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Pluto
Discovered in 1930, Pluto was long considered the ninth planet. The planet is much smaller than mercury, or even the Earth's moon. Pluto is also unlike the terrestrial planets, gas giants and ice giants that comprise the solar system's other planets. Pluto's properties ignited a debate that has raged for decades among astronomers as to whether Pluto truly was a planet or whether it was something different.
Kuiper Belt
The Kuiper Belt is a region of our solar system, beyond Neptune, in which thousands of objects are orbiting the Sun. These objects are called Kuiper Belt objects or KBOs. Many believed that Pluto should be considered the largest KBO, not a planet. In 2005, astronomers identified a KBO even larger than Pluto, named Eris. This discovery brought the debate over whether these two KBOs were ninth and 10th planets to a crescendo.
Dwarf Planets
After the discovery of Eris, the International Astronomical Union set about the task of classifying the new object. In 2006, this worldwide association of astronomers released a resolution that defined planets and a new category, dwarf planets. Pluto, Eris and several other large KBOs are now recognized as dwarf planets, though not all astronomers agreed with the new classification. According to the IAU resolution, dwarf planets share three properties.
Dwarf Planet Orbits
The orbit of a dwarf planet is similar to a regular planet. Dwarf planets orbit around the Sun. Furthermore, dwarf planets do not orbit other objects. This differentiates them from satellites, or moons, of another planet. This represents a shared property between planets and dwarf planets.
Dwarf Planet Shape
Unlike asteroids and comets, dwarf planets contain enough mass to produce a nearly spherical shape, caused by gravitational attraction. In the case of the asteroid belt Ceres, this requires a diameter of greater than around 560 miles. For objects in the Kuiper Belt, this mass requires a diameter greater than about 250 miles. For a comparison, the diameter of the Earth is around 7,900 miles, and the Moon is around 2,160 miles. This also represents a shared property between planets and dwarf planets.
Gravitational Domination
The property that represents the key difference between planets and dwarf planets is known as gravitational domination. Unlike planets, dwarf planets do not have enough mass to clear their neighborhood of the solar system, or orbital path. Dwarf planets orbit the Sun in belts that still contain thousands of small objects. Originally, the orbit of the planets also contained such objects; however, they contained enough mass to either absorb them or scatter them.