Why is Pluto called a "dwarf" planet?

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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...

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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.

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