Neptune

Neptune is the eighth planet from the Sun. It orbits the Sun every 165 years at a mean distance of 30.1 astronomical units. It has a diameter of 48,000 kilometres and a mass 17 times that of the Earth. It is the furthest of the giant gaseous planets from the Sun and has a rotation period of about 19 hours.

Astronomers have studied Neptune since September 23, 1846, when Johann Gottfried Galle, of the Berlin Observatory, and Louis d'Arrest, an astronomy student, discovered the eighth planet on the basis of mathematical predictions by Urbain Jean Joseph Le Verrier. Similar predictions were made independently by John Couch Adams. (Galileo Galilei had seen Neptune during several nights of observing Jupiter, in January 1613, but didn't realise he was seeing a new planet.) Still, any knowledge and understanding of Neptune was limited by the astronomer's ability to see the distant object, almost 4.5 billion kilometres (2.8 billion miles) from Earth.

Scarcely a month after Galle and d'Arrest first saw Neptune, British astronomer William Lassell spotted a satellite orbiting the planet and named it Triton. Triton, almost the size of Earth's Moon, is the only large satellite in the solar system to circle a planet in a retrograde direction -- in a direction opposite to the rotation of the planet. That phenomenon led some astronomers to surmise that Neptune had captured Triton as it travelled through space several billion years ago.

Earth-based telescopic observations of Neptune over the last few years showed tantalising hints of dynamic cloud structures on the distant planet, from which scientists could estimate the speed of winds circling the planet. Against that background, Voyager's scientists prepared for the first encounter with Neptune, perhaps the only close-up look at Neptune in the lifetime of many of us. What they found forced scholars to rewrite the astronomy textbooks, and scientists to adjust their views of the solar system's other giant planets.

Neptune is a dynamic planet, even though it receives only 3 percent as much sunlight as Jupiter does. Several large, dark spots are reminiscent of Jupiter's hurricane-like storms. The largest spot is big enough for Earth to fit neatly inside. Designated the Great Dark Spot by its discoverers, the feature appears to be an anticyclone similar to Jupiter's Great Red Spot. Neptune's Great Dark Spot is comparable in size, relative to the planet, and at the same latitude (the Great Dark Spot is at 22░ south latitude) as Jupiter's Great Red Spot. However, Neptune's Great Dark Spot is far more variable in size and shape than its Jupiter counterpart. Bright, wispy "cirrus-type" clouds overlaying the Great Dark Spot at its southern and north-eastern boundaries may be analogous to lenticular clouds that form over mountains on Earth. At about 42░ south, a bright, irregularly shaped, eastward-moving cloud circles much faster than does the Great Dark Spot, "scooting" around Neptune in about 16 hours. This "scooter" may be a cloud plume rising between cloud decks.

From the Earth Neptune can be seen only as a small greenish-blue disc. Almost all of our detailed knowledge of Neptune comes from the close encounter by The Voyager 2 spacecraft in 1989. Voyager 2 had visited Jupiter, Saturn, and Uranus; Neptune was its last calling post. It had taken 12 years to get there and yet passed within 3000 miles of the planet's surface. The signals received on the Earth had a strength of less than 0.0000000000000001 watts and yet the pictures showed fantastic details.

Neptune is orbited by 8 known moons.