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. Round objects that orbit the Sun, including Pluto, that failed the third criterion were demoted to a new class of astronomical objects informally called "dwarf planets." A formal name for this new class of objects has yet to be established. The status of Pluto remains controversial.