Neptune

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Neptune  

Neptune from Voyager 2
Discovery
Discovered by:	Urbain Le Verrier John Couch Adams Johann Galle
Discovery date:	September 23, 1846[1]
Orbital characteristics[2][3]
Epoch J2000
Aphelion	4,553,946,490 km 30.44125206 AU
Perihelion:	4,452,940,833 km 29.76607095 AU
Semi-major axis:	4,503,443,661 km 30.10366151 AU
Eccentricity:	0.011214269
Orbital period:	60,190[4] days 164.79 yr
Synodic period:	367.49 day[5]
Avg. orbital speed:	5.43 km/s[5]
Mean anomaly:	267.767281°
Inclination:	1.767975° 6.43° to Sun's equator
Longitude of ascending node:	131.794310°
Argument of perihelion:	265.646853°
Satellites:	13
Physical characteristics
Equatorial radius:	24,764 ± 15 km[6][7] 3.883 Earths
Polar radius:	24,341 ± 30 km[6][7] 3.829 Earths
Flattening:	0.0171 ± 0.0013
Surface area:	7.6408 × 109 km²[4][7] 14.98 Earths
Volume:	6.254 × 1013 km³[5][7] 57.74 Earths
Mass:	1.0243×1026 kg[5] 17.147 Earths
Mean density:	1.638 g/cm³[5][7]
Equatorial surface gravity:	11.15 m/s²[5][7] 1.14 g
Escape velocity:	23.5 km/s[5][7]
Sidereal rotation period:	0.6713 day[5] 16 h 6 min 36 s
Rotation velocity at equator:	2.68 km/s 9,660 km/h
Axial tilt:	28.32°[5]
Right ascension of North pole:	19 h 57 min 20 s 299.333°[6]
Declination of North pole:	42.950°[6]
Albedo:	0.290 (bond) 0.41 (geom.)[5]
Surface temp.:    1 bar level    0.1 bar	min mean max 72 K[5] 55 K[5]
Apparent magnitude:	8.0 to 7.78[5]
Angular diameter:	2.2" — 2.4"[5]
Adjectives:	Neptunian
Atmosphere [5]
Scale height:	19.7 ± 0.6 km
Composition:	80±3.2% Hydrogen (H2) 19±3.2% Helium 1.5±0.5% Methane ~0.019% Hydrogen deuteride (HD) ~0.00015% Ethane Ices: Ammonia Water Ammonium hydrosulfide(NH4SH) Methane (?)

Note: This article contains special characters.

Neptune (pronounced /ˈnɛptjuːn/^[8]) is the eighth and farthest planet from the Sun in the Solar System. 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 less dense.^[9] The planet is named after the Roman god of the sea. Its astronomical symbol is , a stylized version of Poseidon's trident.

Discovered on September 23, 1846,^[1] Neptune was the first planet found by mathematical prediction rather than regular observation. Unexpected changes in the orbit of Uranus led astronomers to deduce the gravitational perturbation of an unknown planet. Neptune was found within a degree of the predicted position. The moon Triton was found shortly thereafter, but none of the planet's other 12 moons were discovered prior to the twentieth century. Neptune has been visited by only one spacecraft, Voyager 2, which flew by the planet on August 25, 1989.

Neptune's atmosphere is primarily composed of hydrogen and helium along with traces of methane. The methane in the atmosphere, in part, accounts for the planet's blue appearance.^[10] Neptune also has the strongest winds of any planet in the solar system, measured as high as 2,100 km/h.^[11] 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's temperature at its cloud tops is usually close to −218 °C, or 55 K, one of the coldest in the solar system, due to its great distance from the Sun. The temperature in Neptune's centre is about 7,000 °C, which is comparable to the Sun's surface and similar to most other known planets.

Contents 1 History 1.1 Discovery 1.2 Naming 2 Structure 2.1 Mass and composition 2.2 Weather 2.3 Magnetosphere 3 Orbit and rotation 4 Moons 5 Planetary rings 6 Observation 7 Exploration 8 See also 9 References 9.1 Notes 9.2 Book references 10 External links

[edit] History

[edit] Discovery

Main article: Discovery of Neptune

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.^[12] Hence he is not credited with Neptune's discovery. During the period of his first observation in December 1612, it 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.^[13]

In 1821, Alexis Bouvard published astronomical tables of the orbit of Uranus.^[14] 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' 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.^[15][16]

In 1845-6, Urbain Le Verrier, independently of Adams, rapidly developed his own calculations but also experienced difficulties in encouraging 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.^[17][18]

Meantime, Le Verrier by letter urged Berlin Observatory astronomer Johann Gottfried Galle to search with the observatory's refractor. 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. The very evening of the day of receipt of Le Verrier's letter, Neptune was discovered, September 23, 1846, 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.^[17][19]

In the wake 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 Observatory, Greenwich), which had apparently been misappropriated by astronomer Olin J. Eggen for nearly three decades and were only rediscovered (in his possession) immediately after his death.^[20] After reviewing the documents, some historians now suggest that Adams does not deserve equal credit with Le Verrier.^[21]

[edit] 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, who proposed the name Janus. In England, Challis put forth the name Oceanus.^[22]

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.^[23] In October, he sought to name the planet Le Verrier, after himself, and he was patriotically supported in this by the observatory director, François Arago. However, this suggestion met with stiff resistance outside France.^[24] French almanacs quickly reintroduced the name Herschel for Uranus, after that planet's discoverer Sir William Herschel, and Leverrier for the new planet.^[25]

Struve came out in favor of the name Neptune on December 29, 1846, to the Saint Petersburg Academy of Sciences.^[26] 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 and Earth, were named for Roman gods.^[27]

[edit] Structure

[edit] Mass and composition

With a mass of 1.0243 × 10²⁶ kg,^[5] Neptune is an intermediate body between Earth and the largest gas giants: it is seventeen Earth masses but just 1/19th the mass of Jupiter.^[9] 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.^[28] 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",^[29] just as astronomers refer to various extra-solar "Jupiters."

At high altitudes, the atmosphere is 80% hydrogen and 19% helium.^[30] A trace amount of methane is also present. In the stratosphere of Neptune, ultraviolet photolysis of methane produces acetylene and ethane, as observed in the planet's spectra.^[30] Prominent absorption bands of methane occur at wavelengths above 600 nm, in the red and infrared portion of the spectrum. This absorption of red light by the atmospheric methane is part of what gives Neptune its blue hue,^[31] although Neptune's vivid blue differs from Uranus's milder cyan hue. Since Neptune's atmospheric methane content is similar to that of Uranus, some unknown atmospheric constituent is thought to contribute to Neptune's colour.^[10]

The internal structure resembles that of Uranus. The atmosphere forms about 5–10% of the planet's mass, extends perhaps 10–20% of the way towards the core and reaches pressures of about 10 GPa. Increasing concentrations of methane, ammonia, and water are found in the lower regions of the atmosphere.^[30]

Gradually this darker and hotter area blends into a superheated liquid mantle, where temperatures reach 2–5,000 K. The mantle is equivalent to 10–15 Earth masses, and is rich in water, ammonia, methane, and other compounds.^[1] As is customary in planetary science, this mixture is referred to as an ice even though it is a hot and highly dense fluid. This fluid, which has a high electrical conductivity, is sometimes called a water–ammonia ocean.^[32]

The core of Neptune is composed of iron, nickel and silicates, with an interior model giving a mass about 1.2 times the Earth's mass.^[33] The pressure at the centre is 7 Mbar—millions of times more than that on the surface of the Earth, and the temperature may be 5,400 K.^[30][34] Comparing its rotational speed to its degree of oblateness indicates that its mass is less concentrated towards the centre than Uranus.^{[citation needed]}

Orbiting so far from the sun, Neptune receives very little heat with the uppermost regions of the atmosphere reaching a minimum temperature of 52 K. At a depth where the atmospheric pressure equals 1 bar, the temperature is 72 K.^[35] Deeper inside the layers of gas, however, the temperature rises steadily. As with Uranus, the source of this heating is unknown, but the discrepancy is larger: Neptune is the farthest planet from the Sun, yet its internal energy is sufficient to drive the fastest planetary winds seen in the Solar System. Neptune radiates about 2.61 times as much energy as it receives from the Sun, which means the internal heat source generates 161% of the solar input.^[36] Several possible explanations have been suggested, including radiogenic heating from the planet's core,^[37] dissociation of methane into hydrocarbon chains under atmospheric pressure,^[38][37] and convection in the lower atmosphere that causes gravity waves to break above the tropopause.^[39][40]

[edit] Weather

One difference between Neptune and Uranus is the typical level of meteorological activity. When the Voyager spacecraft flew by Uranus in 1986 that planet was visually quite bland, while Neptune exhibited notable weather phenomena during its 1989 Voyager fly-by. 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 dynamic storm systems, with winds reaching supersonic speeds of up to around 2,100 km/h.^[42] Even more typical winds in the banded equatorial region can possess speeds of around 1,200 km/h.^[43]

In 1989, the Great Dark Spot, a cyclonic storm system the size of Eurasia, was discovered by NASA's Voyager 2 spacecraft. The storm resembled the Great Red Spot of Jupiter. 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.^[44] The reason for the Great Dark Spot's disappearance is unknown. One possible theory is that heat transfer from the planet's core disrupted the atmospheric equilibrium and disrupted existing circulation patterns.^{[citation needed]}

The Scooter is another storm, a white cloud group further south than the Great Dark Spot. Its nickname was bestowed when it was first detected in the months leading up to the Voyager encounter in 1989: it moved faster than the Great Dark Spot. Subsequent images showed clouds that moved even faster than Scooter. The Small Dark Spot is a southern cyclonic storm, the second most intensive storm during the 1989 encounter. It initially was completely dark, but as Voyager approached the planet, a bright core developed and is seen in most of the highest resolution images.^{[citation needed]}

In 2007 it was discovered that Neptune's south pole was about 10 °C warmer than the rest of Neptune which averages approximately −200 °C. The warmth differential is enough to let methane gas, which elsewhere lies frozen in Neptune's upper atmosphere, to leak out through the south pole and into space. The relative "hot spot" is due to Neptune's tilt in its orbit which has exposed the south pole to the Sun for the last 40 years, a Neptunian year being 165 Earth years. As Neptune slowly moves towards the sun, the south pole will be darkened and the north pole illuminated, causing the methane release to shift to the north pole.^[45]

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, meaning they have a larger solid core and are also made of ices. Neptune is very cold, with temperatures as low as −224 °C (49 K) recorded at the cloud tops in 1989.^{[citation needed]}

[edit] Magnetosphere

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 centre. 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. This field may be generated by convective fluid motions in a thin shell of electrically conducting liquids, resulting in a dynamo action. ^[46]

The magnetic field at the equatorial surface of Neptune is estimated at 1.42 μT, for a magnetic moment of 2.16 × 10¹⁷ Tm³. Neptune's magnetic field has a complex geometry that includes relatively large contributions from non-dipolar components, including a strong quadropole moment that may exceed the dipole moment in strength. By contrast, Earth, Jupiter and Saturn only have relatively small quadropole moments and their fields are less tilted from the polar axis. The large quadropole moment of Neptune may be the result of offset from the planet's center and geometrical constraints of the field's dynamo generator.^[47][48]

Neptune's bow shock, where the magnetosphere begins to slow the solar wind, occurs at a distance of 34.9 times the radius of the planet. The magnetopause, where the pressure of the magnetosphere counterbalances the solar wind, lies at a distance of 23–26.5 times the radius of Neptune. The tail of the magnetosphere extends out to at least 72 times the radius of Neptune, and very likely much further.^[47]

[edit] Orbit and rotation

Neptune's orbital period (year) lasts approximately 164.79 Earth years^[4] and has yet to complete one full orbit since its discovery in 1846. It will return to its original heliocentric point of discovery on July 12, 2011,^[49] but it will not appear at its exact discovery position in our sky due to the Earth being in a different location in its 365.25 day orbit. From the Earth, Neptune goes 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. On August 20, 2010, Neptune will come to opposition on the same night as the discovery year of 1846.^{[citation needed]}

Its sidereal rotation period (day) is roughly 16.11 hours long.^[4] Since its axial tilt of 28 degrees is not dissimilar to the Earths (23 degrees), the variation in the length of its days over the course of its long year is not any more extreme.

[edit] Moons

Main article: Moons of Neptune

Neptune has 13 known moons.^[5] 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 was once a dwarf planet in the Kuiper belt. It is close enough to Neptune to be locked into a synchronous orbit, and is slowly spiraling inward and eventually will be torn apart when it reaches the Roche limit. Triton is the coldest object that has been measured in the solar system, with temperatures of −235 °C (38 K).

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

Neptune's second known satellite (by order of discovery), the irregular moon Nereid, has one of the most eccentric orbits of any satellite in the solar system. The eccentricity of 0.7512 gives it an apoapsis that is seven times its periapsis distance from Neptune.^[50]

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 occulted a star. This had been 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.^[51][52] 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 discovery of Solar System planets and their moons

[edit] Planetary rings

Main article: Rings of Neptune

Faint azure coloured rings have been detected around the blue planet, but are much less substantial than those of Saturn. When these rings were discovered by a team led by Edward Guinan, it was thought that they might not be complete. However, this was disproved by Voyager 2.

These planetary rings have a peculiar "clumpy" structure,^[53] the cause of which is not currently understood but which may be due to the gravitational interaction with small moons in orbit near them.^{[citation needed]}

Evidence that the rings are incomplete first arose in the mid-1980s,^[54] when stellar occultations 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.^[55]

New 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 ring might disappear in as little as one century.^[56]

[edit] Observation

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; the blue colour comes from the methane in its atmosphere.^[57] 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.

[edit] Exploration

Main article: Exploration of Neptune

The closest approach of Voyager 2 to Neptune occurred on August 25, 1989. Since this was the last major planet the spacecraft could visit, it was decided to make a close flyby of the moon Triton, regardless of the consequences to the trajectory, similarly to what was done for Voyager 1's encounter with Saturn and its moon Titan.

The probe also discovered the Great Dark Spot, which has since disappeared, according to Hubble Space Telescope observations. Originally thought to be a large cloud itself, it was later postulated to be a hole in the visible cloud deck.

Neptune turned out to have the strongest winds of all the solar system's gas giants. In the outer regions of the solar system, where the Sun shines over 1000 times fainter than on Earth (still very bright with a magnitude of -21), the last of the four giants defied all expectations of the scientists.

One might expect that the farther one gets from the Sun, the less energy there would be to drive the winds around. The winds on Jupiter were already hundreds of kilometres per hour. Rather than seeing slower winds, the scientists found faster winds (over 1600 km/h) on more distant Neptune.

One suggested cause for this apparent anomaly is that if enough energy is produced, turbulence is created, which slows the winds down (like those of Jupiter). At Neptune however, there is so little solar energy that once winds are started they meet very little resistance, and are able to maintain extremely high velocities.^{[citation needed]} Nonetheless, Neptune radiates more energy than it receives from the Sun,^[58] and the internal energy source of these winds remains undetermined.

The images relayed back to Earth from Voyager 2 in 1989 became the basis of a PBS all-night program called Neptune All Night.^[59]

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.^[60]

[edit] See also

• Planets in astrology – Neptune
• Neptune in fiction
• Neptune Orbiter – Proposed space probe to Neptune to be launched after 2030.
• Neptune Trojans – Asteroids orbiting in Neptune's Lagrangian points.
• Neptune in music – One of the seven movements in Gustav Holst's orchestral suite, The Planets.

[edit] References

[edit] Notes

[edit] Book references

• Baum, Richard; Sheehan, William (2003). In Search of Planet Vulcan: The Ghost in Newton's Clockwork Universe. Oxford University Press. ISBN 0738208892. 
• Cruikshank, Dale P. (1996). Neptune and Triton. University of Arizona Press. ISBN 0-8165-1525-5. 
• Littmann, Mark (2004). Planets Beyond, Exploring the Outer Solar System. Courier Dover Publications. ISBN 0486436020. 
• Miner, Ellis D.; Wessen, Randii R. (2002). Neptune: The Planet, Rings, and Satellites. Springer-Verlag. ISBN 1-85233-216-6. 
• Moore, Patrick (2000). The Data Book of Astronomy. CRC Press. ISBN 0-7503-0620-3. 

[edit] External links

Find more about Neptune on Wikipedia's sister projects:
	Dictionary definitions
	Textbooks
	Quotations
	Source texts
	Images and media
	News stories
	Learning resources

• Smith, Bradford A. "Neptune." World Book Online Reference Center. 2004. World Book, Inc. (NASA.gov)
• NASA's Neptune fact sheet
• Neptune Profile by NASA's Solar System Exploration
• Planets - Neptune A kid's guide to Neptune.
• Neptune from Bill Arnett's nineplanets.org

Future missions to Neptune

• Popular Science article
• Universe Today
• Scientific American Magazine (December 2004 Issue) The Case of the Pilfered Planet

v • d • e Neptune
Major Moons	Triton · Nereid
Characteristics	Rings of Neptune · Great Dark Spot · Neptune Trojans · Moons
Discovery	John Couch Adams · Johann Gottfried Galle · Heinrich d'Arrest · William Lassell · Urbain Le Verrier
Exploration	Voyager program · Voyager 2
Neptune Trojans	2001 QR322 · 2004 UP10 · 2005 TN53 · 2005 TO74 · 2006 RJ103 · 2007 RW10
Other	Neptune-crosser asteroid · Neptune in fiction

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