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space Neptune orbits about 30 times as far from the Sun as Earth does. It takes about 165 years to complete each loop. Most of the time, it is the eighth planet from the Sun, but because of Pluto's odd-shaped orbit, Neptune is actually the farthest out of the nine planets for about 20 years out of every 248 years.

Like the other three "gas giant" planets -- Jupiter, Saturn and Uranus -- Neptune has no solid surface. Its atmosphere contains hydrogen and helium with enough methane to give it a bluish tint. Winds on Neptune blow faster than on any other planet.

The discoveries of Neptune and its largest moon, Triton, came less than a month apart in 1846. Mathematicians had accurately predicted where Neptune could be discovered, based on its gravitational effect on Uranus.

NASA's Voyager 2 spacecraft flew over Neptune's north pole in 1989. It studied a hurricane-like storm as large as the Earth and discovered six moons. Most surprisingly, the spacecraft spotted signs of active geysers on Triton. The geysers are believed to be burps of gaseous nitrogen on a world with the coldest known surface temperature of any planet or moon, about minus 235 degrees Celsius (minus 391 degrees Fahrenheit).

For more information, visit NASA's Solar System Exploration site.

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  Fast Facts - Neptune:

Mean Distance from Sun
4,498,252,900 km (2.80 x 109 miles) or 30.07 astronomical units
55,528 km (34,503.5 miles)
Volume (Earth = 1)
1.02 x 1026 kg (3.61 x 1027 ounces) or 17.2 (Earth = 1)
1.76 gm/cm3
Surface gravity tba (1100 cm/s (2)
1.19 (Earth = 1)
Rotation period (length of day in Earth days)
0.67 (19.1 Earth hours)
Revolution period (length of year)
163.72 Earth years
Mean surface temperature tba
-200.15 C (-328.27° F)
Natural satellites
Naiad, Thalassa, Despina, Galatea, Larissa, Proteus, Triton, Nereid

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