n.
Eighth planet from the Sun, discovered in 1846 and named after the Roman god of the sea.
It has an average distance from the Sun of 2.8 billion mi (4.5 billion km), taking 165 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 apparent solid surface but may have a frozen, rocky core. Its atmosphere contains substantial amounts of methane gas, whose absorption of red light causes Neptune's deep blue 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 may be storms similar to Jupiter's Great Red Spot . Neptune receives little solar radiation, but temperature measurements of about -353 °F (-214 °C) suggest an internal heat source. Neptune's weak magnetic field traps solar wind and cosmic ray s in a belt around the planet. Neptune has a system of rings, made largely of dust-sized particles, and eight known satellites; the largest is Triton , almost as big as Earth's Moon.