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Planets and moons


An image of Neptune made by Voyager 2 in August 1989An image of Neptune made by Voyager 2 in August 1989 Neptune is the eighth planet from the Sun—the most distant of the true planets. One of the gas giants—sometimes grouped with Uranus as one of the two "ice giants"—it is the fourth largest planet in the Solar System. Little was known about Neptune until Voyager 2 paid it a visit in 1989. The bluest planet in the Solar System, like Uranus it is almost completely featureless, although clouds and storms can sometimes be seen on its surface. Neptune has 14 known moons, with Triton, the only one that is spherical, rather than irregular, in shape, by far the largest.


Neptune was discovered by German astronomer Johann Galle in 1846. Noting that Uranus was being pulled slightly off course, astronomers concluded that the pull of gravity from another large planet farther away from the Sun was responsible. The new planet was located using mathematical calculations.

An image of Neptune's faint rings made by Voyager 2An image of Neptune's faint rings made by Voyager 2


Voyager also confirmed that Neptune, in common with all the other gas giants, had rings. There are four extremely faint rings, two broad, two narrow, all composed of dark, icy fragments ranging in size from dust to blocks the size of a small car. The outermost ring consists of material clumped together like sausages on a string.

Wispy clouds in Neptune's atmosphereWispy clouds in Neptune's atmosphere


The first person to observe Neptune was Galileo, in 1612. But he mistook Neptune for a star. Galileo seems to have noticed that the "star" he saw had moved relative to the other stars around it.

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