Pluto is the smallest and the furthest planet from the Sun in our solar system (5915.8 million km or 3676 million miles). The nearest planet to Pluto is Neptune, with a distance of 4504 million km from the Sun.
Pluto is often considered to be more similar to a moon than a planet. As you can see in the image, Pluto is even smaller than Earth's moon. Because of Plutos small size and other features that appear to be unlike most of the other planets in our solar system, scientists were considering demoting Pluto from planet status. However, at this time Pluto IS still considered one of the nine planets in our solar system, but has been demoted to the status of dwarf planet.
One reason that Pluto is still classified as a planet is that Pluto has moons of its own. The largest of these, named Charon, was discovered in 1978 and has a diameter of 737 miles (Pluto was discovered in 1930). Astronomers believe that Charon resulted when a large planet-sized object hit Pluto and knocked a large chunk of it into space.
In 2005 two more moons were discovered in orbit around Pluto. These moons were designated Nix and Hydra. These two moons are much smaller than Charon.
Another reason that scientists classify Pluto as a planet is that it is round like a planet. Most asteroids and other space objects are irregularly shaped.
A Day on Pluto
Compared to here on Earth, a day on Pluto lasts a long time - Pluto makes one complete rotation on its axis every 6.9 Earth days. It makes one complete orbit around the Sun every 248 Earth years.
Because of its distance from the Sun, Pluto is cold, with a surface temperature of approximately -375 °F (-225 °C) (Spinrad, 2004).
Because Pluto is so far away from Earth, astronomers don't know very much about it. Scientists can often infer properties about distant planets by observing the way other space objects near them behave when they can't take direct measurements.
Astronomers believe that Pluto is an icy world, that may be covered with frozen methane gas. Pluto's atmosphere is also thought to consist of mostly methane (Spinrad, 2004).
The NASA New Horizons spacecraft is expected to reach Pluto by 2015 and then astronomers will know much more about Pluto than they currently do.
Spinrad, Hyron. "Pluto." World Book Online Reference Center. 2004. World Book, Inc. http://www.worldbookonline.com/wb/Article?id=ar435500.
Photo of Pluto: Courtesy of NASA Glenn Research Center.