The status of Pluto has been controversial since its discovery in 1930. It bears little resemblance to the four gas giant planets that precede it. Pluto is also the smallest "planet." Its diameter of 2,300 kilometers (1,400 miles) is smaller than the Moon (3,476 kilometers, 2,160 miles). The orbit of Pluto further distinguishes it from the other planets, all of which orbit within six degrees of the ecliptic plane. Pluto's orbit is tilted 16 degrees from the ecliptic. It is also far more elliptical than the other planets. Pluto is also the only planet that crosses the orbit of another planet (Neptune).

In August of 2006, the delegates at the general assembly of the International Astronomical Union (IAU) debated the definition of a planet. On August 24, the delegates voted in favor of a new definition of a planet based on three criteria. (1) A planet must be large enough for its own gravity to have made it round. (2) A planet must orbit the Sun but not be a satellite of another planet. (3) A planet must have "cleared" smaller objects from its orbit, i.e., a planet should have a mass that dominates its orbital zone.

In June of 2008, the IAU announced a new name for bright dwarf planets similar to Pluto that lie, on average, beyond the orbit of Neptune. These celestial bodies are now called "plutoids" in honor of the former planet. Currently, only Pluto and the dwarf planets Eris, Haumea, and Makemake are recognized as plutoids, although it is likely more plutoids will be confirmed or discovered in the future. Ceres, a dwarf planet that orbits the Sun in the asteroid belt, and satellites of plutoids are not recognized in this new category. The status of plutoids remains controversial.