Generally, rocky (more rarely metallic or icy) bodies, either irregular or regular in shape, mostly large asteroids, some small moons, about 51 to 1,000 km along the longest axis. These small worlds also tend to be more spherical in shape, reflecting their larger sizes and greater gravity. (Hence intermediate between Asteroidal and Terrestrial Classes).
There are minor planets in every solar system, bodies that represent the embryos of growing planets in a young solar system, and the still born remains of planets in old systems. Yet, despite their small size, these worlds are capable of possessing unique characteristics in nearly every facet that the larger, Terrestrial planets have. Most have barren, cratered surfaces, but many possess surface deposits of ice, and some even have tenuous atmospheres.
In most cases, Planetoidal bodies initially begin to form in independent orbits, and seem to be well on the way to forming a true planet. However, various circumstances can arrest this development, and either leave the Planetoid alone in a planetary orbit, or surrounded by other Planetoids and asteroids, as in a belt. In the Sol System, for example, the Asteroid Belt contains many Planetoids, the largest of which is Ceres. Quite possibly Ceres, or its large cousin Vesta, are the remnants of a failed planet. Other Planetoids can form in the furthest reaches of a solar system, and make up the majority of objects to be found in the local Kuiper belt. Some of these may find their way into the realm of the outer planets and remain as wanderers, or even be captured and become Jovian moons.
Kuiper Belt - Text by M. Alan Kazlev In the Solar System, the region beyond Neptune. It includes more than 70,000 small objects, a number up to asteroidal size. It is located from 30 to 50 A.U.'s and was discovered in 1992. The Kuiper belt may be the source of the short-period comets (like Halley's comet). The Kuiper belt was named for the Dutch-American astronomer Gerard P. Kuiper, who predicted its existence in 1951. In general, any belt beyond the outermost large planet of a solar system, consisting of mainly icy objects (ice dwarfs and ice planetisimals).