Inclinations of the uranian rings

Abstract

Rings formed with the birth of a planet are thought to arise after planetary condensation, from material remaining within the outer accretion limit—which has a radius of approximately two planetary radii, depending on the density of the material and the relative size of the particles^1,2. Although the initial distribution of particles would be spherical, collisions would cause rapid evolution of the particle orbits into circular orbits in the equatorial plane of the planet³. Also, gaps might form at the position of strong resonances with the planet's satellites, in an otherwise broad ring. Observations of planetary rings at high spatial resolution have revealed a more complex situation. The nine confirmed uranian rings are narrow, not broad, and most have elliptical orbits, not circular. Furthermore, the Voyager observations have shown that Saturn's rings consist of a multitude of narrow ringlets, at least some of which have elliptical orbits. Here we report an additional feature that we have found for the uranian ring orbits, not predicted by current theoretical models: at least seven of the nine rings have orbits inclined to the equatorial plane of Uranus.