Although Neptune is a huge planet, it is not visible from Earth without the use of a telescope because it is so far away. Named for the ancient Roman god of the sea, Neptune looks blue from the Earth. The planet was discovered with a telescope in 1846.
Neptune is about 3 billion miles (4.8 billion kilometers) from the sun. It is usually the eighth planet from the sun. However, about every 228 years Pluto's orbit crosses paths with Neptune's. Then Neptune is the ninth and farthest planet in the solar system for a period of 20 years. This last occurred from 1979 to 1999.
Neptune is a huge planet. Its diameter is about 31,000 miles (50,000 kilometers). Neptune's volume is so large that it could hold almost 60 Earths.
Not much is known for certain about Neptune's interior. Like Jupiter, Saturn, and Uranus, it is called a gas giant because it is made up mostly of gases. It is believed that under hundreds of miles of these gases there is a core made of rock and ice that have been turned into liquid by the high pressures and temperatures of the planet. The core is about the size of the Earth.
The stormy atmosphere of Neptune is made up of mostly hydrogen and some helium. This composition is similar to that of the sun and the other gas giants. Methane gas makes up about 2 percent of the atmosphere. The methane gives Neptune its bluish color.
In Neptune's cloud level its temperatures average about –328° F (–200° C). The core, however, can reach temperatures of about 4,000° F (2,200° C). Neptune's winds are nine times stronger than the Earth's.
Neptune has eight known satellites, or moons. These satellites are all named after Greek and Roman gods associated with the sea. The largest satellite, Triton, is unusual because it revolves around Neptune in a direction that is opposite that of the planet's rotation. All other large satellites in the solar system revolve in the direction of their planet's rotation. Scientists believe that Triton might have formed as an independent planet but approached Neptune too closely and was captured by the gas giant's gravity.
At least four narrow rings encircle Neptune. The rings are made of dust-sized particles and may have formed when small meteorites crashed into Neptune's satellites. It is believed that these rings are relatively young and may not last for long.
Like all planets, Neptune has two types of motion, known as orbit and spin. Neptune takes about 165 Earth years to orbit (travel around) the sun. In other words, one Neptune year is equal to 165 Earth years. Neptune has not completely orbited the sun since it was discovered in 1846. Neptune spins around its axis rapidly, about every 16 hours. Therefore, one day in Neptune is equal to about 16 hours. (For more information on orbit and spin, see planets.)
In the 1840s two separate astronomers used mathematical calculations to predict where Neptune might be. They communicated their prediction to others, and in 1846 the German astronomer Johann Gottfried Galle located the planet.
The United States National Aeronautics and Space Administration (NASA) launched Voyager 2 in 1977 to study several planets. In 1989 the spacecraft came within about 3,000 miles (4,800 kilometers) of Neptune. Voyager 2 discovered a magnetic field and three rings around the planet. It also observed intense atmospheric storms and five of the planet's satellites. A large dark oval the size of the Earth was also seen in Neptune's southern hemisphere. A giant storm system, this spot was named the Great Dark Spot. When NASA's Hubble Space Telescope passed by Neptune in 1994, the spot was no longer there. However, a few months later a similar spot was seen in Neptune's northern hemisphere, indicating that the planet's atmosphere changes quickly.
