Why is Pluto called a "dwarf" planet?
Pluto is the second most massive dwarf planet (after Eris) in the solar system. It is the tenth most massive body orbiting the sun. Originally, Pluto was classified as a planet. Pluto is now considered the largest member of a distinct population known as the Kuiper Belt. Pluto is composed primarily of rock and ice, and has one moon, Charon. Pluto is roughly one-fifth of the Earth's moon's mass and one-third of it's volume.
From it's discovery in 1930 until 2006, Pluto was considered the solar system's ninth planet. In the late 1970's, following the discovery of minor planet 2060 Chiron in the outer solar system and the recognition of Pluto's relatively low mass, it's status as a major planet began to be questioned. On August 24, 2006, the International Astronomical Union (IAU) defined what it means to be a "planet" within the solar system. This definition excluded Pluto as a planet and added it as a member of the newly formed "dwarf planet" class, along with Eris and Ceres.
Pluto's planet status remains a matter of debate among astronomers. Dr. Alan Stern, now Principal Investigator of NASA's New Horizons mission to Pluto, initially coined the term "dwarf planet" in 1991 to designate a third class of planets in addition to terrestrials and jovians--small planets large enough to be rounded by their own gravity but not large enough to gravitationally dominate their orbits. He never meant for dwarf planets to not be considered planets at all. The 2006 IAU decision represents only one viewpoint in an ongoing debate on what constitutes a planet. It reflects the position of one camp of astronomers, the dynamicists, who believe that an object has to gravitationally dominate its orbit to be a planet. Only four percent of the IAU even voted on this. Their decision was immediately opposed in a petition of hundreds of professional astronomers. The latter group, the geophysicists concentrate on individual objects themselves rather than on how those objects affect other objects. To geophysicists, if an object is large enough to be rounded by its own gravity, a state known as hydrostatic equilibrium, and it orbits a star, it is a planet. Dwarf planets are simply small planets. Because Eris and Ceres are also in hydrostatic equilibrium, geophysicists consider them planets too.
Pluto is estimated to be about 70% rock, and is geologically differentiated into core, mantle, and crust, just like Earth is. It has geology and weather, all characteristics of planets and not of shapeless asteroids. Additionally, Pluto has four moons: its large moon Charon and three tiny moons, two discovered in 2005, named Nix and Hydra, and one discovered in 2011, known as P4. Pluto can be considered to be both a planet and a Kuiper Belt Object. The first tells us what it is, and the second tells us where it is.