An international team of astronomers has discovered a ring around Quaoar. This large trans-Neptunian object, a potential dwarf planet, thus becomes the eighth body in orbit around the Sun around which rings have been discovered and the fourth outside the giant planets.
With an estimated diameter of 1,110 kilometres, (50,000) Quaoar, discovered in 2002, is the 30e largest body in the Solar System and the 14e larger among those directly orbiting the Sun. In particular, Quaoar is the 7e largest trans-Neptunian object (including Charon), presumably large enough to qualify as a dwarf planet. As a classic (non-resonant) object from the Kuiper belt, it is named after a creator deity, in this case that of the mythology of the Tongvas, an Amerindian people from the Los Angeles basin.
In 2006, a satellite was discovered around Quaoar: Weywot, a body 160 kilometers in diameter which orbits at 24 times the radius of Quaoar, therefore approximately 13,000 kilometers. In Tongva mythology, Weywot is the god of the sky, created by Quaoar when he sang the Song of Creation.
Now it’s a ring that was found around Quaoar. Besides the four giant planets, three minor planets were already known to sport rings, namely the centaurs (10199) Chariclo and (2060) Chiron and the dwarf planet (136108) Haumea. However, the position of Quaoar’s ring surprised the astronomers who discovered it.
A surprisingly distant ring
Quaoar’s ring is too small and faint to be seen directly in an image. Instead, Bruno Morgado and his collaborators made their discovery by observing between 2018 and 2021 star occultations, that is, the passage of Quaoar between us and stars. These events, which were predicted, lasted less than a minute, but were each unexpectedly preceded and followed by two dips in brightness, indicating a ring system around Quaoar.
Until now, all known dense rings have been located relatively close to their parent body, inside the Roche boundary, where tidal forces prevent material of reasonable density from aggregating into a satellite. For example, the main rings around Saturn are within three planetary radii. What makes the ring system around Quaoar remarkable is that it lies 7.4 radii from the central body, i.e. 4,100 kilometers, well outside the boundary of Roche, which is the outer limit of the area where the ring systems were thought to be able to survive. Indeed, taking a density of 400 kg/m3 typical of small internal Saturnian satellites, the classical Roche limit of Quaoar is close to only 1,780 kilometers. This finding implies that the Roche limit does not always determine where ring material can survive and has therefore forced a rethinking of ring formation theories.
An irregular ring sculpted by resonances?
Unlike Chariclo’s ring, Quaoar’s ring is highly irregular. Because of this, it resembles Saturn’s F ring, which contains azimuthal structures (clusters) or even local opaque structures interpreted as small, kilometer-sized moons. This lumpy nature is probably caused by the presence of thousands of small bodies (from 1 to 0.1 kilometer) which collide and produce micrometric to centimetric particles which are reaccreted in a few months on the parent bodies. A similar process could explain the uneven structure of Quaoar’s ring. The Adams ring around Neptune also shows similar variations.
Simulations made by the researchers show that elastic collisions, based on laboratory experiments, can keep a ring away from the central body. An intriguing feature is the fact that Quaoar’s ring orbits near 1:3 spin-orbit resonance with Quaoar, meaning that the ring circles Quaoar once while Quaoar circles it three times- even. This property is shared by the Chariclo and Haumea rings, suggesting that this resonance plays a key role in ring confinement for small bodies. Quaoar’s ring otherwise seems close to the 6:1 resonance with Weywot, i.e. the ring goes around Quaoar six times while Weywot makes one. This other resonance could therefore also play a role in the presence of this ring.