Rings of Quaoar

In 1610, the rings of Saturn were observed for the first time when Galileo Galilei pointed his telescope toward the great gas giant, though his instrument was not powerful enough to discern the true nature of these magnificent structures. In 1655, Dutch astronomer Christiaan Huygens was the first to accurately describe Saturn's rings as discs of particles surrounding the planet. In the following centuries, rings would also be discovered around Jupiter, Uranus, and Neptune. Until recently, it was unknown that rings are not exclusive to the larger bodies in the Solar System.

In 2013, a two-ring system was discovered around the Centaur object Chariklo. Then, a ring around the dwarf planet Haumea was discovered in 2017. Using data from stellar occultations between 2018 and 2021, the same team discovered a ring around the Trans-Neptunian dwarf planet candidate Quaoar, though at an unusually far distance. The discovery of this ring was published in Nature in February 2023. Now, the team has announced that Quaoar's ring system is more complex than previously thought, having discovered a second ring orbiting closer to Quaoar than the first, but still questionably far away. The existence of these rings challenges previously held notions about where ring systems can exist relative to the body they orbit.

Rings are structures that attract people's attention, especially the majestic rings of Saturn.

"Rings are structures that attract people's attention, especially the majestic rings of Saturn," said Chrystian Luciano Pereira, a PhD student with the Observatório Nacional, Brazil and lead author on the paper. "Our work demonstrates that small bodies have rings that are even more curious than those observed on giant planets. In addition, our work includes the participation of citizen astronomers, who helped make this unexpected astronomical discovery."

On 9 August 2022, the team observed a stellar occultation — when an object in the Solar System passes in front of a star and blocks its light for a few moments — to better understand the first Quaoar ring (Q1R) discovered a few months prior. The results of this observational campaign were published in April 2023 in the journal Astronomy & Astrophysics. With the high-resolution imaging power of the ‘Alopeke instrument on Gemini North, one half of the International Gemini Observatory, operated by NSF’s NOIRLab, the team was able to detect tiny variations in the star’s light as it passed behind Quaoar’s thin and tenuous ring system. During these observations, the team was surprised when they unintentionally discovered a second ring (Q2R) orbiting in between Quaoar and Q1R.

Gemini North on a rare cloudy evening as the telescope (visible through the open vent gates) prepares for nighttime operations.

Credit: International Gemini Observatory/NSF’s NOIRLab/AURA/J. Pollard

Unlike the rings observed around Chariklo, Haumea, and the four giant planets, Quaoar's rings lie in a region well beyond what is known as the Roche limit. According to the theory proposed by French astronomer Édouard Albert Roche in 1848, anything orbiting inside the limit would disintegrate to form a ring of particles, whereas outside of the limit such rings of particles would rapidly cluster into compact 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. And yet, despite being outside of the Roche limit, both rings remain as streams of particles rather than conglomerating into solid bodies. How they are able to sustain this structure is still uncertain, though it is speculated that the relationship between the rotation speed of Quaoar and the orbital speeds 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 rings is the variability in Q1R’s width and opacity. Observations of Q1R during the stellar occultation event revealed two distinct regions of the ring. In one region, the stream of particles is a narrow, confined structure that is approximately 5 kilometers wide and is opaque, meaning it’s quite dense. In another region, the stream is wider, with an average width of 90 kilometers, and has a thinner dispersion of particles that is less than 1% as opaque as the densest region. “This type of structure had not yet been detected around small bodies in the Solar System,” says Pereira. One explanation for this confinement is that Q1R is being influenced by the presence of Weywot, a small moon that orbits Quaoar. Q2R, on the other hand, has a consistent width of about 10 kilometers throughout its structure.

This type of structure had not yet been detected around small bodies in the Solar System.

The variability in width and opaqueness of Q1R, why Q2R doesn’t exhibit the same behavior, and how both are retaining their particle structure all adds to the curious nature of Quaoar’s rings. It’s possible that small, as-of-yet-undiscovered “shepherd” satellites are confining the rings and keeping them in place.

Future work on the precise determination of Quaoar's shape, as well as new observations of its rings, will be important for a better understanding of the dynamic system and the role that resonances play in the maintenance and confinement of the rings. But regardless of what forces are at play here, the existence of Q1R and Q2R imply that the classical notion of the Roche limit may need to be revised for small planetary bodies.

“Roche's proposed theory is robust to explain how a satellite is disrupted to form a ring when it gets too close to the central body,” says Pereira. “A better understanding of this process would help us better understand the formation and evolution of our Solar System.”