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Our Solar System .... Pluto.

Although Pluto was discovered in 1930, limited information on the distant dwarf-planet delays a realistic understanding of its characteristics.
Pluto still remains one of the planets that has not been visited by a spacecraft, yet an increasing amount of information is unfolding about this peculiar planet.
The uniqueness of Pluto's orbit, rotational relationship with its satellite, spin axis, and light variations all give the planet a certain appeal.
Pluto is usually farther from the Sun than any of the planets; however, due to the eccentricity of its orbit, it is closer than Neptune for 20 years out of its 249 year orbit.
Pluto crossed Neptune's orbit January 21, 1979, made its closest approach September 5, 1989, and remained within the orbit of Neptune until February 11, 1999.

This will not occur again until September 2226.

Pluto's rotation period is 6.387 days, the same as its satellite Charon. Pluto is the only planet to rotate synchronously with the orbit of its satellite so they continuously face each other as they travel through space.
Their diameters could be measured directly to within about 1 percent by recent images provided by the Hubble Space Telescope. These images resolved the objects to clearly show two separate disks. The improved optics allowed us to measure Pluto's diameter as 2,274 kilometres and Charon's diameter as 1,172 kilometres, just over half the size of Pluto. Their average separation is 19,640 km. That's roughly eight Pluto diameters.

Pluto and Charon's origin remains in the realm of theory.

Pluto's icy surface is 98% nitrogen. Methane and traces of carbon monoxide are also present. The solid methane indicates that Pluto is colder than 70 Kelvin. Pluto's temperature varies widely during the course of its orbit since Pluto can be as close to the sun as 30 AU and as far away as 50 AU. There is a thin atmosphere that freezes and falls to the surface as the planet moves away from the Sun.

Pluto was officially labelled the ninth planet by the International Astronomical Union in 1930 and named for the Roman god of the underworld. It was the first and only planet to be discovered by an American, Clyde W. Tombaugh.
The path toward its discovery is credited to Percival Lowell who founded the Lowell Observatory in Flagstaff, Arizona and funded three separate searches for "Planet X." Lowell made numerous unsuccessful calculations to find it, believing it could be detected from the effect it would have on Neptune's orbit. Dr. Vesto Slipher, the observatory director, hired Clyde Tombaugh for the third search and Clyde took sets of photographs of the plane of the solar system (ecliptic) one to two weeks apart and looked for anything that shifted against the backdrop of stars.
This systematic approach was successful and Pluto was discovered by this young 24 year old Kansas lab assistant on February 18, 1930.

Pluto is actually too small to be the "Planet X" Percival Lowell had hoped to find. Pluto's was a serendipitous discovery.

Because of the number of larger objects found orbiting in the Kuiper Belt, the International Astronomical Union in 2006 was forced to backstep on it's decision of 1930 and de-classify Pluto as a planet and place it in the category of Dwarf Planet.

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