a planet a celestial body a plutoid trans-Neptunian Object a dwarf planet in the Solar System named after the Roman god of the underworld (known as Hades to the Greeks), orbiting the Sun in an eccentric orbit at a distance of between 30 to 49 AU. It is believed to be composed mostly of ice and rock, is about one-fifth the size of the Earth's moon, and has one small satellite named Charon (notable for being the largest satellite in proportion to its planet (or dwarf planet) in the solar system), two really small additional moons named Hydra and Nix, which were spotted in 2005 by the Hubble Space Telescope, and two even smaller unnamed moons, spotted in 2011 and 2012 by Hubble.
 As a holiday resort?
Due to its immense distance and small size, Pluto has never been visited by any man-made device. Neither Voyager 1 nor Voyager 2 went near the dwarf planet. However, in 2006 the New Horizons[wp] probe was launched with the aim of getting to Pluto by 2015 before moving on to try and study the rest of the Kuiper belt. As Pluto's discoverer Clyde Tombaugh said, "He's got to go one long, cold trip..."
 Dwarf planet
Few people other than a couple astronomers gave much of a damn about Pluto until 2005 when the "10th" planet Eris was discovered. As Eris was larger (by roughly a third, mass-wise) than Pluto but almost three times further away, speculation raged about whether the solar system had any more surprises up its sleeve and whether there could be hundreds of planets like this orbiting the sun. Indeed, two other very distant bodies, Haumea and Makemake, were discovered around the same time. Combined with the already known Ceres (the asteroid-belt-dwelling planetoid) and a few other larger bodies discovered since 2000, this made the solar system look a little crowded. Thus, The International Astronomical Union (IAU) decided to coin the term "dwarf planet" and actually define "planet" for the first time—believe it or not, it really wasn't given a proper definition until that recently. Out of the various criteria proposed (remembering that astronomy is science and needs to be objective) there was no definition available that would keep just 9 planets. Either Eris, Ceres, and potentially many more would be added (noting that Ceres was classed as a planet until the 1860s and was used as evidence to support the Titus-Brode law), or Pluto would be lost. The key to this decision was the third criterion for classification of a planet: the object must have cleared its orbit of other debris and asteroids. None of the bodies in question had achieved this, and so would not be eligible for planetary status. Pluto was reclassified, leading to considerable drama both among astronomers and the general public (who probably felt sorry for Pluto or something like that).
Possibly to calm the drama amongst those weeping for Pluto's demotion, or possibly to appease those who prefer the term over "dwarf planet", another category of "plutoid" was also introduced. This refers to the four dwarf planets in the outer solar system which rank alongside Pluto. Pluto was important again.
Pluto was probably also demoted due to the fact that its name sounded like two famous cartoon characters.
 King of the Plutinos
Plutinos are Kuiper belt objects in a 2:3 orbital period resonance with Neptune. Pluto was the first plutino discovered; the rest of the plutinos (all discovered later) are named after it.
Pluto is by far the largest of all the plutinos; the second largest plutino, Orcus, is less than half of Pluto's diameter.
 Legislative planetology
Pluto is still legally a planet in some places - such as the state of Illinois. In 2009, the Illinois Senate passed a resolution declaring 13 March 2009 "Pluto Day" in the state to honour Pluto's discovery (which is more or less OK) and re-establishing it "with full planetary status" (which is daft). The list of "whereas"-es cited in support of the decision only adds to the hilarity. Unfortunately, the resolution does not provide any instructions about how it's going to be enforced. Can you get arrested in Illinois for calling Pluto a dwarf planet?
 The Plutonian language
In 1969, Patrick Moore made a one-off special for the BBC, called One Pair of Eyes, featuring what Moore called "independent thinkers". These included a man from the Aetherius Society who claimed to be able to speak Venusian and Plutonian.
- Is Pluto a planet?, an accessible video explanation by C.G.P.Grey, suitable for showing to scientifically illiterate relatives who bug you about it