Quaoar, the dwarf planet with the impossible giant ring

The Quaoar ring is too big and no jeweler can do anything about it. Thanks to recent surveys, Bruno Cicardi, an astrophysicist at the Paris Observatory, and his colleagues have discovered that this dwarf planet, located beyond the orbit of Neptune, has a ring. Quaoar is the fourth known small body in the solar system to have one. But even more surprising is that this ring is located very far from Quaoar… in a place we didn’t expect to find it!

The four giant planets in the solar system are surrounded by rings, the most famous of which is certainly the rings of Saturn. For a long time, it was believed that only large planets could have stable rings. But in 2013, the surveillance campaign turned everything upside down.

The researchers used the occultation method, which consists of studying the decrease in the light flux received from a star when an object passes in front of it. Depending on the attenuation, it is possible to deduce the properties of the object obscured by the star. This technique is used to study the outer planets but also some bodies in the solar system. In 2013, astronomers became interested in the asteroid Chariclo, which they suspected was spewing matter out like a comet. But observations revealed stellar disappearances on either side of the asteroid. “We then understood that Chariclo was surrounded by a ring,” explains Bruno Cicardi.

This discovery then opened up a field of possibilities: It is possible that other objects in the outer solar system also have a ring. Indeed, two similar discoveries followed each other, in 2015 and 2017, with Chiron and Hauméa. Then, in 2023, with Quaoar.

But Quaoar’s ring differs from all others ever observed because it lies at a distance from the dwarf planet seven times greater than its radius. This means well beyond the Roche limit (named after astronomer Edward Roche), which is set at two or three times the radius of the central body. Below this limit, the tidal effects exerted by the body are so greater than the gravitational cohesion forces that occur between particles of matter in this region, that they cannot clump together. Then they form a ring of debris. Bruno Cicardi says: “Outside the Roche bounds, one would expect this material to coalesce to form a satellite.”

How do we explain the Qawar ring? Laboratory experiments in the 1980s showed that in an extremely cold environment, such as that of the outer solar system, particles of matter similar to those that make up icy bodies, such as Quaoar, have more elastic collisions (that is, they lose less energy). In other words, after a collision, two particles bounce off more and move away from each other more quickly. So much so that they can’t accumulate, even if they exceed Roche’s limit.

But ring formation is not enough to maintain it. So that it does not spread over time and disappear, it must be limited. Specific studies are still needed. However, researchers have found that Quaoar’s ring and the dwarf planet itself resonate. When a ring particle makes three turns, Quaoar makes one. Same for Chariclo and Hauméa… Resonance effects are known to have a major role in trapping some of Saturn’s rings. “The coincidence is very unsettling,” Bruno Cicardi says.