Depending upon what accuracy you need, it should be pretty easy to get. Pluto takes 248 years to orbit the Sun. We've just observed Pluto during the half of its year that it's moving closer. There is no evidence that there is any planet in our solar system beyond the orbit of Pluto. Dr. Eric Christian Shepherd Moons Didn't the rings of Saturn become a flat disk because of gravitational forces of two bodies pulling at each other, balanced by centrifical forces pu lling them apart.
Shepherd moons keep ring edges sharp in the radial direction.
They have nothing to do with the fact that rings form in planes. The reason rings, solar systems, and galactic disks form in planes is just a function of the original angular momentum of the system. You can look at the Nine Planets web site for more information on shepherd moons. I recently heard some curious discussion about all the planets in our solar system becoming aligned in the year 2004. There is speculation on this having an effect on the Earth's magnetic poles and perhaps the Earth actually fipping over on its axis.