Pluto, in this sense, is the small icy world that goes around the Sun, usually out beyond Neptune. For a long time schoolbooks called it the ninth planet. In 2006 astronomers agreed it did not fit their tighter rule for what a planet is, so they classed it as a dwarf planet; though actually, a lot of people still say "planet" out of habit, and the argument never really went quiet. It is dark, far, and slow: one trip around the Sun takes about 248 Earth years. The name is the old name of the Roman god of the underworld (from Greek Plouton). A child, Venetia Burney, is often credited with suggesting that name for the newly found world in 1930, because it was so far and so dim. English also has a Disney dog called Pluto, and the god himself — same spelling, different things. This entry is the one in the sky.