Neptune

Template:Planet Infobox/Neptune

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Neptune is the outermost gas giant in our solar system. For most of its 165 year orbit it is the eighth planet from the Sun and occasionally the ninth due to Pluto's eccentricity. It is the fourth largest planet by diameter and the third largest by mass; Neptune is more massive than its near twin Uranus as its stronger gravitational field has compressed it to a higher density. The planet is named after the Roman god of the sea. Its astronomical symbol is a stylized version of the god's trident (♆).

Neptune's atmosphere is primarily composed of hydrogen and helium, with traces of methane that account for the planet's blue appearance. Neptune's blue colour is much more vivid than that of Uranus, which has a similar amount of methane, and an unknown component is presumed to cause the intense colour. ^[1] Neptune also has the strongest winds of any planet in the solar system, with estimates as high as 2,500 km/h. 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.

Faint dark colored rings have been detected around the blue planet, but are much 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 possesses nine confirmed moons and four awaiting confirmation. Neptune's largest moon, Triton, is notable for its retrograde orbit, extreme cold (38K), and extremely tenuous (14 microbar) nitrogen/methane atmosphere.

Discovered on September 23, 1846, Neptune is notable for being the only planet discovered based on mathematical prediction rather than regular observations. Perturbations in the orbit of Uranus led astronomers to deduce Neptune's existence. It 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.^[2]

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 had mistaken 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;^{[citation needed]} 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.^[3]

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.^[4] 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).^[5]

Physical characteristics

File:GDS Neptune.jpg

The Great Dark Spot, as seen from Voyager 2.

Relative size

At 1.0243×10²⁶ kg Neptune is an intermediate body between Earth and the largest gas giants: it is seventeen Earth masses but just 1/18th the mass of Jupiter. It and Uranus are often considered a sub-class of gas giant termed "ice giants", given their smaller size and important differences in composition relative to Jupiter and Saturn. In the search for extra-solar planets Neptune has been used as a metonym: discovered bodies of similar mass are often referred to as "Neptunes"^[6] just as astronomers refer to various extra-solar "Jupiters."

Composition

Orbiting so far from the sun, Neptune receives very little heat with the uppermost regions of the atmosphere at −218 °C (55 K). 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.

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. There is no solid surface and the atmosphere, extending perhaps 10 to 20 percent of the way towards the center, is mostly hydrogen and helium at high altitudes (80% and 19%, respectively). Increasing concentrations of methane, ammonia, and water are found as the atmosphere 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.

Magnetic field

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.

Weather

File:PIA02219 modest.jpg

Great Dark Spot (top), Scooter (middle white cloud), and the Wizard's eye (bottom).

Neptune's atmosphere has the highest wind speeds in the solar system, thought to be powered by the flow of internal heat, and its weather is characterized by extremely violent hurricanes, whose winds reach up to 2000 km/h. In 1989, the Great Dark Spot (GDS) was discovered by NASA's Voyager 2 spacecraft. However, in 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 reason for the Great Dark Spot's disappearance is unknown. Many scientists believe heat transfer from the planet's core disrupted the atmospheric equilibrium and disrupted existing circulation patterns. The Scooter is another storm desribed as a white cloud south of the Great Dark Spot. The Wizard's eye (Great Dark Spot 2) is a southern hurricane, the second most intensive hurricane on the planet.

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.

Though Neptune's atmosphere is much more dynamic than that of Uranus, both planets are made of the same gases and ices. Uranus and Neptune are not strictly gas giants similar to Jupiter and Saturn, but are rather ice giants.

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.^[7]

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.^[8]

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

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, Triton has a retrograde orbit, indicating that it was captured, and probably represents the largest known example of a Kuiper Belt object (although clearly no longer in the Kuiper Belt). 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.^[9] ^[10] As Neptune was the Roman god of the sea, the planet's moons have been named after lesser sea gods.

For a timeline of discovery dates, see Timeline of natural satellites

Trojan asteroids

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.

Classification of orbits as resonant (1:1 in this case) requires high precision orbital data which can only be obtained from a number of observations spanning a significant arc (i.e. period of time). As of April 2006, out of the last three, only 2005 TN53 and 2005 TO74 are listed by the Minor Planet Center.^[11] 2005 TN74 proved to be a scattered disk object.

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. Interestingly, Neptune emits more light than it receives from the sun^[12]

With an orbital period of 165 years, Neptune will soon return to the approximate position in the sky where Galle discovered it. This will happen three different times. 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. This is explained by the concept of retrogradation. 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.

In its orbit around the Sun, Neptune will return to its original point of discovery in August 2011.

Notes

^ "Neptune overview," Solar System Exploration, NASA.
^ T. R. Spilker and A. P. Ingersoll (November 9, 2004). Outstanding Science in the Neptune System From an Aerocaptured Vision Mission. 36th DPS Meeting, Session 14 Future Missions.
^ William Sheehan, Nicholas Kollerstrom, Craig B. Waff (December 2004). The Case of the Pilfered Planet - Did the British steal Neptune? Scientific American.
^ Second report of proceedings in the Cambridge Observatory relating to the new Planet (Neptune) (1847). Astronomische Nachrichten, volume 25, p.309. Found at articles.adsabs.harvard.edu.
^ Using Eyepiece & Photographic Nebular Filters, Part 2 (October 1997). Hamilton Amateur Astronomers at amateurastronomy.org.
^ "Trio of Neptunes", Astrobiology Magazine, May 21, 2006.
^ Gazetteer of Planetary Nomenclature Ring and Ring Gap Nomenclature (December 8, 2004). USGS - Astrogeology Research Program.
^ Neptune's rings are fading away (March 26, 2005). New Scientist.
^ Holman, Matthew J. et. al. (August 19, 2004). Discovery of five irregular moons of Neptune. Nature, p. 865 - 867.
^ Five new moons for planet Neptune (August 18, 2004). BBC News.
^ List Of Neptune Trojans (May 6, 2006) at cfa-www.harvard.edu/iau/.
^ "Astronomy picture of the day 18 June 2006," NASA.

References

Adams, J. C., "Explanation of the observed irregularities in the motion of Uranus, on the hypothesis of disturbance by a more distant planet", Monthly Notices of the Royal Astronomical Society, Vol. 7, p. 149, November 13, 1846.
Airy, G. B., "Account of some circumstances historically connected with the discovery of the planet exterior to Uranus", Monthly Notices of the Royal Astronomical Society, Vol. 7, pp. 121-144, November 13, 1846.
Challis, J., Rev., "Account of observations at the Cambridge observatory for detecting the planet exterior to Uranus", Monthly Notices of the Royal Astronomical Society, Vol. 7, pp. 145-149, November 13, 1846.
Dale P. Cruikshank (1995). Neptune and Triton. ISBN 0-816-51525-5.
Galle, "Account of the discovery of the planet of Le Verrier at Berlin", Monthly Notices of the Royal Astronomical Society, Vol. 7, p. 153, November 13, 1846.
Ellis D. Miner et Randii R. Wessen (2002). Neptune: The Planet, Rings, and Satellites. ISBN 1-852-33216-6.
Smith, Bradford A. "Neptune." World Book Online Reference Center. 2004. World Book, Inc. Accessed at nasa.gov.