Pluto, as seen by New Horizons. Image courtesy of NASA.
Pluto is a dwarf planet which orbits in an outer region of the solar system known as the Edgeworth-Kuiper belt. Its formal minor-planet designation is 134340 Pluto.
From its discovery in 1930 until 2006, Pluto was considered to be the solar system's outermost known planet. However, it is much smaller than the other planets, measuring 18% of the diameter of the Earth, and less than half the diameter of the next smallest planet, Mercury.
In the early 2000s, its status as a planet became increasingly untenable due to theoretical predictions of large numbers of other similarly-sized objects in the Edgeworth-Kuiper belt.
Pluto was at last downgraded from its status as a planet in 2006, and given the newly-generated title of dwarf planet. This was prompted by the discovery of another Edgeworth-Kuiper belt object, which was 27% more massive than Pluto. In view of its role in history, that object was named Eris, after the goddess of strife of discord in Greek mythology; Eris was also classified as a dwarf planet.
Pluto is a faint and distant object. At around magnitude 14, it is beyond the reach of all but the largest amateur telescopes. Pluto's largest moon, Charon, is not much smaller than Pluto itself, measuring a little under 10% of the diameter of the Earth, and appears 2–3 magnitudes fainter. Two other much smaller moons, Nix and Hydra, were discovered in 2005.
Pluto has only ever been visited once by one spacecraft: the NASA New Horizons probe flew past it in 2015 before going on to study other Edgeworth-Kuiper belt objects.