Pluto

Discovered in 1930 by C. Tombaugh, Pluto is still a little known planet, because its distance and its dimensions prevent a good observation with the telescope, and because it was never approached by an automatic probe, as instead it happened for all the others planets of the Solar System.

Smaller than the Moon, the planet seems covered by frozen methane, with a temperature near to -230°C. Its orbit is so eccentric that from 1979 to 1999 it is nearer to the Sun than Neptune: this characteristic has let hypothesize its "capture" by the Solar System in a more recent epoch in comparison to the formation of the other planets.

Pluto has a satellite, Charon: because of the proximity of the two bodies and the small difference of their dimensions, they rotate around the barycentre of the system with equal orbital periods and equal periods of revolution. This makes the system Pluto-Charon the only example of "double planet" in the Solar System.