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Neptune is the eighth planet from the Sun, is
a colder twin to Uranus, a blue ball composed largely of hydrogen, methane, ammonia, and water.Neptune is the eighth and farthest
planet from the Sun in our Solar System. Named for the Roman god of the sea, it is the fourth-largest planet by diameter and
the third-largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which
is 15 Earth masses and not as dense.
Discovered on September 23, 1846,[1] Neptune was the first planet found by mathematical
prediction rather than by empirical observation. Unexpected changes in the orbit of Uranus led Alexis Bouvard to deduce that
its orbit was subject to gravitational perturbation by an unknown planet. Neptune was subsequently observed by Johann Galle
within a degree of the position predicted by Urbain Le Verrier, and its largest moon, Triton, was discovered shortly thereafter,
though none of the planet's remaining 12 moons were located telescopically until the 20th century. Neptune has been visited
by only one spacecraft, Voyager 2, which flew by the planet on August 25, 1989.
Neptune is similar in composition
to Uranus, and both have compositions which differ from those of the larger gas giants Jupiter and Saturn. Neptune's atmosphere,
while similar to Jupiter's and Saturn's in that it is composed primarily of hydrogen and helium, along with traces of hydrocarbons
and possibly nitrogen, contains a higher proportion of "ices" such as water, ammonia and methane. 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.[16] 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 (55 K). Temperatures at the planet's centre, however, are approximately 5,400 K
(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.
Galileo's drawings show that he first observed
Neptune on December 28, 1612, and again on January 27, 1613. On both occasions, Galileo mistook Neptune for a fixed star when
it appeared very close—in conjunction—to Jupiter in the night sky; hence, he is not credited with Neptune's discovery.
During the period of his first observation in December 1612, Neptune was stationary in the sky because it had just turned
retrograde that very day. This apparent backward motion is created when the orbit of the Earth takes it past an outer planet.
Since Neptune was only beginning its yearly retrograde cycle, the motion of the planet was far too slight to be detected with
Galileo's small telescope. However, in July 2009 University of Melbourne physicist David Jamieson announced new evidence suggesting
that Galileo was at least aware that the star he had observed had moved relative to the fixed stars.
In 1821, Alexis
Bouvard published astronomical tables of the orbit of Neptune's neighbor Uranus. Subsequent observations revealed substantial
deviations from the tables, leading Bouvard to hypothesize that an unknown body was perturbing the orbit through gravitational
interaction. In 1843, John Couch Adams calculated the orbit of a hypothesized eighth planet that would account for Uranus's
motion. He sent his calculations to Sir George Airy, the Astronomer Royal, who asked Adams for a clarification. Adams began
to draft a reply but never sent it and did not aggressively pursue work on the Uranus problem.
Urbain Le VerrierIn
1845–46, Urbain Le Verrier, independently of Adams, developed his own calculations but also experienced difficulties
in stimulating any enthusiasm in his compatriots. In June, however, upon seeing Le Verrier's first published estimate of the
planet's longitude and its similarity to Adams's estimate, Airy persuaded Cambridge Observatory director James Challis to
search for the planet. Challis vainly scoured the sky throughout August and September.
In the meantime, Le Verrier
by letter urged Berlin Observatory astronomer Johann Gottfried Galle to search with the observatory's refractor. Heinrich
d'Arrest, a student at the observatory, suggested to Galle that they could compare a recently drawn chart of the sky in the
region of Le Verrier's predicted location with the current sky to seek the displacement characteristic of a planet, as opposed
to a fixed star. The very evening of the day of receipt of Le Verrier's letter on September 23, 1846, Neptune was discovered
within 1° of where Le Verrier had predicted it to be, and about 12° from Adams' prediction. Challis later realized
that he had observed the planet twice in August, failing to identify it owing to his casual approach to the work.
Shortly after its discovery, Neptune was referred
to simply as "the planet exterior to Uranus" or as "Le Verrier's planet". The first suggestion for a name
came from Galle, who proposed the name Janus. In England, Challis put forward the name Oceanus. Claiming the right to name
his discovery, Le Verrier quickly proposed the name Neptune for this new planet, while falsely stating that this had been
officially approved by the French Bureau des Longitudes. In October, he sought to name the planet Le Verrier, after himself,
and he had loyal support in this from the observatory director, François Arago. However, this suggestion met with stiff
resistance outside France. French almanacs quickly reintroduced the name Herschel for Uranus, after that planet's discoverer
Sir William Herschel, and Leverrier for the new planet.
Struve came out in favor of the name Neptune on December
29, 1846, to the Saint Petersburg Academy of Sciences. Soon Neptune became the internationally accepted name. In Roman mythology,
Neptune was the god of the sea, identified with the Greek Poseidon. The demand for a mythological name seemed to be in keeping
with the nomenclature of the other planets, all of which, except for Earth, were named for Greek and Roman mythology.Neptune's
internal structure resembles that of Uranus. Its atmosphere forms about 5 to 10 percent of its mass and extends perhaps 10
to 20 percent of the way towards the core, where it reaches pressures of about 10 GPa. Increasing concentrations of methane,
ammonia and water are found in the lower regions of the atmosphere.
The Great Dark Spot, as seen from Voyager 2In
1989, the Great Dark Spot, an anti-cyclonic storm system spanning 13000×6600 km, was discovered by NASA's Voyager 2
spacecraft. The storm resembled the Great Red Spot of Jupiter. Some five years later, however, on November 2, 1994, the Hubble
Space Telescope did not see the Great Dark Spot on the planet. Instead, a new storm similar to the Great Dark Spot was found
in the planet's northern hemisphere.
The Scooter is another storm, a white cloud group farther south than the Great
Dark Spot. Its nickname is due to the fact that when first detected in the months before the 1989 Voyager 2 encounter it moved
faster than the Great Dark Spot. Subsequent images revealed even faster clouds. The Small Dark Spot is a southern cyclonic
storm, the second-most-intense storm observed during the 1989 encounter. It initially was completely dark, but as Voyager
2 approached the planet, a bright core developed and can be seen in most of the highest-resolution images.
Neptune's dark spots are thought to occur in the troposphere
at lower altitudes than the brighter cloud features, so they appear as holes in the upper cloud decks. As they are stable
features that can persist for several months, they are thought to be vortex structures. Often associated with dark spots are
brighter, persistent methane clouds that form around the tropopause layer. The persistence of companion clouds shows that
some former dark spots may continue to exist as cyclones even though they are no longer visible as a dark feature. Dark spots
may dissipate when they migrate too close to the equator or possibly through some other unknown mechanism.
Neptune
has 13 known moons. The largest by far, comprising more than 99.5 percent of the mass in orbit around Neptune and the only
one massive enough to be spheroidal, is Triton, discovered by William Lassell just 17 days after the discovery of Neptune
itself. Unlike all other large planetary moons in the Solar System, Triton has a retrograde orbit, indicating that it was
captured rather than forming in place; it probably was once a dwarf planet in the Kuiper belt. It is close enough to Neptune
to be locked into a synchronous rotation, and it is slowly spiraling inward because of tidal acceleration and eventually will
be torn apart, in about 3.6 billion years, when it reaches the Roche limit. In 1989
Neptune is never visible to
the naked eye, having a brightness between magnitudes +7.7 and +8.0, which can be outshone by Jupiter's Galilean moons, the
dwarf planet Ceres and the asteroids 4 Vesta, 2 Pallas, 7 Iris, 3 Juno and 6 Hebe. A telescope or strong binoculars will resolve
Neptune as a small blue disk, similar in appearance to Uranus.
Because of the distance of Neptune from the Earth,
the angular diameter of the planet only ranges from 2.2–2.4 arcseconds; the smallest of the Solar System planets. Its
small apparent size has made it challenging to study visually. Most telescopic data was fairly limited until the advent of
Hubble Space Telescope and large ground-based telescopes with adaptive optics. From the Earth, Neptune goes through apparent
retrograde motion every 367 days, resulting in a looping motion against the background stars during each opposition. These
loops will carry it close to the 1846 discovery coordinates in April and July 2010 and in October and November 2011.
Observation
of Neptune in the radio frequency band shows that the planet is a source of both continuous emission and irregular bursts.
Both sources are believed to originate from the planet's rotating magnetic field. In the infrared part of the spectrum, Neptune's
storms appear bright against the cooler background, allowing the size and shape of these features to be readily tracked.
The internal structure of Neptune:
1. Upper atmosphere,
top clouds
2. Atmosphere consisting of hydrogen, helium and methane gas
3. Mantle consisting of water, ammonia and
methane ices
4. Core consisting of rock and iceGradually this darker and hotter region condenses into a superheated liquid
mantle, where temperatures reach 2,000 K to 5,000 K. The mantle is equivalent to 10 to 15 Earth masses and is rich in water,
ammonia and methane. As is customary in planetary science, this mixture is referred to as icy even though it is a hot, highly
dense fluid. This fluid, which has a high electrical conductivity, is sometimes called a water-ammonia ocean. At a depth of
7000 km, the conditions may be such that methane decomposes into diamond crystals that then precipitate toward the core.
The core of Neptune is composed of iron, nickel and silicates, with an interior model giving a mass about 1.2 times
that of the Earth. The pressure at the centre is 7 Mbar (700 GPa), millions of times more than that on the surface of the
Earth, and the temperature may be 5,400 K.Bands of high-altitude clouds cast shadows on Neptune's lower cloud deckModels suggest
that Neptune's troposphere is banded by clouds of varying compositions depending on altitude. The upper-level clouds occur
at pressures below one bar, where the temperature is suitable for methane to condense. For pressures between one and five
bars (100 and 500 kPa), clouds of ammonia and hydrogen sulfide are believed to form. Above a pressure of five bars, the clouds
may consist of ammonia, ammonium sulfide, hydrogen sulfide and water. Deeper clouds of water ice should be found at pressures
of about 50 bars (5.0 MPa), where the temperature reaches 0 °C. Underneath, clouds of ammonia and hydrogen sulfide may
be found.
The first of the planetary rings was discovered in 1968 by a team led by Edward Guinan, but it was later
thought that this ring might be incomplete. Evidence that the rings might have gaps first arose during a stellar occultation
in 1984 when the rings obscured a star on immersion but not on emersion. Images by Voyager 2 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 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 W. M. Keck Observatory in 2002 and 2003 show 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.
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