Neptune is currently the most distant planet from the Sun, with an orbital radius of 30 Astronomical Units (30 trillion kilometres) and an orbital period(time it takes to make one revolution around the sun) of 165 years. Its diameter is about four times that of the Earth, which makes it the 4th largest planet. It is slightly smaller than Uranus.

The relatively low density indicates large concentrations of hydrogen and helium, but Uranus and Neptune have much larger concentrations of heavier elements than Jupiter and Saturn. As for all the gas giant planets, models suggest rocky cores of may be 15 Earth masses, but there is no direct confirmation of this.

The bluish colour of the adjacent image is, as for Uranus, because of methane in the atmosphere, which absorbs red light. The period of rotation is about 16 hours, comparable to that of Uranus and much slower than for Jupiter and Saturn.

The temperatures at the cloud tops are about -216 degrees Celsius, slightly warmer than Uranus. Neptune, like Jupiter and Saturn but unlike Uranus, has an internal heat source and produces 2.7 times more heat than it absorbs.

Astronomers sometimes categorize Uranus and Neptune as «ice giants” in order to emphasize these distinctions. The interior of Neptune, like that of Uranus, is primarily composed of ices and rock. Traces of methane in the outermost regions in part account for the planet’s blue appearance.

In contrast to the relatively featureless atmosphere of Uranus, Neptune’s atmosphere, is notable for its active and visible weather patterns. At the time of the 1989 Voyager 2 flyby, for example, the planet’s southern hemisphere possessed a Great Dark Spot comparable to the Great Red Spot on Jupiter.

These weather patterns are driven by the strongest sustained winds of any planet in the Solar System, with recorded wind speeds as high as 2,100 km/h. Because of its great distance from the Sun, Neptune’s outer atmosphere is one of the coldest places in the Solar System, with temperatures at its cloud tops approaching −218 °C.

Temperatures at the planet’s centre, however, are approximately (5,000 °C). Neptune has a faint and fragmented ring system, which may have been detected during the 1960s but was only indisputably confirmed in 1989 by Voyager 2 space craft.

Neptune has a planetary ring system, though one much less substantial than that of Saturn. The rings may consist of ice particles coated with silicates or carbon-based material, which most likely gives them a reddish colour.

The three main rings were named as the narrow Adams Ring, 63000 km from the centre of Neptune, the Le Verrier Ring, at 53000 km, and the broader, fainter Galle Ring, at 42000 km. A faint outward extension to the Le Verrier Ring has been named Lassell; it is bounded at its outer edge by the Arago Ring at 57000 km.

The first of these planetary rings was discovered in 1968 by a team of scientists led by Edward Guinan, but it was later thought that this ring might be incomplete.

Images by Voyager 2 spacecraft in 1989 settled the issue by showing several faint rings. These rings have a clumpy structure, the cause of which is not currently understood but which may be due to the gravitational interaction with small moons in orbit near them.

The outermost ring, Adams, contains five prominent arcs now named Courage, Liberté, Egalité 1, Egalité 2 and Fraternité (Courage, Liberty, Equality and Fraternity).

The existence of arcs was difficult to explain by scientists because the laws of motion would predict that arcs would spread out into a uniform ring over very short timescales. Astronomers now believe that the arcs are corralled into their current form by the gravitational effects of Galatea, (a moon just inward from the ring).

Earth-based observations announced in 2005 appeared to show that Neptune’s rings are much more unstable than previously thought. Images taken from the earth Observatory in 2002 and 2003 showed considerable decay in the rings when compared to images by Voyager 2. In particular, it seems that the Liberté arc might disappear in as little as one century.

Like the other gas giants, its rotation on its own axis is faster than the speed of sound here on planet Earth.

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