Pluto

Pluto (134340) 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.Pluto - Wikipedia Although Pluto was discovered in 1930, limited information on the distant object delayed a realistic understanding of its characteristics. Pluto is the second largest known dwarf planet and tenth largest orbiting the Sun. From its time of discovery in 1930 to 2006 it was considered to be the ninth planet in the solar system, but because additional objects have been discovered including Eris which is 27% more massive, the IAU reclassified Pluto and the other objects as dwarf planets.Pluto - Views of the Solar System
== Surface ==
Pluto is thought to be made of mostly ice. It probably also has a small rocky core which might contain some metals. The ice on the surface of Pluto is made of frozen nitrogen, methane and carbon monoxide.http://coolcosmos.ipac.caltech.edu/ask/163-What-is-Pluto-made-of-
Pluto's surface is characterized by mountains, valleys, plains, and craters. The temperature on Pluto can be as cold as -375 to -400 degrees Fahrenheit (-226 to -240 degrees Celsius).
Pluto's mountains can be as tall as 6,500 to 9,800 feet (2 to 3 kilometers) and are big blocks of water ice, sometimes with a coating of frozen gases like methane. And long troughs and valleys as long as 370 miles (600 kilometers) add to the interesting features of this faraway dwarf planet.
Craters as large as 162 miles (260 kilometers) in diameter dot some of the landscape on Pluto, with some showing signs of erosion and filling. This suggests tectonic forces are slowly resurfacing Pluto.
The most prominent plains observed on Pluto appear to be made of frozen nitrogen gas and show no craters. These plains do show structures suggesting convection (blobs of material circulating up and down).
== Atmosphere ==
Pluto has a thin, tenuous atmosphere that expands when it comes closer to the sun and collapses as it moves farther away—similar to a comet. The main constituent is molecular nitrogen, though molecules of methane and carbon monoxide have also been detected.
When Pluto is close to the sun, its surface ices sublimate (changing directly from solid to gas) and rise to temporarily form a thin atmosphere. Pluto's low gravity (about six percent of Earth's) causes the atmosphere to be much more extended in altitude than our planet's atmosphere. Pluto becomes much colder during the part of each year when it is traveling far away from the sun. During this time, the bulk of the planet's atmosphere may freeze and fall as snow to the surface.
== Moons ==
Pluto has five known moons: Charon, Nix, Hydra, Kerberos and Styx. This moon system might have formed by a collision between Pluto and another similar-sized body early in the history of the solar system.
==References==
Category:Dwarf planets
Category:Kuiper belt objects