Neptune is the outermost gas giant in our solar system. For most of its 165 year orbit it is the eighth planet from the Sun and occasionally the ninth due to Pluto's eccentricity. It is the fourth largest planet by diameter and the third largest by mass; Neptune is more massive than its near twin Uranus as its stronger gravitational field has compressed it to a higher density. The planet is named after the Roman god of the sea. Its astronomical symbol is a stylized version of the god's trident (♆).

Neptune's atmosphere is primarily composed of hydrogen and helium, with traces of methane that account for the planet's blue appearance. Neptune's blue colour is much more vivid than that of Uranus, which has a similar amount of methane, and an unknown component is presumed to cause the intense colour. "Neptune overview," Solar System Exploration, NASA. Neptune also has the strongest winds of any planet in the solar system, with estimates as high as 2,500 km/h. At the time of the 1989 Voyager 2 flyby, it had in its southern hemisphere a Great Dark Spot comparable to the Great Red Spot on Jupiter.

Faint dark colored rings have been detected around the blue planet, but are much less substantial than those of Saturn. When these rings were discovered by a team lead by Edward Guinan, it was thought that they might not be complete but this was disproved by Voyager 2. Neptune possesses nine confirmed moons and four awaiting confirmation. Neptune's largest moon, Triton, is notable for its retrograde orbit, extreme cold (38K), and extremely tenuous (14 microbar) nitrogen/methane atmosphere.

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NPR Topics: Space

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