Beyond Neptune, the solar system does not end. It continues into the Kuiper Belt, a vast ring of icy bodies that extends from about 30 to 50 astronomical units from the Sun. It is similar to the asteroid belt, but much larger and much colder. Its objects are made of rock, water ice, methane, and ammonia.

The Kuiper Belt is home to Pluto, Haumea, and Makemake, three of the five recognized dwarf planets. It also contains hundreds of thousands of smaller icy bodies. Some have moons. Some have rings. Some are binaries. The diversity is surprising for such a cold, distant region.

The belt is a remnant of the solar system's formation. Its objects never coalesced into a planet because the region is too sparse and the orbits too stable. They preserve the chemistry of the early solar nebula. Studying them tells us what the building blocks of the planets were made of.

The Kuiper Belt is also the source of short-period comets. Gravitational interactions with Neptune occasionally send an icy body inward, where it becomes a comet. Halley's Comet is a Kuiper Belt object, and so are many of the comets that visit the inner solar system.