Identification

Pluto is a dwarf planet which is further from the sun than the other eight planets. Pluto is made of rock and ice, and scientists theorize that its frozen surface is made of 98 percent nitrogen, with small amounts of methane and carbon monoxide. It has a very thin methane atmosphere.

Considerations

Pluto's average surface temperature was determined in 2006 by radio telescopes to be 43 Kelvin, or -382 degrees Fahrenheit. The temperature appears to vary from about -400°F to -360°F, caused by factors such as seasons, an eccentric orbit, significant planetary tilt, and night versus day. Because of Pluto's distance from Earth, it has been difficult to obtain extensive information about it, and astronomers do not yet know what the average temperatures during the night and day are. A NASA mission called New Horizons that was sent out in January 2006 and is scheduled to reach Pluto in 2015 should provide more specific information about its temperatures.

Considerations

Pluto's rotation takes a little longer than six Earth days, and it orbits the sun every 248 Earth years. Because of its highly elliptical orbit, Pluto comes closer to the sun than Neptune once every 228 years and then remains closer for 20 years. This last occurred between 1979 and 1999.

Features

Pluto has three moons, named Charon, Nix and Chara. Charon was discovered in 1978 and is large enough that some astronomers think of Pluto and Charon as a binary system. The other two moons were discovered in 2005 after the Hubble Space Telescope took pictures of them. All the moons are a neutral gray color like Earth's moon.

Expert Insight

Pluto was discovered in 1930 and was classified as a planet until 2006. At that point, Pluto was reclassified as a dwarf planet, a new category created by the International Astronomical Union. Pluto is now officially considered the largest member of a group of asteroids called the Kuiper Belt, although many astronomers continue to disagree with the reclassification. The decision has been an ongoing controversy and had been made official only after years of debate.

Two other dwarf planets in our solar system are named Eris and Ceres, with Eris being larger than Pluto. Pluto is even smaller than Earth's moon.

The smallest planet besides Pluto is Mercury, with its mass being 25 times greater than Pluto's.