Trans-Neptunian object · Dwarf-planet candidate (classical KBO) · orbits Sun
Quaoar is a classical Kuiper belt object about half the size of Pluto, following one of the most circular orbits in the outer Solar System. In 2023 astronomers announced something baffling around it: a dense ring orbiting far beyond the distance where rings are supposed to be able to survive.
Its ring, found when Quaoar briefly blotted out background stars, is the first known to defy the Roche limit.
Its nearly circular orbit (e ≈ 0.04) never strays far from 43 AU - a textbook classical Kuiper belt object.
Crystalline water ice and ammonia hydrate on its surface hint at geologically recent cryovolcanism.
The ring that should be a moon
Inside a body’s Roche limit, tides tear moonlets apart, so debris stays a ring; outside it, ring material should clump into a moon within decades. Quaoar’s ring sits at over seven Quaoar radii - more than twice the classical Roche limit - yet it has not coalesced. Something, perhaps resonances with Quaoar’s spin or its moon Weywot, keeps sweeping the material apart, and the textbooks are being revised.
Physical characteristics
Diameter
1,090 km
Mass
1.200e+21 kg
Geometric albedo
0.12
Known moons
1
Atmosphere and composition
Atmosphere
None detected.
Composition
Rock and water ice; spectra show crystalline water ice, ammonia hydrate and traces of methane.
Discovery
Discovered by
Chad Trujillo & Mike Brown
Year
2002
How
Palomar Observatory 48-inch Samuel Oschin Telescope