Neptune

Neptune

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Discovery
Discovered by	Urbain Le Verrier John Couch Adams Johann Galle
Discovered on	September 23, 1846
Orbital characteristics ( Epoch J2000)
Semi-major axis	4,498,252,900 km 30.068 963 48 AU
Orbital circumference	28.263 T m 188.925 AU
Eccentricity	0.008 585 87
Perihelion	4,459,631,496 km 29.810 795 27 AU
Aphelion	4,536,874,325 km 30.327 131 69 AU
Orbital period	60,223.3528 d (164.88 a)
Synodic period	367.49 d
Avg. Orbital Speed	5.432 km/s
Max. Orbital Speed	5.479 km/s
Min. Orbital Speed	5.385 km/s
Inclination	1.769 17° (6.43° to Sun's equator)
Longitude of the ascending node	131.721 69°
Argument of the perihelion	273.249 66°
Number of satellites	13
Physical characteristics
Equatorial diameter	49,528 km [1] (3.883 Earths)
Polar diameter	48,681 km (3.829 Earths)
Oblateness	0.0171
Surface area	7.619×109 km2
Volume	6.254×1013 km3 (57.74 Earths)
Mass	1.0243×1026 kg (17.147 Earths)
Mean density	1.638 g/cm3
Equatorial gravity (At 1 bar)	11.15 m/s2 (1.14 g)
Escape velocity	23.5 km/s
Rotation period	16.11 h (16 h 6 min 36 s) 1
Rotation velocity	2.68 km/s = 9660 km/h (at the equator)
Axial tilt	28.32°
Right ascension of North pole	299.33° (19 h 57 min 20 s)
Declination	42.95°
Albedo	0.41
Surface temp.	min mean max 50 K 53 K N/A K
Adjective	Neptunian
Atmospheric characteristics
Surface pressure	≫100 MPa
Hydrogen - H2	80% ±3.2%
Helium - He	19% ±3.2%
Methane - CH4	1.5% ±0.5%
Hydrogen Deuteride - HD	192 ppm
Ethane - C2H6	1.5 ppm

Note: This article contains special characters.

Neptune is the eighth or, occasionally, the ninth planet from the Sun due to Pluto's eccentric orbit , and the outermost gas giant in our solar system. Although the smallest of the gas giants, Neptune is more massive than Uranus: Its stronger gravitational field has compressed it to a higher density.

Faint dark rings have been detected around the blue planet, but are less substantial than those of Saturn. When these rings were discovered, it was thought that they might not be complete, but this was disproved by Voyager 2. Neptune also has 2,000 km/h winds of hydrogen, helium, and methane that gives the planet its blue appearance. At the time of the 1989 Voyager 2 flyby, it had in its southern hemisphere a Great Dark Spot comparable to the Great Red Spot on Jupiter. Neptune possesses nine confirmed moons and four awaiting confirmation. Neptune's largest moon, Triton, is notable for its retrograde orbit, extreme cold (38 K), and extremely tenuous (14 microbar) nitrogen/methane atmosphere.

Neptune is named after the Roman god of the sea because of its blue methane clouds. Its astronomical symbol is a stylized version of the god's trident (♆).

Discovered on September 23, 1846, Neptune has been visited by only one spacecraft, Voyager 2, which flew by the planet on August 25, 1989. In 2003, there was a proposal to NASA's "Vision Missions Studies" to implement a "Neptune Orbiter with Probes" mission that does Cassini-level science without fission-based electric power or propulsion. The work is being done in conjunction with JPL and the California Institute of Technology.

Discovery

Galileo's astronomical drawings show that he had first observed Neptune on December 27, 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. Believing it to be a fixed star, he cannot be credited with its discovery. At the time Galileo first observed Neptune on December 28, 1612, it was stationary in the sky because it had just turned retrograde that very day; because it was stationary in the sky and only beginning the planet's yearly retrograde cycle, its motion was far too slight to be detected with Galileo's small telescope. Had Neptune been moving at its regular/average speed when Galileo first observed it in 1612 and 1613, he would have most likely realized that it was a planet and not a fixed star due to Neptune's relatively rapid normal motion along the ecliptic compared to the extremely slow motion of the fixed stars.

In 1821, Alexis Bouvard published astronomical tables of the orbit of Uranus. Subsequent observations revealed substantial deviations from the tables, leading Bouvard to hypothesize some perturbing body. In 1843, John Couch Adams calculated the orbit of an eighth planet that would account for Uranus' 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.

In 1846, Urbain Le Verrier, independently of Adams, produced his own calculations but also experienced difficulties in encouraging any enthusiasm in his compatriots. However, in the same year, John Herschel started to champion the mathematical approach and persuaded James Challis to search for the planet.

After much procrastination, Challis began his reluctant search in July 1846. However, in the mean time, Le Verrier had convinced Johann Gottfried Galle to search for the planet. Though still a student at the Berlin Observatory, Heinrich d'Arrest suggested that a recently drawn chart of the sky, in the region of Le Verrier's predicted location, could be compared with the current sky to seek the displacement characteristic of a planet, as opposed to a fixed star. Neptune was discovered that very night, September 23, 1846, within 1° of where Le Verrier had predicted it to be, and about 10° 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.

In the aftermath of the discovery, there was much nationalistic rivalry between the French and the British over who had priority and deserved credit for the discovery. Eventually an international consensus emerged that both Le Verrier and Adams jointly deserved credit. However, the issue is now being re-evaluated by historians with the rediscovery in 1998 of the "Neptune papers" (historical documents from the Royal Greenwich Observatory), which had apparently been misappropriated by astronomer Olin Eggen for nearly three decades and were not rediscovered (in his possession) until immediately after his death. After reviewing the documents, some historians now suggest that Adams did not in fact deserve equal credit with Le Verrier.

Naming

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. He proposed the name Janus. In England, Challis put forth the name Oceanus, particularly appropriate for a seafaring people. In France, Arago suggested that the new planet be called Leverrier, a suggestion which was met with stiff resistance outside France. French almanacs promptly reintroduced the name Herschel for Uranus and Leverrier for the new planet.

Meanwhile, on separate and independent occasions, Adams suggested altering the name Georgian to Uranus, while Leverrier (through the Board of Longitude) suggested Neptune for the new planet. Struve came out in favor of that name on December 29, 1846, to the Saint Petersburg Academy of Sciences. Soon Neptune became the internationally accepted nomenclature. 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 Uranus, were named in antiquity.

In the Chinese, Korean, Japanese, and Vietnamese languages the planet's name is literally translated as the sea king star (海王星).

Physical characteristics

Orbiting so far from the sun, Neptune receives very little heat — in fact the uppermost regions of the atmosphere are −218 °C (55 K). Because Neptune is a gas giant, there is no solid surface; as one ventures deeper and deeper inside the layers of gas, however, the temperature rises steadily. It is thought that this may be leftover heat generated by infalling matter during the planet's birth, now slowly radiating away into space. Neptune's atmosphere has the highest wind speeds in the solar system, up to 2000 km/h, thought to be powered by this flow of internal heat.

The internal structure resembles that of Uranus. There is likely to be a core consisting of (molten) rock and metal, surrounded by a mixture of rock, water, ammonia, and methane. The atmosphere, extending perhaps 10 to 20 percent of the way towards the center, is mostly hydrogen and helium at high altitudes, but has increasing concentrations of methane, ammonia, and water as it approaches and finally blends into the liquid interior. The pressure at the center of Neptune is millions of times more than that on the surface of Earth. Comparing its rotational speed to its degree of oblateness indicates that it has its mass less concentrated towards the center than does Uranus.

Neptune also resembles Uranus in its magnetosphere, with a magnetic field strongly tilted relative to its rotational axis at 47° and offset at least 0.55 radii (about 13,500 kilometres) from the planet's physical center. Comparing the magnetic fields of the two planets, scientists think the extreme orientation may be characteristic of flows in the interior of the planet and not the result of Uranus' sideways orientation.

One difference between Neptune and Uranus is the level of meteorological activity. Uranus is visually quite bland, while Neptune's high winds come with notable weather phenomena. The Great Dark Spot, a cyclonic storm system the size of Eurasia, was captured by Voyager 2 in the 1989 flyby. The storm resembled the Great Red Spot of Jupiter, but was shown to have disappeared in June 1994. However, a newer image of the planet taken by the Hubble Space Telescope on November 2, 1994, revealed that a smaller storm similar to its predecessor had formed over Neptune’s Northern Hemisphere. Unique among the gas giants is the presence of high clouds casting shadows on the opaque cloud deck below.

Planetary rings

Neptune has a faint planetary ring system of unknown composition. The rings have a peculiar "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.

Evidence that the rings are incomplete first arose in the mid- 1980s, when stellar occultation experiments were found to occasionally show an extra "blink" just before or after the planet occulted the star. Images by Voyager 2 in 1989 settled the issue, when the ring system was found to contain several faint rings. The outermost ring, Adams, contains three prominent arcs now named Liberté, Egalité, and Fraternité (Liberty, Equality, and Fraternity). The existence of arcs is very difficult to understand because the laws of motion would predict that arcs spread out into a uniform ring over very short timescales. The gravitational effects of Galatea, a moon just inward from the ring, are now believed to confine the arcs.

Several other rings were detected by the Voyager cameras. In addition to the narrow Adams Ring 63,000 km from the centre of Neptune, the Leverrier Ring is at 53,000 km and the broader, fainter Galle Ring is at 42,000 km. A faint outward extension to the Leverrier Ring has been named Lassell; it is bounded at its outer edge by the Arago Ring at 57,000 km.

New Earth-based observations announced in 2005 appeared to show that Neptune's rings are much more unstable than previously thought. In particular, it seems that the Liberté ring might disappear in as little as one century. The new observations appear to throw our understanding of Neptune's rings into considerable confusion.

Name of ring	Radius (km)	Width (km)	Notes
1989 N3R ('Galle')	41,900	15	Named after Johann Galle
1989 N2R ('Leverrier')	53,200	15	Named after Urbain Le Verrier
1989 N4R ('Lassell')	55,400	6	Named after William Lassell
Arago Ring	57,600	-	Named after François Arago
Liberté Ring Arc	62,900	-	"Leading" arc
Égalité Ring Arc	62,900	-	"Equidistant" arc
Fraternité Ring Arc	62,900	-	"Trailing" arc
Courage Ring Arc	62,900	-
1989 N1R ('Adams')	62,930	<50	Named after John Couch Adams

Natural satellites and Trojan asteroids

Neptune has 13 known moons. The largest by far, 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, it has a retrograde orbit. It is close enough to Neptune to be locked into a synchronous orbit, and is slowly spiraling inward. Triton is the coldest object that has been measured in the solar system.

Triton, compared to Earth's Moon
Name ( Pronunciation key)		Diameter (km)	Mass (kg)	Orbital radius (km)	Orbital period (days)
Triton	trye'-tən ˈtraɪtən	2700 (80% Luna)	2.15×1022 (30% Luna)	354,800 (90% Luna)	-5.877 (20% Luna)

Neptune's second known satellite, the irregular moon Nereid, has one of the most eccentric orbits of any satellite in the solar system.

From July to September 1989, Voyager 2 discovered six new Neptunian moons. Of these, the irregularly shaped Proteus is notable for being as large as a body of its density can be without being pulled into a spherical shape by its own gravity. Although the second most massive Neptunian moon, it is only one quarter of one percent of the mass of Triton. Neptune's innermost four moons, Naiad, Thalassa, Despina, and Galatea, orbit close enough to be within Neptune's rings. The next farthest out, Larissa was originally discovered in 1981 when it had blocked a star. This was attributed to ring arcs, but when Voyager 2 observed Neptune in 1989, it was found to have been caused by the moon. Five new irregular moons discovered between 2002 and 2003 were announced in 2004 (for a timeline of discovery dates, see Timeline of natural satellites).

As of 2005, there are two known Trojan asteroids of Neptune which have the same orbital period as the planet. They lie in the elongated, curved regions around the L₄ and L₅ Lagrangian points 60° ahead of and behind Neptune. These are 2001 QR₃₂₂ and 2004 UP₁₀.In 2005, three more suspected Neptune Trojans were spotted: 2005 TN₅₃, 2005 TN₇₄, and 2005 TO₇₄. Better orbits are required before they can be truly labeled as Neptune Trojans.

Appearance and visibility from Earth

Neptune is never visible with the naked eye. The brightness of Neptune is between magnitudes +7.7 and +8.0, so a telescope or binoculars are required to observe it. With the use of a telescope it appears as a small blue-green disk, similar in appearance to Uranus; the blue-green colour comes from the methane in its atmosphere.

With an orbital period of 165 years, Neptune will soon return to the approximate position where Galle discovered it, on three different dates. These are April 11, 2009, when it will be in prograde motion; July 17, 2009, when it will be in retrograde motion; and finally for the last time for the next 165 years, on February 7, 2010. Like all planets in the solar system beyond Earth, Neptune undergoes retrogradation at certain points during its synodic period. In addition to the start of retrogradation, other events within the synodic period include astronomical opposition, the return to prograde motion, and conjunction to the Sun.

See also: Aspects of Neptune

Neptune in fiction and film

Neptune has been used as a reference and setting in fiction and films. The first fictional visit of Neptune, portrayed as glacial but nevertheless inhabited, occurred in Spirito gentil (1889). In Olaf Stapledon's 1930 epic novel Last and First Men, Neptune is the final home of the highly evolved human race. Samuel R. Delany's 1976 novel Triton has humanity colonizing several parts of the solar system, including Neptune's largest moon. Neptune was the intended destination of the mining ship Red Dwarf in the books based on the BBC sitcom of that name, but an accident on board sends it into deep space instead. Although used purely as a backdrop, the planet was the setting of the 1997 sci-fi/horror film Event Horizon.

The planet was also the home of various alien species and characters. In H. P. Lovecraft's Cthulhu Mythos (1928), Neptune is known as "Yaksh" and is inhabited by curious fungoid creatures ( Clark Ashton Smith's The Family Tree of the Gods, 1944). In the anime Bishoujo Senshi Sailor Moon (1992), Sailor Neptune is the soldier representing the planet. A Pisces, she has wavy deep green hair and her image colours represent those of Neptune. On her forehead is the symbol of Neptune, which resembles a trident, Poseidon's weapon. Her attacks represent Poseidon's ocean powers (e.g. Deep Submerge). Her talisman is the Deep Aqua Mirror, also bearing the glyph and colors of Neptune. In the animated TV series Futurama (1999-2003), Neptunians are a purple-skinned, four-armed race of humanoids that coexist peacefully with humans across the solar system. They appear to hold two of their four arms with another Neptunian for unknown reasons. Elzar the cook, is also a Neptunian. Robot Santa's base is also located on Neptune, in his 'Death Fortress' at the north pole of Neptune. In the Japanese Anime Urusei Yatsura , Neptune is an icy, cold place which is the home of Oyuki, one of Lum's childhood alien friends.

Special characters

Neptune ( satellites)
Naiad | Thalassa | Despina | Galatea | Larissa | Proteus | Triton | Nereid
S/2002 N 1 | S/2002 N 2 | S/2002 N 3 | Psamathe | S/2002 N 4
See also: Pronunciation key | Rings of Neptune

The Solar System
Planets: Mercury - Venus - Earth (Moon) - Mars - Jupiter - Saturn - Uranus - Neptune - Pluto
Other: Sun - Asteroid belt - Kuiper belt - Scattered disc - Oort cloud
See also astronomical objects and the solar system's list of objects, sorted by radius or mass.