Neptune, Eighth planet from the Sun, discovered in 1846 and named for the Roman god of the sea. It has an average distance from the Sun of 2.8 billion mi (4.5 billion km), taking nearly 164 years to complete one orbit and rotating every 16.11 hours. Neptune has more than 17 times Earth’s mass, 58 times its volume, and 12% stronger gravity at the top of its atmosphere. It has an equatorial diameter of 30,775 mi (49,528 km). Neptune consists largely of hydrogen and helium. It has no solid surface; its interior is believed to consist of a fluid mixture of rock, ices, and gas. Its atmosphere contains substantial amounts of methane gas, whose absorption of red light causes Neptune’s deep blue-green colour. The Voyager 2 space probe in 1989 discovered winds of over 1,570 mi/hour (700 m/second), the fastest known for any of the Sun’s planets, and dark spots that appear to be storms similar to Jupiter’s Great Red Spot. Neptune receives little solar radiation, but it radiates substantially more energy than it receives, which indicates an internal heat source. Neptune’s weak magnetic field traps charged particles in a belt around the planet. Neptune has a system of rings, made up largely of dust-size particles, and at least 13 moons; the largest is Triton, almost as big as Earth’s Moon.
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