Two regions of icy bodies beyond Neptune
The Kuiper Belt is a disk of icy bodies orbiting the Sun between 30 and 55 astronomical units (AU), roughly 4.5 to 8 billion kilometers away. It contains thousands of known objects larger than 100 kilometers and likely millions smaller than that. Pluto, reclassified as a dwarf planet in 2006, is the largest known Kuiper Belt object. Other famous residents include Eris, Makemake, and Haumea (all dwarf planets), and countless smaller icy planetesimals.
The Oort Cloud is a spherical shell of comets extending from about 2,000 AU to 100,000 AU, surrounding the entire planetary system. It is invisible to telescopes but revealed by occasional comets that fall inward toward the Sun. The Oort Cloud likely contains billions of comets, the remnants of the early solar system's accretion.
Comet origins and long-period orbits
Comets are icy bodies that vaporize when heated by the Sun, releasing dust and gas into dramatic tails. Short-period comets (orbits under 200 years) originate in the Kuiper Belt. Long-period comets come from the Oort Cloud; gravitational perturbations from passing stars can nudge Oort objects into orbits bringing them near the Sun. Comets with periods of thousands of years were on Oort Cloud trajectories before their recent infall.
The Kuiper Belt and Oort Cloud formed in the inner solar system during accretion, then were scattered outward by gravitational interactions with Jupiter and Saturn. This late heavy bombardment (ending around 3.8 billion years ago) may have delivered water and organic chemistry to the early Earth.