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When the astronomers had discovered and studied the orbit of Uranus, they began to see some anomalies in its orbit, which made them suspect the presence of yet another planet beyond the orbit of Uranus. They began to calculate where they would find this new wanderer of the solar system. Indeed they found it in 1846 (by Johann Galle) in the constellation of Aquarius. It was found almost where they thought it would be. Thus, Neptune is the first planet which had its presence calculated before the discovery.
Neptune orbits the sun at a distance of about 4.5 billion kilometres and takes about 165 years to complete an orbit. Due to Pluto's very eccentric orbit, it sometimes crosses Neptune's orbit, making Neptune the furthest planet away from the sun during a period of 20 years each 248 years.
Like Uranus and the other jovian planets, Neptune is a gas giant. It is sometimes considered as Uranus' twin planet as they have many similar properties, such as size, composition and rotation around the axis. Neptune is mostly known for its beautiful blue color, but also for the tremendeous speeds the winds has in the atmosphere. The speed of the wind can reach more than 500 m/s, which is the fastest in the solar system. They are thought to be powered by internal heat generated from matter falling into it in the past. Neptune is also known for its moon Triton.
Neptune has more meteorological activity than Uranus. On pictures taken by Voyager 2 one can see more features than are visible on Uranus. For instance, there is a large blue spot on Neptune, similar to the one on Jupiter.

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Exploring Neptune
Neptune's Ring System
The Interior

Neptune's Moons

Neptune's Rings

Like the other Jovian planets, Neptune too has rings. They are weak and few in quantity and were discovered the same way as was Uranus's rings: when the planet passed in front of a start it appeared to "blink". Unlike Uranus, some of Neptune's rings are incomplete, they are arcs. The existence of the arcs perplexes astronomers, because the laws of motion predict that arcs the arcs would spread out into a uniform ring over very short timescales.
The outermost ring, "Adams", contains three prominent arcs that are called Liberté, Egalité, and Fraternité (Liberty, Equality, and Fraternity).

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Neptune's Interior

Neptune harbors an Earth-sized rocky and icy core at the center. Outside of that is a large mantle of water, ammonia and methane ices. The atmosphere consists mainly of molecular hydrogen, but also of helium and methane.
Neptune has a magnetic field that is inclined 47° to its rotation axis and does not pass through its center, just like Uranus. Some scientists believe that the magnetic field is generated within the icy cores that surround the central, rocky core. Its possible that the pressure is so large in that area that the elements become ionized and become conducting. The same process may be found at the center of Uranus.
A few years back it was suggested that diamonds could form in the atmosphere of Neptune. They would rain down like hailstones. The diamonds are thought to be made from the large quantities of methane in the atmosphere.
Like the other jovian planets, Neptune has no surface. Its atmosphere is its surface.

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Neptune's Moons

Neptune has many moons, such as Nereid, Despina and Naiad, but Triton is the undoubtedly most known by people. Not only does it have many geyser-like ice volcanoes, but it is also known for the fact that its retrograde orbit (opposite direction of orbit than the other moons) will make it break up and form a ring around Neptune (in a few million years) as it continues its slow, but deadly spiraling inwards. Its retrograde motion and its different composition suggests that it is a captured moon. Maybe it originates from the Kuiper belt outside of Neptune's orbit, which is a smaller version of the asteroid belt. Triton, which is the largest moon of Neptune was discovered only 17 days after the discovery of the planet itself. Triton's icy surface reflects so much of the little sunlight that reaches it that the moon is one of the coldest objects in the solar system, about -240 degrees Celsius.
During a Voyager 2 flyby an ejection from an ice volcano was detected. The liquid (mixture of nitrogen, methane and dust) rose 8 kilometres above the surface and fell down 140 km away. These ejections are however made in a very cold environment, making them rapidly shift state to solid ice.

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Exploring Neptune

NASA has sent a few probes to study both Jupiter and Saturn, but so far, only Voyager 2 has visited Neptune. Little is known about Uranus and Neptune. However, a new mission is under investigation. It might launch between 2016-2018 and arrive somewhere around 2035. It will be propelled by nuclear-powered fission technology, which will be able to carry a great scientific payload. It will investigate global atmospheric circulation dynamics, meteorology and chemistry. On Triton, two landers will gather atmospheric and geochemical information near geysers on the surface. Also, three probes will be dropped into Neptune's atmosphere at different latitudes to study the planet.
Neptune is a planet far away from the sun. The sun has little impact on it, and it has not experienced as many comet/asteroid impacts as the other planets. It is more in its original state, hence astronomers want to study this planet in order to learn more about the creation of our solar system. More.

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Next: Pluto.


Moons: 13, Triton is the most famous.
Average distance from sun:
4 498 252 900 km.
Equatorial Radius: 24,764 km.
Mass: 1.0244 x 10^26 kg
Density: 1.76 g/cm^3
Escape Velocity: 23.710 km/s.
Length of day: 16.11 hours.
Length of year: 164.79 Earth years.
Mean Orbit Velocity: 5.478 km/s.
Equatorial Inclination to Orbit: 1.769 degrees.
Effective Temperature: -214 °C.
Atmosphere: Hydrogen 85%, Helium 13%, Methane 2%.
Misc: Discovered by Johann Gotfried Galle on September 23, 1846.

Source: NASA.

Space art: Neptune

Space Art 1: Neptune as seen with its rings.

Illustration: Interior of Neptune
Illustration 1: The interior of Neptune. Image credit: NASA.

Space art: Triton, Moon of Neptune

Above: A geyser erupting from the surface Neptune's largest moon, Triton. Triton is one of very few places in the solarsystem where we can find active geysers. Unlike geysers on Earth, these consist of nitrogen. In the background the moons Nereid and Proteus are visible.

This illustration is available upon request, as a Print (4000x5500 pixels, 300 dpi).


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