Pluto is discovered in 1930 by Clyde W. Tombaugh who was an astronomer which was considered
as the ninth planet.
There are 3 criteria of the IAU (International Astronomical Union) to meet to be considered as full-sized planet, they are:
1) The planet's orbit must be around the sun.
2) It must have mass to assume hydrostatic equilibrium (a nearly round shape)
3) It has "cleared the neighborhood" around its ordit.
But in 2006 was demoted to a dwarf planet because it has not "cleared the neighborhood" around its orbit. "Cleared the neighborhood" means it must have no other bodies of comparable size infuencing it gravitationally. Since Pluto shares its orbital with Kuiper Belt objects for example plutinos, its not considered a planet.
Pluto has 5 known moons, they are Charon, Hydra, Nix, Kerberos and Styx.
One third of Pluto is water which is in a form of ice and is 3 times more water than all of earth's ocean.
It has a strange elliptical orbit meaning it distance from the sun varies between 4.4 and 7.3 billion km. At times it can be nearer to the Sun than Neptune.
Pluto sometimes has an atmosphere. Means when the orbit is closer to the Sun, the ice melts and forms an atmosphere mainly nitrogen which oozes tardily out of the planet. When it travels away further away from the sun, the atmosphere freezes back to its solid state.
Data from the NASA New Horizon probe indicates that pluto has a lot of frozen nitrogen, methane and carbon mixed in. It is seen from the probe it has frozen methane on its surface. Along the Southern hemisphere, the Cthulhu Regio,a dark region maybe comprised of "tholins", complex hydrocarbons.
Credit:www.loc.gov/rr/scitech/mysteries/pluto.html, space-facts.com/pluto/, www.nasa.gov/ & www.space.com)