Pluto is unusual, and we knew it for a long time before reclassifying it as a dwarf planet.

An extremely elliptical orbit causes its distance from the sun to range from 29 to 49 AU. At its closest approach to the sun, Pluto comes inside the orbit of Neptune. This kind of orbit is more characteristic of asteroids and comets.

But Pluto is large enough to have an atmosphere. And as Pluto moves outward once again, that atmosphere likely freezes and falls to the planet's surface.

Unlike the gas giant planets that lie inside its orbit, Pluto's composition is similar to that of Triton, one of the moons of Neptune. Triton could be very much like Pluto—a Pluto that's been captured by Neptune's gravity. Both are made of water ice, nitrogen, and carbon monoxide—a composition they share with the objects in the Kuiper Belt.

Pluto has a diameter of 2,300 kilometers (1,430 miles). The discovery of two Kuiper Belt objects, Quaoar at 1,250 kilometers (775 miles) and Sedna at 1,600 kilometers (1,000 miles), followed by the discovery of even larger Eris at 2,326 km (1,445 miles), Makemake at 1,502 km (933 miles), and Haumea at 1,450 km (900 miles) a short while later, gave credence to the idea that Pluto is but the largest object of the Kuiper Belt found thus far. About 100 Kuiper Belt objects have been identified. There may be as many as 40,000! Odds are that many, like Eris, are larger than Pluto.

But, if for no other reason than respect for the painstaking and precise work of a young man named Clyde Tombaugh decades ago, Pluto may always be categorized by some as the ninth planet in our solar system.