- Pluto (center) and its moon, Charon (lower left), are shown in a picture taken by the European …
The smallest and coldest planet in the solar system, Pluto was the last of the nine planets to be discovered. It was named after the ancient Roman god of the underworld. Pluto is very far from the Earth. The planet is difficult to observe, even with the most powerful telescopes. Astronomers know much less about Pluto than they know about the other planets. Pluto's average distance from the sun is about 3.6 billion miles (5.9 billion kilometers). A person standing on Pluto would see the sun only as a bright star. Pluto is usually the outermost planet in the solar system and the ninth planet from the sun. However, about every 228 years, Pluto's orbit, or path around the sun, crosses with Neptune's. Then Pluto is the eighth planet in the solar system. This last occurred from 1979 to 1999. Physical featuresPluto is tiny for a planet. Its diameter is about 1,490 miles (2,390 kilometers). This is less than half that of the next smallest planet, Mercury. Pluto's unusual features make it difficult to classify. It is not one of the inner, rocky planets like Earth. However, it is also not one of the outer, gaseous planets like Jupiter. Some astronomers believe that Pluto should not be considered one of the major planets. In some ways Pluto is more like an icy comet or moon. Pluto may have a rocky core that is under ice. Some areas of Pluto's surface are very bright, while others are dark. The bright regions are probably frozen nitrogen. Pluto seems to have polar ice caps as well. It is believed that the ice on Pluto thaws a bit when Pluto's orbit is closer to the sun. Pluto has a thin atmosphere. As the planet moves away from the sun, its atmosphere freezes and falls to the surface like snow. Pluto is so far from the sun that it receives only a little sunlight. The average temperature on the surface of Pluto is thought to be about –356° F (–215° C). The temperature varies greatly depending on how far the planet is from the sun. Orbit and spinLike all planets, Jupiter has two types of motion, known as orbit and spin. Pluto orbits (travels around) the sun about every 248 Earth years. However, Pluto's orbit is oval and the least circular of all the planets. As a result, Pluto's distance from the sun varies greatly throughout its year. Pluto spins on its axis very slowly, completing one revolution only about every 6.5 days. Only Mercury and Venus spin more slowly. Like Uranus, Pluto is tilted in such a way that it spins on its side. Pluto also spins in a clockwise direction like Uranus and Venus. This means that a person on Pluto would see the sun rise in the west and set in the east, which is the opposite of what happens on Earth. CharonPluto has only one known satellite, or moon: Charon. Some astronomers think that Pluto and Charon are a double-planet system. Pluto and Charon are the closest in size of any planet and moon in the solar system. In addition, the two are very close together. Although little is known about Charon, many astronomers believe that its surface is probably covered with ice. ObservationIn the early 20th century some astronomers began to suspect that there might be a ninth planet in the solar system. In 1930 U.S. astronomer Clyde Tombaugh discovered Pluto. He recognized the new planet by its motion in the sky. Pluto's moon Charon was discovered in 1978 when it showed up as a small bump on photographs of Pluto. Unlike the other planets in the solar system, no spacecraft from Earth has visited Pluto. However, scientists have learned some important things about the planet by using the Hubble Space Telescope and other telescopes and satellites that orbit the Earth. |