Pluto
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Pluto is the farest planet from the star. It is 5'870'000'000 km far from the Sun. Its diametre is about 2'390 km. This planet is still mysterious, it is to far away for us, that wa can't study it well for the moment. It volumic mass is 1.75kg/litre. We don't know of what it is composed, but we think that there are probably silicates and iced methane. The average of the temperature there is -225°C. The composition of the atmosphere if there is one, is unknown for the moment. Pluto is smaller than seven of the solar system's moons (the Moon, Io, Europa, Ganymede, Callisto, Titan and Triton).
Fortunately, Pluto has a satellite, Charon. By good fortune, Charon was discovered (in 1978) just before its orbital plane moved edge-on toward the inner solar system. It was therefore possible to observe many transits of Pluto over Charon and vice versa. By carefully calculating which portions of which body would be covered at what times, and watching brightness curves, astronomers were able to construct a rough map of light and dark areas on both bodies.
Pluto's orbit is highly eccentric. At times it is closer to the Sun than Neptune (as it was from January 1979 thru February 11 1999). Pluto rotates in the opposite direction from most of the other planets.
Scientists think that Pluto probably came from an other solar system and had been captured by our system, this is an hypothese on the orbit of Pluto and its origins.
