Neptune, also known as the Blue Giant for its large size and vivid color, is the outermost planet in our Solar System. It joins Uranus, Saturn, and Jupiter as one of the giant planets. Giant planets are unimaginably huge, stunningly beautiful, and sometimes a little weird. They are made mostly of gases instead of solid materials, but Neptune likely has regions of ices beneath its atmosphere.
Discovery
The five planets visible to the naked eye had been known since ancient times. Neptune, however, was not discovered until the age of the telescope. Irregularities in the orbit of Uranus led French mathematician Joseph Le Verrier and British astronomer John Couch Adams to independently calculate where an unknown body affecting Uranus might be. Using Le Verrier’s prediction, German astronomer Johann Galle found Neptune in 1946 while observing the sky at the Berlin Observatory .
Characteristics of Neptune
Neptune is an ice giant. An ice giant is a giant planet that is made up of a thick soup of water, methane, and ammonia which scientists refer to as "ices." Neptune’s core is rocky, and its mantle is made up of the icy water and ammonia associated with the ice giants. Its striking blue hue is due to the haze that is created by methane and hydrogen sulfide gases in its atmosphere.
All four giant planets have rings. Jupiter, Uranus, and Neptune have only a few faint, narrow rings that are difficult to observe.
As with Uranus, Neptune’s rings were discovered when astronomers observed them blocking the light from a star. Six rings were later identified in Voyager 2 images. The outermost ring contains bright areas called arcs, where material is clumped together. These structures puzzle scientists because the material should even out.
Large storms swirl in the atmospheres of the ice giants. Neptune’s frigid winds rage at up to 1,200 mph (2,000 km/h). While exploring Neptune in 1989, Voyager 2 observed a storm larger than the planet Earth. Astronomers called it the Great Dark Spot. By 1994, the Hubble Space Telescope revealed that the spot had disappeared, but another large storm soon developed.
Just beneath the Great Dark Spot in this Voyager image is a bright cloud feature called the Scooter, because its sped around Neptune faster than the dark spot. To the south of the Scooter is a similar but smaller storm named Dark Spot 2.
Neptune has 14 moons. The Voyager spacecraft revealed the moons of the giant planets to be surprisingly diverse worlds in their own right. Further exploration has unveiled intricate surfaces both young and old, volcanoes and impact features, huge plumes and geysers, deep canyons, subsurface oceans, and even clues to possible environments that might be friendly to simple forms of life.
Triton is a large and very unusual moon. It orbits Neptune in the opposite direction from most other moons. It has active geysers that release nitrogen. Its icy surface appears fairly young, which means that an episode of warming may have erased earlier features. All these oddities could mean that Triton formed elsewhere—in the Kuiper Belt—and was captured by Neptune’s gravity.
In this image, arrows indicate geysers caused by jets of nitrogen.