A distant planetoid hosts a curiously distant ring

Discovered in 2002, Quaoar is a small, elongated planetoid in the solar system’s Kuiper belt, well beyond the orbit of Neptune. It has a moon named Weywot. An international team including Bruno Morgado of the Federal University of Rio de Janeiro now reports that Quaoar also has a ring. The problem: That ring is so far from Quaoar that it should not exist. The discovery exposes a rare exception to the Roche limit, beyond which a massive object’s tidal forces are typically too feeble to prevent the accretion of smaller bodies into larger ones.

Quaoar’s ring revealed itself to Morgado and colleagues as a series of subtle shadows in the light curves of four distant stars when Quaoar passed between them and Earth. The measurements suggest that the ring is densely populated, with rocky or icy boulders up to kilometers in size that cluster into particularly opaque clumps. Finding the ring is not totally surprising: Whereas the most prominent planetary rings surround Saturn and other gas giants (see the Quick Study by Carl Murray, Physics Today, August 2007, page 74 ), rings have also been spotted around small solar-system bodies such as Haumea, another Kuiper belt object. Far more puzzling is the radius of the ring. The ring is located 4100 km from Quaoar’s center, more than twice as distant as the planetoid’s Roche limit. At that distance, Quaoar’s tidal forces should not be strong enough to prevent the ring material from accreting via its mutual gravitational attraction into additional moons, a process that should not take more than a couple of decades. Although Saturn, for example, hosts faint, dusty rings past its Roche limit, Quaoar appears to be the first known planetary body to support a dense ring so far beyond.

Morgado and colleagues look to other small ringed objects to help explain the Quaoar system. They note that the rings of both Haumea and the asteroid Chariklo exist close to regions where the ring particles complete one orbit for every three rotations of the parent body. Those planetoids’ irregular structure—in the form of an ellipsoidal shape or uneven topography—could lead to gravitational perturbations at those locations that prevent the ring material from accreting. For Haumea and Chariklo, the 1:3 spin–orbit resonances coincide with distances near their Roche limits. But for Quaoar, the resonance arises well beyond the Roche distance, very close to the discovered ring location. A separate resonance caused by Weywot’s gravity occurs around that same region. The researchers also ran simulations to show how certain particle compositions could lead to sufficiently elastic collisions that would inhibit accretion and prolong the ring’s life span.

Quaoar is not the only planetoid that the researchers are studying with the help of background starlight. They hope to find more that sport rings. (B. E. Morgado et al., Nature 614, 239, 2023 .)