"Pluto and its moon Charon, which would both have been planets under the initial definition proposed Aug. 16, now get demoted because they are part of a sea of other objects that occupy the same region of space." http://www.msnbc.msn.com/id/14489259/

It was removed from the planets because it is known as a dwarf planet, and has a different orbit than most planets. In the last few years, new objects like pluto were discovered in the outer solar system in a somewhat scattered fashion, and scientists have decided to name Pluto and other masses minor planets while all the rest of the planets remain major planets.

Any planet with a diameter between something like 900km to 4,000km is classified as a dwarf planet. Basically, it's big enough to be quite spherical but not quite spherical enough to be a planet. Pluto has a diameter of about 2,400km and Mercury has a diameter of about 5,000km.

1) "From its discovery in 1930 until 2006, Pluto was counted as the Solar System's ninth planet. In the late 20th and early 21st centuries, however, many objects similar to Pluto were discovered in the outer solar system, notably the scattered disc object Eris, which is 27% more massive than Pluto. On August 24, 2006 the IAU defined the term "planet" for the first time. This definition excluded Pluto, which the IAU reclassified as a member of the new category of dwarf planets along with Eris and Ceres. After the reclassification, Pluto was added to the list of minor planets and given the number 134340." Source and further information: http://en.wikipedia.org/wiki/Pluto 2) "A planet, as defined by the International Astronomical Union (IAU), is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals." Source and further information: http://en.wikipedia.org/wiki/Planet Further information: http://en.wikipedia.org/wiki/2006_definition_of_planet 3) "A dwarf planet, as defined by the International Astronomical Union (IAU), is a celestial body orbiting the Sun that is massive enough to be rounded by its own gravity but which has not cleared its neighbouring region of planetesimals and is not a satellite. More explicitly, it has to have sufficient mass for its self-gravity to overcome rigid body forces in order to assume a hydrostatic equilibrium and acquire a near-spherical shape." Source and further information: http://en.wikipedia.org/wiki/Dwarf_planet