Neptune is the eighth and is the farthest of the gas giants in our Solar System. Planet Neptune was named after the Roman god of the sea. It is the fourth-largest planet by diameter and the third-largest by mass. Neptune is very big and has an equatorial diameter of 49,500 kilometers (30,760 miles). It is that big that if Neptune was hollow, it could contain nearly 60 Earths. Neptune’s mass is seventeen times the mass of Earth, however just 1/19th that of Jupiter. Only Jupiter exceeds Neptune’s surface gravity, making these two gas giants the only planets in the Solar System that have higher surface gravity than Earth. Neptune’s equatorial radius is 24,764 km, which is almost four times that of the Earth.
Neptune was discovered on September 23, 1846. It was the first planet found by mathematical prediction and not by empirical observation. Surprising changes in the orbit of Uranus showed the way to Alexis Bouvard to assume that its orbit was subject to gravitational perturbation by an unidentified planet. Neptune was consequently observed by Johann Galle within a degree of the position calculated by Urbain Le Verrier. Neptune’s largest moon, Triton, was discovered soon thereafter, although none of the planet’s remaining 12 moons were sited telescopically until the 20th century.
Visibility of Neptune
Neptune is never observable to the naked eye. It can be seen through a telescope or strong binoculars, which will show Neptune as a small blue disk. Neptune’s appearance is similar to Uranus. Since Neptune is so far away from Earth, the angular diameter of the planet as visible from Earth is only 2.2–2.4 arc seconds. Because of its small size, it is a very challenging job to study it visually. Most telescopic data was fairly restricted until the arrival of Hubble Space Telescope and large ground-based telescopes with adaptive optics. In the radio frequency band, observation of Neptune shows that the planet is a source of both nonstop emission and irregular bursts. Both sources are likely to originate from the planet’s rotating magnetic field. In the infrared part of the spectrum, Neptune’s storms appear bright against the cooler surroundings, permitting the size and shape of these storms to be readily tracked.
Neptune’s Composition and Atmosphere
Neptune and Uranus are similar in make up and both have compositions different from those of the larger gas giants Jupiter and Saturn. Neptune’s atmosphere is however similar to Jupiter’s and Saturn’s. It is composed primarily of hydrogen and helium, along with traces of hydrocarbons and potentially nitrogen. It contains a higher proportion of “ices” such as water, ammonia and methane.
The interior of Neptune, similar to Uranus, is mainly composed of ices and rock. The first two thirds of Neptune consist of a mixture of molten rock, water, liquid ammonia and methane. The outer third is a combination of heated gases comprised of hydrogen, helium, water and methane. Traces of methane in the outermost regions result in planet’s blue appearance.
Neptune’s Great Dark Spot
Like a typical gas planet, Neptune has rapid winds restricted to bands of latitude and large storms or vortices. Neptune’s winds are the strongest and fastest in the solar system, reaching up to 2000 km/hour. Neptune is a dynamic planet with a number of large, dark spots. The largest spot, called the Great Dark Spot, is almost the size of the earth and is similar to the Great Red Spot on Jupiter. Voyager discovered a small, irregularly shaped, eastward-moving cloud scooting around Neptune every 16 hours or so. This scooter as it has been called could be a plume rising over a deeper cloud deck.
Neptune’s Rings
Neptune has a set of four rings, though one much less extensive than those of Saturn. Neptune rings are narrow and very faint, and are made up of dust particles considered to have been made by tiny meteorites colliding into Neptune’s moons. From ground based telescopes the rings looked like arcs but from Voyager 2 the arcs were found to be the bright spots or clumps in the ring system. The real cause of the bright clumps is unknown. The outermost ring is called Adams (which includes three prominent arcs now named Liberty, Equality and Fraternity), after that is an unnamed ring co-orbital with Galatea, followed by Leverrier (whose outer extensions are called Lassell and Arago), and lastly the faint but broad Galle.
Magnetic Field of Neptune
Just like Uranus, the magnetic field of Neptune is highly tilted at 47 degrees from the rotation axis and offset at least 0.55 radii, or about 13,500 km from the planet’s physical centre. Evaluating the magnetic fields of the two planets, scientists assume the extreme orientation may be feature of flows in the interior of the planet and not the result of that planet’s sideways orientation or of any probable field reversals at either planet.
[images: www.nasa.org]
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