A ring that shouldn't exist
In the icy darkness of the Kuiper Belt, more than 6 billion kilometers from the Sun, orbits the dwarf planet 50000 Quaoar. Discovered on June 4, 2002, by astronomers Chad Trujillo and Michael Brown, Quaoar is a trans-Neptunian object with a diameter of about 1,110 kilometers, roughly half the size of Pluto. It is a minor world that presents a major astronomical puzzle. In February 2023, observations revealed that Quaoar possesses a dense ring of material. This discovery was made not by direct imaging—the ring is too faint and distant—but through a stellar occultation. Astronomers, using the European Space Agency's CHEOPS space telescope among other instruments, watched as Quaoar passed in front of a distant star, noting the dips in starlight that indicated an orbiting ring.
The existence of a ring is not unique; Saturn, Jupiter, Uranus, Neptune, and even smaller bodies like the dwarf planet Haumea have rings. The paradox with Quaoar's ring system is its location. The main, densest ring, designated Q1R, orbits at a distance of about 4,100 kilometers from Quaoar's center. This is seven times the dwarf planet's radius. Scientific principles of celestial mechanics, specifically the Roche limit, dictate that this ring should not be there.
Defying the Roche limit
The Roche limit, calculated by French astronomer Édouard Roche in 1848, defines the distance from a celestial body within which tidal forces will pull apart any smaller object held together only by its own gravity. Inside this boundary, material forms stable rings. Outside the Roche limit, the self-gravity of the particles is expected to overcome the primary body's tidal forces, causing them to accrete into a moon. This process should be rapid; models suggest that a ring outside the Roche limit would coalesce into a moonlet within just a few decades.
Quaoar's Roche limit is calculated to be approximately 1,780 kilometers from its center. Yet, its primary ring orbits at 4,100 kilometers, more than double the supposed boundary for stable rings. This finding changes models of ring dynamics. One hypothesis suggests that at the frigid temperatures of the Kuiper Belt, icy particles in the ring might be more "bouncy." Collisions between them would then be more likely to result in them ricocheting off each other rather than sticking together. Another theory involves orbital resonances, where the gravitational influence of Quaoar's known moon, Weywot, could be helping to maintain the ring's structure. Weywot, with a diameter of about 170 kilometers, orbits much farther out at a distance of about 14,500 km. Further observations in 2023 revealed a second, inner ring (Q2R) also orbiting outside the Roche limit, deepening the mystery of this unexpected system.