Pluto 
was considered the smallest planet in the solar system until 2006. In that year a large group of scientists decided that Pluto was not a true planet. They voted to call Pluto a dwarf planet instead. Pluto is very far from Earth. It is difficult to observe from Earth, even with the most powerful telescopes.
Pluto orbits, or travels around, the Sun at an average distance of about 3.6 billion miles (5.9 billion kilometers). It is usually farther from the Sun than the planet Neptune. However, Pluto’s path around the Sun is shaped like an oval, not a circle. Its distance from the Sun varies greatly as it orbits. About every 228 years Pluto’s orbit takes it closer to the Sun than Neptune. This last occurred from 1979 to 1999.
Pluto’s oval orbit, small size, and other unusual features make it unlike the eight planets. In the 1990s scientists discovered a ring of millions of small, icy objects. These objects orbit the Sun beyond Neptune. Pluto seems to be similar to some of them.
Pluto’s diameter, or distance through its center, is about 1,430 miles (2,300 kilometers). This is less than half the diameter of the smallest planet, Mercury.
Scientists think that Pluto consists of rocky material and frozen gases. Some areas of Pluto’s surface are very bright, while others are dark. The bright regions are probably a frozen gas called nitrogen.
Pluto is so far from the Sun that it receives only a little sunlight. Scientists believe that the average temperature on its surface is about −387° F (−233° C).
Like the planets, Pluto has two types of motion: orbit and spin. Pluto completes one orbit around the Sun every 248 Earth years. That means that a year on Pluto lasts 248 Earth years. Pluto spins about its center slowly. It completes one rotation in about 6.5 Earth days, so a Pluto day lasts about 6.5 Earth days. Pluto is tilted so that it spins nearly on its side.
Pluto has five known moons: Charon, Nix, Hydra, Kerberos, and Styx. Charon is much larger than the other four.
In 1930 U.S. astronomer Clyde Tombaugh discovered Pluto with the use of a telescope. Scientists believed that Pluto was the ninth planet in the solar system. In 2006 the United States launched a spacecraft, called New Horizons, to explore Pluto. The journey to Pluto was expected to take about nine years. Later in 2006 scientists decided that Pluto should be called a dwarf planet rather than a planet. This brought the number of planets down from nine to eight.
