Neptune is the 8th planet from the sun. Most of the time Pluto is the only planet more distant than Neptune. Every 248 years, however, Pluto's orbit carries it closer to us than Neptune, and for 20 years Neptune will be the planet farthest from the sun.
Neptune's existence was predicted based on mathematical calculations before it was even discovered--a first for the planets of our solar system.
Neptune is one of the two planets in our solar system that is too dim and distant to be visible to the naked eye. It takes 165 Earth years for Neptune to travel once around the sun, so if you lived on Neptune, your year would last 165 Earth years. A Neptune day lasts 16 hours and 7 minutes.
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Neptune's mass is over 17 times that of Earth, and its volume is over 57 times that of our planet. It's thought to be composed primarily of hydrogen, helium, and silicate minerals; its surface is covered with clouds. Beneath the clouds lies a ocean of dense, highly compressed gas, and then a layer of liquid around a small core of ice and rock about the size of Earth.
Neptune has 11 known moons. The largest, Triton, has the coldest known temperature for any planet or moon in the solar system.