Narrow rings: a scattering billiard model

We propose a generic mechanism for the formation of narrow rings in rotating systems. For this purpose we use a system of discs rotating about a common center lying well outside the discs. A discussion of this system shows that narrow rings occur, if we assume noninteracting particles. A saddle-center bifurcation is responsible for the relevant appearance of elliptic regions in phase space, that will generally assume ring shapes in the synodic frame, which will suffer a precession in the sidereal frame. Finally we discuss possible applications of this mechanism and find that it may be relevant for planetary rings as well as for semiclassical considerations.