Figure; Hubble Space Telescope image of Pluto and its four moons.

Technically Pluto (diameter 2306 km) it is now called a dwarf-planet, which is a classification that also includes the largest Kuiper Belt Objects Haumea (1150 km), Makemake (1500 km) and Eris (<2340 km) as well as the asteroid Vesta (974 km).

Astronomers who study the far solar system say that if Pluto had been discovered today, it would not be called a planet but instead a Kuiper Belt Object, along with all the others that have been found way out there beyond Neptune. Since 1992 when astronomers first began discovering the tiny faint streaks of these distant chunks of ice and rock in deep photographs of the sky, Pluto has been joined by such behemoths as Quaoar (780 miles across) and Varuna (550 miles across). There are probably more out there to be discovered!

This answer was updated in 2011. See my books: The Astronomy Cafe (1998) and Back to the Astronomy Cafe (2003) for more FAQs in printed form. Author: Dr. Sten Odenwald, Copyright 2011 Return to Ask the Astronomer