There probably is a not-too-difficult physics formula to calculate the gravitational field of a disc. I tried the following but couldn’t figure it out. I’m sure there are more simple and direct ways of determining the gravitational dynamics of a disc.
Gravitational field of a disc:
http://www.physicsforums.com/showthread.php?t=39461
...then again, there probably aren’t many solid/rigid, disc-shaped objects in nature or elsewhere massive enough to even exert a gravitational field. There are large dust discs and galactic discs.