Pluto and its satellite Charon are often referred to as a double planet system. The two objects revolve about a common center of gravity located between the two bodies (closer to Pluto, as it has more mass).
Pluto is the outermost planet in our solar system and its existence was first predicted by astronomic calculations based on slight perturbations in the orbit of Uranus. In 1930, Pluto was finally found near the position predicted by the American astronomer Tombaugh.
However, its discovery was rather an accident. Today, scientists theorize that the gravitational_forces of the small planet are not sufficient to explain the irregularities observed in the orbit of Uranus. Due to the fact that the gravitational center of the double planet system of Pluto and Charon is located outside the planet, the rotation period of Pluto is equal to the period of revolution of Charon.
The two bodies are therefore always in the same position to each other. One of the theories on the origin of the planet maintains that Pluto and Charon originate from a moon of Jupiter that escaped the planet's field of gravitation. The main reasons presented for this view is the obvious fact that Pluto has very little in common with the other outer planets. Its composition, structure and size are rather moon-like.
Pluto's orbit is extremely elliptic. Its distance to the Sun thus fluctuates between 4.4 and 7.4 billion kilometers. It takes Pluto 148 Earth years to complete a full revolution around the Sun. For approximately 20 of those years, Pluto is actually not the outermost planet in the solar system.
Sometimes, its orbit crosses that of Neptune, which was the case between 1979 and 1999, when Neptune thus became the outermost planet in the system. For the next 228 years however, Pluto will hold its position as the planet farthest from the Sun. There is very little known of the composition of the planet and its satellite. It is, however, believed that both have a rocky core covered by a mantle of ice.
During periods when Pluto is closer to the Sun, an extremely thin atmosphere of nitrogen and methane develops. Temperatures are believed to be around minus 230┬░ Celsius. Both in terms of mass and size (diameter 2300 kilometers), Pluto is smaller than our Moon. Its density is lower than that of ice. Pluto and Charon are only 19,400 kilometers apart.
