Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System (after Eris) and the tenth-most-massive body observed directly orbiting the Sun.
Originally classified as the ninth
planet from the Sun, Pluto was recategorized as a dwarf planet and
plutoid due to the discovery that it is one of several large bodies
within the newly charted Kuiper belt.
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.
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.
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 new category “dwarf planet” along with Eris and
Ceres.
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.
Pluto has four known moons, the largest
being Charon discovered in 1978, along with Nix and Hydra, discovered in
2005, and the provisionally named S/2011 P 1, discovered in 2011.
Pluto and Charon are sometimes described
as a binary system because the barycenter of their orbits does not lie
within either body. However, the IAU has yet to formalise a definition
for binary dwarf planets, and as such Charon is officially classified as
a moon of Pluto.
