Editing Ring system (section)

== Formation ==
There are three ways that thicker planetary rings have been proposed to have formed: from material originating from the [[protoplanetary disk]] that was within the [[Roche limit]] of the planet and thus could not coalesce to form moons, from the debris of a [[natural satellite|moon]] that was disrupted by a large impact, or from the debris of a moon that was disrupted by [[tidal stress]]es when it passed within the planet's Roche limit. Most rings were thought to be unstable and to dissipate over the course of tens or hundreds of millions of years, but it now appears that Saturn's rings might be quite old, dating to the early days of the Solar System.<ref name="NASA2007"/>

Fainter planetary rings can form as a result of meteoroid impacts with moons orbiting around the planet or, in the case of Saturn's E-ring, the ejecta of [[cryovolcano|cryovolcanic]] material.<ref name="Spahn"/><ref name="Helfenstein2006"/>

Ring systems may form around [[centaur (minor planet)|centaurs]] when they are [[Tidal force|tidally disrupted]] in a close encounter (within 0.4 to 0.8 times the [[Roche limit]]) with a giant planet. For a [[Planetary differentiation|differentiated body]] approaching a giant planet at an initial relative velocity of 3−6&nbsp;km/s with an initial rotational period of 8 hours, a ring mass of 0.1%−10% of the centaur's mass is predicted. Ring formation from an undifferentiated body is less likely. The rings would be composed mostly or entirely of material from the parent body's icy mantle. After forming, the ring would spread laterally, leading to satellite formation from whatever portion of it spreads beyond the centaur's Roche Limit. Satellites could also form directly from the disrupted icy mantle. This formation mechanism predicts that roughly 10% of centaurs will have experienced potentially ring-forming encounters with giant planets.<ref name="Hyodo2016"/>