Neptune: a World in the Deep Freeze

Ever hear of Neptune, the gas giant planet?  It's one of those really "far out" places because it truly is extremely distant from Earth and the Sun. It orbits in what planetary scientists think of as the "deep freeze" of the solar system, containing icy and gas-giant worlds.

Take a Tour of Neptune

Planetary scientists began to study this distant frontier in the late 1970s and only a very few spacecraft have traversed it. The Voyager 2 mission ventured out past Neptune in 1989, giving us a very short look at a very interesting world. The planet shares similarities with its nearest neighbor, Uranus, but is also similar in many ways to the king of the planets, Jupiter. It also has rings, just as Saturn does, although Neptune's set is more tenuous and thin and made of darker materials.

Like the other worlds of the outer solar system, Neptune has moons. The largest is Triton, an interesting place in its own right. Among other things, Triton spews geysers of nitrogen and hydrocarbons out from beneath the surface.

Astronomers now study Neptune using Hubble Space Telescope, plus ground-based instruments on Maunakea in Hawai'i, as well as in Chile and Australia. They search out changes in the atmosphere and watch for the sudden appearance and disappearance of storms and cloud features. Neptune has also been dubbed an "ice giant" due to the many frozen aerosols (icy particles) in its atmosphere. 

Let's learn some more facts about this ice giant world that some are calling a gateway to the most distant reaches of the solar system.

Edited by Carolyn Collins Petersen.

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Neptune is (usually) the Most Distant Planet from the Sun

Neptune as seen by the Voyager spacecraft. NASA

Neptune's orbit takes it outside the orbit of dwarf planet Pluto. During that time, Neptune is the most distant planet. It is not visible to the naked eye from Earth, but anyone with a reasonably good backyard-type telescope should be able to find it.

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It Takes Neptune about 165 Earth Years to Orbit the Sun

Neptune in order of distance from the Sun. NASA Planetary Photojournal

Because of its great distance, Neptune moves very slowly around the Sun. It takes about 165 years on Earth for the blue giant to make one orbit.

If you try to observe Neptune, you'll notice that it appears to move through the same part of the sky for years at a time!

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Neptune is the Only Planet in the Solar System to be Discovered Mathematically

Another View of Neptune from Voyager 2. NASA

Neptune actually HAD been seen through telescopes for years before it was officially identified as a planet. Those who observed it accidentally didn't realize it was a planet. Unless you observe the world consistently for many nights, its motion is hard to detect against the backdrop of stars.

In the 1800s, scientists noticed that something was perturbing the orbit of the planet Uranus, leading them to mathematically predict the location of another world. That predicted position was confirmed by astronomers and Neptune was found.

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Neptune Has Giant Storms

Neptune - 8th Planet From the Sun
Neptune showing off what looks like a giant storm, called the Great Dark spot. NASA

Both Uranus and Neptune have giant storms that whirl through their upper cloud decks. So do Saturn and Jupiter. Astronomers often call these two worlds "ice giants" because of the presence of ices in their atmospheres and interiors. When Voyager 2 passed by Neptune, it captured views of two such storms, dubbed "Dark Spot" and "Dark Spot 2". A smaller atmospheric disturbance was nicknamed "Scooter" due to its fast motion. 

Neptune's atmosphere is made up largely of hydrogen and helium gases. There are also small amounts of methane, water, ammonia and other gases and compounds. The ices in its atmosphere are more abundant than at Uranus (the other ice giant). 

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Neptune has Rings

Neptune's rings, as seen by Voyager 2. NASA/LPI

When we think of planetary rings, we imagine those of Saturn. But in fact all of the outer gas and ice giant planets have rings. The rings around Jupiter, Uranus, and Neptune are much more difficult to see and often require specialized instruments to fully detect.

Neptune has five main rings which look more like "arc" shaped objects from a distance. The Voyager 2 mission took detailed images of the rings, and since then, astronomers have determined that they contain a lot of dust, plus some mixtures of icy particles. 

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Triton, Neptune's Largest Moon, is a bit Backward

Neptune on Triton's Horizon
Neptune on Triton's Horizon. NASA

Neptune has at least 13 moons, and most of them are quite small. The largest is Triton, which is quite unique. It is the only moon in the solar system that orbits its host planet opposite the orbital direction of the other moons. This has led scientists to suspect that it was actually captured from the Kuiper Belt, instead of forming around the planet. Such a capture may have been a common event back in the early days of the solar system.