单选题
Pluto
Pluto is unique among the planets in our solar system for several reasons. It was the last planet to be discovered, and is also the smallest and furthest from the Sun. Pluto was discovered in 1930 and has an estimated diameter of about 1400 miles. Because of its great distance from the Sun, it takes Pluto over 248 years to complete an entire orbit. There is considerable variation in the distance of Pluto from the Sun during the course of its orbit. It gets as close as 2.75 billion miles and travels as far as 4.6 billion miles from the Sun. Because Pluto"s orbit is highly elliptical, its orbit passes within that of Neptune for about 20 years of the time it takes to complete an orbit.
Pluto"s great distance from Earth makes it difficult to know with complete certainty its physical characteristics. It is believed that it has a thick methane atmosphere a few kilometers deep and that it is covered with frost and ice. The covering of methane frost is redder towards the equator and bluer at the poles. Pluto is thought to have a large core, likely of silicate rock, which accounts for the planet"s high density.
New information is still being learned about Pluto as technology becomes more sophisticated. As recently as 1978, it was discovered that Pluto has a satellite. This satellite, named Charon, has a diameter of about 700 miles and takes just over six days to complete a revolution of Pluto, which is about the same time that it takes Pluto to complete a rotation.
Recently, however, Pluto"s status as a planet has hotly been debated amongst astronomers. More traditional scientists have tried to defend Pluto"s position amongst our solar system"s planetary array on account of traditional values and beliefs. After all, some argue, it is unfair to downgrade Pluto to a mere asteroid just because of its smaller size when it obviously is a dense corpus with its own satellite.
Arguments to the contrary also abound. Many pose that Pluto is a mere object of the Kuiper belt due to the fact that it is constantly pulled into Neptune"s orbital sphere. It is only such drag from a more powerful planet that corrects Pluto"s own course and prevents it from colliding with other celestial bodies. The latter argument won out, and Pluto has recently been removed from textbooks from the list of other planets of our solar system.
Glossary
Pluto:
the second smallest planet and the farthest known from the sun, has the most elliptical orbit of all the planets
elliptical:
rounded like an egg
silicate:
any of numerous compounds containing silicon, oxygen, and one or more metals
Charon:
the only satellite of Pluto