When Pluto was first discovered in 1930 by American astronomer Clyde Tombaugh (1906–1997), it was considered the ninth planet of our solar system. During the late 1990s and the beginning of the twenty-first century, astronomers began to discover more objects orbiting beyond Neptune in the area known as the transneptunian region. On the night of October 21, 2003, Mike Brown from Caltech, Chad Trujillo from the Gemini Observatory, and David Rabinowitz from Yale University discovered a new object more massive than Pluto with its own satellite. The Internaional Astronomical Union (IAU) began to debate the question of what constitutes a planet. In 2006, the IAU approved a new definition of a planet. This definition states that a planet is:

1. An object in orbit around the sun
2. An object with sufficient mass (large enough) to have its self-gravity pull itself into a round (or near spherical) shape
3. Has cleared the neighborhood around its orbit of other objects.

Pluto no longer meets the definition of a planet because of its size and it is in the transneptunian region, a zone of other similarly sized objects. Instead, Pluto is a dwarf planet.