After 76 years of classification as a planet, Pluto was demoted in 2006 to a dwarf planet, in part because of its size but also because of its minor gravitational effects on the bodies around it. It remains one of the most well-known non-planetary bodies in the solar system.
Radius, diameter and circumference
Pluto has a mean radius of 715 miles (1,151 kilometers), less than 20 percent that of Earth. Unlike many of the planets in the solar system, such as Earth, Pluto does not bulge at its center; its radius is the same at its poles and at its equator. The diameter of the planet is 1,430 miles (2,302 km), only about two-thirds the diameter of the moon.
If you were to take a walk around the equator of Pluto, you would travel 4,494 miles (7,232 km). That is only about 1,400 miles farther than a trip across the United States, from Augusta, Maine, to Los Angeles, Calif.
Pluto is thought to have a rocky core covered by ice, which would mean that its surface features would change with temperature as it travels closer to and farther from the sun. In fact, as the ice melts, the atmosphere of the tiny body expands outward.
Unfortunately, the distance to Pluto makes studying the dwarf planet a challenge. This should be remedied when the New Horizons probe arrives in 2015.
Density, mass and volume
Although all of the planets beyond Mars are gas giants, Pluto is small and rocky. The tiny body has a mass of only 1.31 x 1022 kilograms, about two-tenths of a percent of Earth's. It has a volume of 1.5 billion cubic miles (6.4 billion cubic km).
Pluto's small size and low mass mean that it has a density of 2.05 grams per cubic centimeter, about 40 percent of Earth's density.
Demoted from planetary status
Since its discovery in 1930, Pluto's status as a planet has been debated. It is less massive than seven of the moons in the solar system — Earth's moon, the four Galilean moons of Jupiter, Neptune's moon Triton, and Saturn's moon Titan. [Infographic: Pluto's 5 Moons Explained]
In 2003, the icy body of Eris was found far beyond the Kuiper Belt. Originally, it appeared to be larger than Pluto. The discovery sparked a debate about what it meant to be a planet.
In 2006, the International Astronomical Union came up with four criteria that cause an object to be classified as a dwarf planet. A dwarf planet:
- Orbits the sun
- Has enough mass to assume a nearly round shape
- Is not a moon
- Has not cleared the neighborhood around its orbit
Under this criteria, Pluto's low mass does not directly keep it from full planetary status, but the fact that it fails to sweep clean the area surrounding it. Of course, the reason it can't clear out the Kuiper Belt it orbits through is because it lacks the gravitational force to do so, a fact related to its mass.
— Nola Taylor Redd, SPACE.com Contributor