Like planets, dwarf planets owe their circular shape to gravitational attraction. Unlike planets, dwarfs cannot gather or clear objects in their orbital path. Unlike asteroids, comets and other celestial bodies, dwarf planets meet a planet's three defining criteria: 1. Orbits the sun, 2. Is not a satellite and 3. Has enough mass to be nearly round. Dwarf planets are found either in the asteroid or Kuiper belts. These two belts are the source for the solar system's comets. Within the Kuiper Belt there are at least 70,000 objects with Pluto's composition. There are potentially hundreds that are bigger than Pluto, each having dwarf planet status.
Pluto's light weight status causes other problems for it. Because of its strong gravitational influence, a planet locks its satellites in a synchronous obit. Pluto cannot. It faces the same side of its satellite Charon. They form a binary dwarf planet pairing. As the Kuiper Belt region is explored, astronomers are sure to find more binary dwarf planet pairings.
On average, Neptune is 30 astronomical units (AU) from the sun. Nothing beyond that distance is a planet. The Kuiper Belt is the region of space that extends from Neptune's orbit out to 55 AU. 1 AU is 149,597,870.691 kilometers. It is the average distance of the Earth from the sun. Pluto averages 39.3 AU distance from the sun, placing it within the belt.
There are eight other dwarf planets separating Pluto from the last planet. Ranging in size from 440 up to 1,100 kilometers, these dwarf planets have solid ice interiors. Although no longer a full planet, Pluto has moved to a much larger family: the dwarf planets.