Very small satellites whose gravity helps keep the particles in a planet's ring system from dispersing are referred-to as "shepherd moons". They are generally in orbit at the edges of the rings, but might also lie within a large ring. Simply put, they act like snowplows, pushing aside the ice and small rocks that make up the rings. These moons have been found in orbits around Saturn and Uranus, and are often identified by examining gravitationally induced ripples in a ring.