Pluto

Pluto is the farthest planet from the sun. It is also the smallest planet – less than half the size of Mercury. Another reason Pluto is unusual is that its moon, Charon, is more than half its size in fact, Charon is the largest satellite relative to its planet in the solar system.

From Pluto’s density, we know that it must be made of rock and ice. A very thin atmosphere of methane has been detected. While Pluto is cover by nitrogen ice, Charon is covered by water ice. Pluto is the only planet that has not been visited by a NASA mission, but plans are underway to finally visit this world and its moon in 2010.

Moon of Pluto

Pluto’s only moon, Charon, was discovered in 1978. Charon’s period of revolution is about 6.4 days. This means that one side of Pluto always faces Charon. In other words, it you stood on the surface of Pluto, Charon would always occupy the same place every night! Because Charon’s orbit around Pluto is tilted with respect to Pluto’s orbit around the sun, as seen from Earth, Charon sometimes eclipses Pluto. But don’t hold your breath; this happens only every 120 years.