Pluto is small because it has a very low force of gravity, caused by its low density at 2.03 g/cm³. Its density is almost half of Mercury's, the smallest of the eight planets in the solar system. Its gravitational force is only 5 percent of Earth's.
Pluto has a mean radius that is 20 percent smaller than Earth's at 715 miles. Unlike many planets in the solar system, Pluto does not bulge at the center. Instead, its radius is the same at the center as it is at its poles and equator. Pluto has a diameter of 1,430 miles, making it even smaller than the moon. Pluto's surface is composed of rock and ice.
A U.S. astronomer discovered Pluto in 1930 and first classified it as a planet, making it the smallest planet in the solar system. It was downgraded to a dwarf planet in 2003 when a larger object, later named Eris, was observed beyond Pluto and thought to be another new planet and a closer look was taken at Pluto's standing as a planet. In 2006, the International Astronomical Union released a set of criteria that are used to classify dwarf planets. These requirements state that a dwarf planet must have just enough mass to make it almost round, must orbit the sun, must not have a moon and must be unable to clear the orbit around it.