Pluto

(symbol of Pluto)

Pluto

Pluto, was the 9th planet of the Solar System but on August 24, 2006, the International Astronomical Union (IAU) defined what it means to be a 'planet' within the Solar System. Thus it was excluded as a planet. It was recategorized due to recent discoveries as a dwarf planet along with Eris and Ceres and is now considered the 2nd largest dwarf planet within the newly charted Kuiper belt. It is the tenth most massive body observed directly orbiting the Sun. 

The definition of planet set in 2006 by the (IAU) states that, in the Solar System, a planet is a celestial body which:

1. is in orbit around the Sun,
2. has sufficient mass to assume hydrostatic equilibrium (a nearly round shape), and
3. has cleared the neighbourhood around its orbit.

Pluto has four known moons, the largest being Charon, along with Nix and Hydra, discovered in 2005. Like other members of the Kuiper belt, Pluto is composed primarily of rock and ice and is relatively small: approximately a fifth the mass of the Earth's Moon and a third its volume. It has an eccentric and highly inclined orbit that takes it from 30 to 49 AU (4.4–7.4 billion km) from the Sun. This causes Pluto to periodically come closer to the Sun than Neptune. As of 2011, it is 32.1 AU from the Sun. Pluto and Charon are sometimes described as a binary system because the barycenter of their orbits does not lie within either body.

From its discovery in 1930 until 2006, Pluto was classified as a planet. In the late 1970s, following the discovery of minor planet 2060 Chiron in the outer Solar System and the recognition of Pluto's relatively low mass, its status as a major planet began to be questioned. In the late 20th and early 21st century, many objects similar to Pluto were discovered in the outer Solar System, notably the scattered disc object Eris in 2005, which is 27% more massive than Pluto. After the reclassification, Pluto was added to the list of minor planets and given the number 134340. A number of scientists continue to hold that Pluto should be classified as a planet.