Neptune, gas giant with great dark spot

Neptune is the fourth largest of the planets in the solar system and eighth major planet in order of increasing distance from the Sun. It is similar in size and structure to its neighbour Uranus. Neptune is, on average, about 4.5 billion km (2.8 billion miles) from the sun. It is about 49,400 km (30,700 miles) in diameter, or about 3.8 times as wide as the earth. Even though Neptune's volume is 72 times that of Earth's, its mass is only 17 times Earth's mass.

Like Jupiter and Saturn, Neptune is one of the 'gas giant' planets, composed of a deep atmosphere around a liquid surface and sometimes a solid core. Its atmosphere consists of mostly hydrogen and helium, but up to three percent of its atmosphere is made up of methane, which gives the planet its striking blue colour. There are several cloud features, the most prominent being the great dark spot, a huge storm system which is as big as the Earth. The cloud features are blown around the planet by the fastest winds in the Solar System, at speeds of up to 2,200 km/h (1,370 mph). Beneath the clouds is a mantle of ices and gases and a small rocky core.

Neptune's core contains more rock and metal than the cores of other gas giant planets. The planet has a magnetic field, which is tilted more than 50° to the rotation axis.

Neptune rotates completely on its axis once every 16 hours and orbits the sun once in 164.79 Earth years. Its albedo, or reflectivity, is high (84 percent of the light falling on it is reflected), but it is so far away from the earth its stellar magnitude (a scale used to describe the brightness of an astronomical object; lower numbers correspond to brighter objects) is only 7.8, which means it is never bright enough to be seen with the naked eye from earth. When observed through a telescope, it appears as a greenish-blue disk without any definite surface markings.

Great Dark Spot
The great dark spot and the small dark spot are oval-shaped anticyclones in Neptune's atmosphere that are thought to be caused by the difference in temperature between the heat-producing core and the frigid cloud tops. The fastest winds in the solar system sweep them 'backward' around the planet (in a direction opposite to Neptune's direction of spin). A small cirrus cloud called Scooter lies at a different altitude to the spots where it is less windy; this cloud remains in the same position relative to Neptune's core and is carried by the planet's rotation in the opposite direction to the spots.

Neptune's Moons
Thirteen known satellites - only two of which are observable from Earth - orbit Neptune. Triton, discovered 1846 (the same year Neptune was first observed), is the largest and the brightest with a diameter of 2705 km (slightly smaller than Earth's moon). Triton has a retrograde orbit in relation to its parent planet and also, despite its extremely cold temperature, has a nitrogen atmosphere with some methane and haze, displaying an active surface of geysers spouting an unknown subsurface material. Triton is slowly spiraling in towards Neptune. In 10 million to 100 million years Triton will be close enough to be pulled apart by Neptune's gravitational forces, its remnants adding to Neptune's five rings. Nereid, the second observable satellite (discovered in 1949), has a diameter of 320 km (about 200 miles). Voyager 2 discovered six more moons in 1989, a third irregular moon called Psamathe and a regular moon have just been confirmed, and three Trojans are said to orbit Neptune as well.

Neptune's Rings
Its rings stretch from about 40,000 to 63,000 km (25,000 to 39,000 miles) from the planet. There are one broad and three narrow rings, all very dark. The Adams and Le Verrier Rings are named after astronomers who predicted Neptune's existence and position; the Galle Ring is named for Neptune's discoverer, the German astronomer Johann Galle (1812 - 1910). The Voyager 2 probe discovered clumps of ring material in the Adams Ring, whose existence astronomers cannot explain.

Author: Marc Delehanty