134340 Pluto, more commonly known simply as Pluto, is the second-largest known dwarf planet in the Solar System, and the tenth largest observed body directly orbiting the Sun. It orbits between 29 and 49 AU from the Sun, and was the first Kuiper Belt object (KBO) to be discovered. Approximately one-fifth the mass of the Earth's Moon, Pluto is composed primarily of rock and ice. It has an eccentric orbit that is highly inclined with respect to the planets and takes it closer to the Sun than Neptune during a portion of its orbit.
Pluto and its largest satellite, Charon, could be considered a binary system, because they are closer in size than any of the other known celestial pair combinations in the solar system, and because the barycenter of their orbits does not lie within either body. However, the International Astronomical Union (IAU) has yet to formalize a definition for binary dwarf planets, so Charon is currently regarded as a moon of Pluto. Two smaller moons, Nix and Hydra, were discovered in 2005. Pluto is smaller than several of the natural satellites or moons in our solar system. Around 2015, New Horizons will pass by Pluto, and take the first pictures of the dwarf planet. In 2006, the planet was demoted to dwarf planet due to Pluto not meeting one of the IAU's new regulations.
Pluto was formed around 3.5 billion years ago. Due to its location in the Kuiper Belt, it is believed that the collisions of asteroids caused emmense amounts of heat. This heat fused these asteroids together, creating a celestial object now known as a dwarf planet.
Not much is known about the surface of Pluto, yet artists' depictions describe Pluto's as made of hard rock, for it is located in the Kuiper belt. The rock makes the planet appear very dark, and there are numerous caverns and mountainous areas covering the surface.
Pluto does not have a significant atmosphere. It has a thin envelope of gas that is most likely made up of nitrogen, methane, and carbon monoxide, that develops in equilibrium with solid nitrogen and carbon monoxide ices on the surface as it approaches the Sun. As Pluto moves away from its perihelion (closest point to the Sun), more of its atmosphere freezes and falls to the ground. When it returns to a closer proximity to the Sun, the temperature of Pluto's solid surface will increase, causing the nitrogen ice to sublimate into gas—creating an anti-greenhouse effect. Much as sweat evaporating from the surface of human skin, this sublimation has a cooling effect and scientists have recently discovered, by use of the sub-millimeter array, that Pluto's temperature is 10 K less than they expected.
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