015.2 The Kuiper Belt

A large number of balls of frozen gas orbit between 30 and 100 astronomical units from the Sun.

Their orbits are approximately in the plane of the Solar System.

If undisturbed, their circular orbits keep them outside of the orbit of Neptune.

When Neptune's gravity deflects them inward, they become short-period comets on elliptical orbits that bring them near the Sun with a regular period of less than 200 years.