Neptune

The eighth and farthest planet from the Sun is Neptune. It is always more than 2.5 billion miles (4 billion kilometers) from Earth, making it too far to be seen with the unaided eye. It was the second planet, after Uranus, to be discovered through a telescope but the first planet to be found by people specifically searching for one. In the mid-1800s several astronomers began looking for a planet beyond Uranus, in part because Uranus did not move along its orbit exactly as expected. Scientists thought that these slight differences could be caused by the gravitational pull of another planet, and they were right. Several people can be credited with Neptune’s discovery. Urbain-Jean-Joseph Le Verrier independently calculated the planet’s probable location, while in 1846 Johann Gottfried Galle and his assistant Heinrich Louis d’Arrest were the first to identify it in the night sky. The new planet was named Neptune after the ancient Roman god of the sea.

Relatively little was known about the distant planet until the Voyager 2 spacecraft—the only mission to Neptune—flew by it in 1989. The planet that Voyager uncovered is a stormy, windswept world with a vivid blue hue. Its highly active atmosphere is surprising, since it receives so little sunlight to power its weather systems. Like the other giant outer planets, Neptune has no solid surface. It also has a system of rings and more than a dozen moons