Generalities: it is the farthest planet from the Sun, besides being the smallest planet of the Solar System. Unlike the other external planets, which are constituted by gas, Pluto has a rocky structure.
It is the least  known planet, since it has an essentially stellar aspect when observed with a telescope, because of its small dimensions and of its great distance, without showing any characteristics of its surface. Moreover it is the only planet not to be visited by probes yet. Pluto is the only planet  to have been discovered in our century, because of its extremely weak light. The discovery was completed in 1930 by Tombaugh, who identified it by means of photographic shots, after the theoretical forecast of its existence. The knowledge on Pluto is therefore still limited, even if its diameter and its rotation period  have been esteemed with enough precision.
It seems that Pluto has an atmosphere very poor in methane.
Pluto’s origins  are very discussed, but no hypothesis is completely certain. The most recent one sets it as the founder of great dimensions of a series of small bodies (asteroids), some of which have been  identified, which populate the outskirts of the Solar System. Pluto has a rather different orbit in comparison to the other planets, since it is more eccentric and more sloping. For some periods of its orbit (about twenty years) Pluto is taken to a distance from the Sun which is inferior to Neptune’s one. Up to  1999 Pluto, which  reached the point of least proximity to the Sun in 1989,  will be found in the condition described above.

Observation: to be observed Pluto needs a telescope with an opening of at least 25-30 cm. Its observation is made difficult by the fact that it is necessary to know exactly where it is found in the sky, and to have a stellar map  pointing even to the very weak stars, of magnitude similar to Pluto’s, which shows in this period  a brightness equal to  magnitude 14 . Even with more powerful telescopes  it is impossible to perceive the planetary disk because of its small dimensions.

Satellites: Pluto has a satellite named Charon, which was discovered in 1978. Charon has a diameter esteemed to be  1,200 Km, so that it is big over half the planet. Charon is  at a small distance from Pluto, only 19,000 Km. The satellite has a rotation period of 6.3867 days, equal therefore to that of Pluto itself, the only  case in the Solar System. Charon has then the supremacy to be the greatest satellite in comparison to its planet. It is to be noticed  that, before Charon’s discovery , this supremacy belonged to the Moon.
 
 

- Visibility of planets during this month

- Mutual position of Earth and external planets

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