The main reason is that it is so far away from the Sun, that it is considered part of the Kuiper belt - not the solar system. There are similar sized objects to Pluto in the Kuiper belt system, and thus Pluto was 'demoted' to dwarf planet status.
Pluto is probably nearly 50% water ice. Of course, it was demoted to "dwarf planet" status.
Pluto
Pluto. Poor thing...
In 1853 Vesta, Juno, Ceres and Pallas were demoted from planets to minor planets, and in 2006 Pluto was demoted from planet to dwarf planet.
Pluto.
Pluto.
Pluto is probably nearly 50% water ice. Of course, it was demoted to "dwarf planet" status.
The furthest planet from the Sun in our solar system is Neptune. Formerly it was Pluto (for some of its orbit) but the IAU demoted Pluto to minor planet status.
pluto
The International Astronomical Union "demoted" Pluto from true planet to dwarf or minor planet status in 2006; it is highly unlikely to reverse that decision and change the definition back to include Pluto as a planet in 2015.
There are now only eight planets in the solar system. In 2006, in a controversial decision, Pluto was demoted to the status of a "dwarf planet."
Pluto
Pluto. Poor thing...
On August 24, 2006, the International Astronomical Union redefined the word "planet" and added the category "dwarf planet". End result: Pluto was "demoted" to the status of dwarf planet.
Pluto didn't change; our ideas of Pluto did. It turns out that Pluto is smaller than originally believed, so eventually he was "demoted" to the status (newly-created) of a "dwarf planet". It is unlikely that he will oficially be declared (by the IAU) a planet again.
Mercury is all rock, Pluto is largely ice. Mercury is a planet, Pluto was recently demoted from planet status. Mercury is the closest to the sun, Pluto the farthest away. Mercury is very hot, Pluto is very cold.
In 1853 Vesta, Juno, Ceres and Pallas were demoted from planets to minor planets, and in 2006 Pluto was demoted from planet to dwarf planet.