The Outer Solar System
The region beyond to orbit of Neptune is called the Kuiper Belt. It is the circumstellar disc at the outer margin of the Solar System beyond the planets. It is similar to the Asteroid Belt, but far larger (wider) and many times more massive. The belt extends from Neptune (at about 30 AU [astronomical units] to about 50 AU - one AU is the average distance of the center of the Earth to the center of the Sun.
There are more than 100,000 Kuiper Belt Objects (KBO). The Kuiper Belt includes three recognized planetesimals (or dwarf planets) including Pluto, Haumea, and Makmake. KBOs are probably mostly composed of frozen volatile compounds (ices of methane, ammonia, and water). Pluto's status as a planet has been argued for years - Figure 1.79. Pluto does not behave like other planets; it does not orbit the Sun within the ecliptic plane, and sometimes Pluto's orbit puts it closer to the sun than Neptune.)
Beyond the Kuiper Belt is the hypothetical Oort Cloud - a region that may contain an abundance of icy planetesimals and objects that may surround the Solar System at a distance of between 50,00 and 200,000 AU. The Oort Cloud may be the source of most of the long-period comets that have been observed.