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The Pluto Charon System and the Kuiper Belt.
According to the history of its discovery Pluto was considered to be the ninth and furthest planet of the solar system. A description which had to be revised after recent discoveries. Pluto has characteristics which are untypical of planets. Pluto is much smaller than we originally thought and Charon, its neighbouring planet, is half the size of it. Pluto's mass is about 1/400 that of the earth's or 1/6 of our moon's. Pluto (diameter about 2300 km) and Charon (diameter about 1180 km) orbit each other at a distance of 19,700 km, each turn lasting 6.4 days. The corresponding orbital plane stands nearly perpendicular (118°) to the orbit of both around the sun. One rotation around the sun for both celestial bodies lasts 248 years and this is with an orbit which is slanted 17° to the ecliptical plane, which is an orbit more inclined and eccentric than of all the other planets. Its nearest point to the sun is located at 29.6 AU and its furthest at 49.3 AU. For a short time  Pluto and Charon come closer to the sun than even Neptune, such was the case from 1979 to 1999. Neither planet has been explored by a space probe yet, so knowledge about their qualities are sparse and vague. The NASA probe Pluto-Kuiper-Express should have reached Pluto in 2020,  but due to lack of money it was delayed so much that the right window for a launch was missed. The fact that Pluto reached the status of a planet, which it will maybe lose again, lies in the history of its discovery. Throughout the history of the planets, the understanding and positioning of them (not only of the gods) was dependent on the scientific knowledge of mankind. Our view of the earth itself, which wasn't considered to be a planet 500 years ago, is an excellent example. Pluto's discovery happened similarly to the discovery of Uranus and Neptune due to a well-aimed search, but with false expectations and an actually unobtainable objective. As before, because of orbital disturbances, one expected it to be an unknown planet. 

A pioneer in the research of our solar system was the American Lowell, who made a name for himself exposing the false discoveries of the supposed Mars-canals and Mars-inhabitants at the beginning of the 19th century. 14 years after Lowell's death, the young astronomer C. W. Tombaugh found, whilst carefully scanning photos in the observatory later named after Lowell, an object which had moved opposite the bustling activity of the fixed stars. They thought that they had finally discovered the 9th planet which they had been searching for and on March 13th, 1930 they named it Pluto. This name was chosen, among other possibilities, because its beginning letters were the same as Percical Lowell's initials, who would at that time have been celebrating his 75th birthday. 
Pluto was falsely estimated at the beginning as being the same size as the earth and seven times its mass. Soon people recognized characteristics not typical of a planet. With progress in technology of optical tools, its size was reduced to that of a small planet (asteroid), which we had known about since Piazzi discovered Ceres in 1801. Pluto's moon was discovered in 1978 and was called Charon. Up to now we know around 150 similar objects, which are travelling at an average distance of 40 AU around the sun and which have, like the Pluto-Charon-System a resonance of 2:3 to the rotational period of Neptune. Their diameters are normally several 100 km. They are part of the so-called Kuiper-Belt, of which  the supposedly many thousand members are called Plutinos . In this outer asteroid belt no objects have been found yet which are bigger in size than Pluto. But Pluto is considerably smaller than the earth's moon or either one of Jupiter's,  Saturn's or Neptune's, whose moon is Triton and was probably a Plutino which was caught by Neptune.

If Pluto was discovered for the first time now or if its real size had been correctly estimated at the time of its discovery, it would not really have been counted as a planet but merely as a small planet of the Kuiper-Belt, as Brian Marsden has suggested to the IAU (International Astronomic Union). The retraction of Pluto`s identity as a planet has caused a great wave of displeasure amongst the conservative members of the Lowell Observatory, who believe the name small planet would mean a loss of prestige. In former times mankind quarrelled as to whether the earth or the sun was at the center of the universe, now it seems history is repeating itself.

 Pluto is the Roman name for the Greek god of the underworld  (Hades), who receives the souls of the dead when the ferryman Charon takes them over the river Styx.

Hades/Pluto entführt Persephone in die Unterwelt.
Picture: Hades/Pluto kidnaps Persephone into the underworld