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Neptune |
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--Though it is called the eighth planet, Neptune is sometimes farther from the Sun than Pluto. Neptune is the fourth largest planet in our solar system. Although it is called the eighth planet, Neptune is sometimes farther from the Sun than Pluto; Pluto moves inside Neptune's orbit for almost 20 years every 248 years. Pluto and Neptune are the only two planets that cannot be seen from the Earth without a telescope. Neptune is about 30 times farther from the Sun than our planet. Of the "gas giant" planets, Neptune is the farthest from the Sun. The other "gas giant" planets include Saturn, Jupiter, and Uranus. At about 30,500 miles (49,100 km), Neptune's diameter is about four times greater than the Earth's diameter. Its mass is 17 times greater and its volume is about 72 time greater than our planet's. It takes Neptune 164 Earth years to travel around the Sun. However, the planet rotates quickly on its axis, once about every 16 hours. Neptune has a bluish color because of its thick atmosphere of hydrogen, helium and methane. In fact, the planet was named after the Roman god of water and sea. Scientists think Neptune hydrogen and helium gasses most likely surround a rocky core, which is surrounded by ice and liquid methane. Thick clouds cover the planet's surface, which is not solid. Winds at speeds as great as 700 miles per hour blow Neptune's clouds. Neptune experiences season and temperature changes; the Sun heats the planet's northern and southern halves at different times because Neptune's axis is tilted at about 30 degrees. Five rings and eight known moons surround Neptune. Triton, one of the planet's large moons, is the only large moon in our solar system that orbits its planet in the opposite direction. Scientists believe Triton, only slightly smaller than our planet's moon, may have been a comet that traveled around the Sun and was trapped by Neptune's gravitational pull. Scientists predict the moon will eventually be torn apart by gravity and become a ring. Nereid, another of the planet's moons, has the oddest shaped orbit of any moon in our solar system. Neptune's discovery was a triumph of mathematical astronomy. In 1845, John Couch Adams, an Englishman, calculated the location of an eighth planet based on the disturbance caused in Uranus' orbit. In 1846, Jean Joseph Leverrier, a Frenchman who did not know about Adams work, did the same thing. After less than one year of searching, Neptune was discovered within one degree of its predicted position. Little was known about Neptune before the spacecraft, Voyager 2, visited the planet in 1989.
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