In 2006, the International Astronomical Union defined a planet as a body that is in orbit around the sun, has enough mass to establish a round shape, and has cleared other debris from the neighborhood of its orbit. This ruling famously resulted in the demotion of Pluto to dwarf planet status, since Pluto did not have the gravitational influence to clear other bodies from its orbital path.
When Pluto was initially discovered, it was believed to have more mass than Mercury, making it easy to define this new body as a planet. When astronomers discovered its moon Charon, however, it soon became obvious that Pluto's mass was much smaller than they realized, and what they assumed was Pluto's gravitational field was really the effect of both objects. Pluto remained a planet for some time, however.
The debate intensified in the early 2000s, with the discovery of bodies like Sedna and Eris. Astronomers began to realize that there may be dozens or even hundreds of objects in the solar system, especially in the range of the Kuiper Belt, that could be as large or larger than Pluto. Faced with the prospect of having to add significant numbers of planets to the solar system's official tally, the IAU redefined planethood in 2006 and created a new classification of "dwarf planet" for these smaller bodies.