Pluto is a unique case in our solar system. Discovered in 1930, Pluto remained classified as a planet until 2006 when the International Astronomical Union re-classified it as a “dwarf planet,” saying it didn’t meet all the criteria for being an official planet.
The union noted that Pluto is much smaller than any other planet (it is about half the size of Mercury, the smallest official planet) and it shares its orbit with more than 1,000 other icy bodies in the Kuiper Belt, a collection of small objects circling the solar system beyond Neptune’s orbit.
Pluto consists of a rocky core with an atmosphere comprised of methane, carbon monoxide and nitrogen. Pluto has a long way to go around the Sun, taking 248 Earth years to complete a single orbit. Unlike other members of the solar system, Pluto’s orbit is elliptical rather than circular, which actually brings it closer to the Sun than Neptune.
Research on Pluto is extremely limited due to its distance from the Earth and its small size. In the late 1980s, Pluto and one of its moons, Charon, repeatedly passed in front of one another, allowing scientists to determine the brightness of each and create maps of the surface. 
NASA’s New Horizons spacecraft, which left Earth in 2006, is expected to reach Pluto in July 2015.
(Text by Noel Kirkpatrick)

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