said Pereira. “A better understanding of these processes will help us better understand the formation and evolution of our Solar System.”
