Neptune
Encyclopedia
Neptune is the eighth and farthest planet
from the Sun
in the 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 times the mass of Earth but not as dense. On average, Neptune orbits the Sun at a distance of 30.1 AU
, approximately 30 times the Earth–Sun distance. Its astronomical symbol is ♆, a stylized version of the god Neptune's trident
.
Discovered on September 23, 1846, 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 gravitation
al 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 giant
s, 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 hydrocarbon
s and possibly nitrogen
, contains a higher proportion of "ices" such as water, ammonia
and methane
. Astronomers sometimes categorize Uranus and Neptune as "ice giant
s" 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. For example, at the time of the 1989 Voyager 2 flyby, 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. 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. Temperatures at the planet's centre are approximately 5400 kelvins (5,126.9 °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.
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
. 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 gravitation
al interaction. In 1843, John Couch Adams
began work on the orbit of Uranus using the data he had. Via James Challis, he requested from Sir George Airy
, the Astronomer Royal
, who sent the data in February 1844. Adams continued to work on this in 1845–46 and produced several different estimates of a new planet, but did not respond to a request from Airy about the orbit of Uranus.
In 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 1846, 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.
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.
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. Since 1966 Dennis Rawlins
has questioned the credibility of Adams's claim to co-discovery and the issue was re-evaluated by historians with the return in 1998 of the "Neptune papers" (historical documents) to the Royal Observatory, Greenwich
. After reviewing the documents, they suggest that "Adams does not deserve equal credit with Le Verrier for the discovery of Neptune. That credit belongs only to the person who succeeded both in predicting the planet's place and in convincing astronomers to search for it."
. 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
. 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 favour 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
.
Most languages today, even in countries that have no direct link to Graeco-Roman culture, use some variant of the name "Neptune" for the planet; in Chinese
, Japanese
and Korean
, the planet's name was literally translated as "sea king star" (海王星), since Neptune was the god of the sea.
in 1930, Neptune was the farthest known planet. Upon Pluto's discovery Neptune became the penultimate planet, save for a 20-year period between 1979 and 1999 when Pluto's elliptical orbit brought it closer to the sun than Neptune. The discovery of the Kuiper belt
in 1992 led many astronomers to debate whether Pluto should be considered a planet in its own right or part of the belt's larger structure. In 2006, the International Astronomical Union
defined the word "planet" for the first time, reclassifying Pluto as a "dwarf planet
" and making Neptune once again the last planet in the Solar System.
With a mass of 1.0243 kg
, Neptune is an intermediate body between Earth
and the larger gas giants
: its mass is seventeen times that of the Earth but just 1/19th that of Jupiter
. The planet's surface gravity
is only surpassed by Jupiter
. Neptune's equator
ial radius of 24764 km is nearly four times that of the Earth. Neptune and Uranus
are often considered a sub-class of gas giant termed "ice giants", due to their smaller size and higher concentrations of volatiles
relative to Jupiter and Saturn
. In the search for extrasolar planet
s Neptune has been used as a metonym
: discovered bodies of similar mass are often referred to as "Neptunes", just as astronomers refer to various extra-solar bodies as "Jupiters".
. Increasing concentrations of methane
, ammonia
and water are found in the lower regions of the atmosphere.
The mantle
reaches temperatures of 2,000 K to 5,000 K. It 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 mantle may consist of a layer of ionic water where the water molecules break down into a soup of hydrogen and oxygen ions, and deeper down superionic water
in which the oxygen crystallises but the hydrogen ions float around freely within the oxygen lattice.
The core
of Neptune is composed of iron
, nickel
and silicate
s, 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.
and 19% helium
. A trace amount of methane is also present. Prominent absorption bands of methane occur at wavelengths above 600 nm, in the red and infrared portion of the spectrum. As with Uranus, this absorption of red light by the atmospheric methane
is part of what gives Neptune its blue hue,
although Neptune's vivid azure
differs from Uranus's milder cyan
. Since Neptune's atmospheric methane content is similar to that of Uranus, some unknown atmospheric constituent is thought to contribute to Neptune's colour.
Neptune's atmosphere is sub-divided into two main regions; the lower troposphere
, where temperature decreases with altitude, and the stratosphere
, where temperature increases with altitude. The boundary between the two, the tropopause
, occurs at a pressure of 0.1 bars (10 kPa). The stratosphere then gives way to the thermosphere
at a pressure lower than 10−5 to 10−4 microbars (1 to 10 Pa). The thermosphere gradually transitions to the exosphere
.
Models 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 MPa), where the temperature reaches 0 °C. Underneath, clouds of ammonia and hydrogen sulfide may be found.
High-altitude clouds on Neptune have been observed casting shadows on the opaque cloud deck below. There are also high-altitude cloud bands that wrap around the planet at constant latitude. These circumferential bands have widths of 50–150 km and lie about 50–110 km above the cloud deck.
Neptune's spectra
suggest that its lower stratosphere is hazy due to condensation of products of ultraviolet photolysis of methane, such as ethane and acetylene. The stratosphere is also home to trace amounts of carbon monoxide
and hydrogen cyanide. The stratosphere of Neptune is warmer than that of Uranus due to the elevated concentration of hydrocarbons.
For reasons that remain obscure, the planet's thermosphere is at an anomalously high temperature of about 750 K. The planet is too far from the Sun for this heat to be generated by ultraviolet
radiation. One candidate for a heating mechanism is atmospheric interaction with ions in the planet's magnetic field
. Other candidates are gravity wave
s from the interior that dissipate in the atmosphere. The thermosphere contains traces of carbon dioxide
and water, which may have been deposited from external sources such as meteorite
s and dust.
, with a magnetic field
strongly tilted relative to its rotation
al axis at 47° and offset at least 0.55 radii, or about 13500 km from the planet's physical centre. Before Voyager 2s arrival at Neptune, it was hypothesised that Uranus's tilted magnetosphere was the result of its sideways rotation. In comparing the magnetic fields of the two planets, scientists now think the extreme orientation may be characteristic of flows in the planets' interiors. This field may be generated by convective
fluid motions in a thin spherical shell of
electrically conducting
liquids (probably a combination of ammonia, methane and water) resulting in a dynamo
action.
The dipole component of the magnetic field at the magnetic equator of Neptune is about 14 microteslas
(0.14 G
). The dipole magnetic moment
of Neptune is about 2.2 T·m3 (14 μT·RN3, where RN is the radius of Neptune). Neptune's magnetic field has a complex geometry that includes relatively large contributions from non-dipolar components, including a strong quadrupole
moment that may exceed the dipole moment in strength. By contrast, Earth, Jupiter and Saturn have only relatively small quadrupole moments, and their fields are less tilted from the polar axis. The large quadrupole moment of Neptune may be the result of offset from the planet's center and geometrical constraints of the field's dynamo generator.
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 farther.
Neptune has a planetary ring
system, though one much less substantial than that of Saturn
. The rings may consist of ice particles coated with silicates or carbon-based material, which most likely gives them a reddish hue. The three main rings are the narrow Adams Ring, 63000 km from the centre of Neptune, the Le Verrier Ring, at 53000 km, and the broader, fainter Galle Ring, at 42000 km. A faint outward extension to the Le Verrier Ring has been named Lassell; it is bounded at its outer edge by the Arago Ring at 57000 km.
The first of these 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.
Neptune's weather is characterized by extremely dynamic storm systems, with winds reaching speeds of almost 600 m/s—nearly attaining supersonic
flow. More typically, by tracking the motion of persistent clouds, wind speeds have been shown to vary from 20 m/s in the easterly direction to 325 m/s westward. At the cloud tops, the prevailing winds range in speed from 400 m/s along the equator to 250 m/s at the poles. Most of the winds on Neptune move in a direction opposite the planet's rotation. The general pattern of winds showed prograde rotation at high latitudes vs. retrograde rotation at lower latitudes. The difference in flow direction is believed to be a "skin effect" and not due to any deeper atmospheric processes. At 70° S latitude, a high-speed jet travels at a speed of 300 m/s.
The abundance of methane, ethane and ethyne at Neptune's equator is 10–100 times greater than at the poles. This is interpreted as evidence for upwelling at the equator and subsidence near the poles.
In 2007 it was discovered that the upper troposphere of 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, which elsewhere lies frozen in Neptune's upper atmosphere, leak out as gas through the south pole and into space. The relative "hot spot" is due to Neptune's axial tilt
, which has exposed the south pole to the Sun
for the last quarter of Neptune's year, or roughly 40 Earth years. As Neptune slowly moves towards the opposite side of the Sun, the south pole will be darkened and the north pole illuminated, causing the methane release to shift to the north pole.
Because of seasonal changes, the cloud bands in the southern hemisphere of Neptune have been observed to increase in size and albedo. This trend was first seen in 1980 and is expected to last until about 2020. The long orbital period of Neptune results in seasons lasting forty years.
, 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, 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.
of -221.4 °C. At a depth where the atmospheric pressure
equals 1 bars (100 kPa), the temperature is -201.15 °C. Deeper inside the layers of gas, the temperature rises steadily. As with Uranus, the source of this heating is unknown, but the discrepancy is larger: Uranus only radiates 1.1 times as much energy as it receives from the Sun; while Neptune radiates about 2.61 times as much energy as it receives from the Sun. 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. Several possible explanations have been suggested, including radiogenic heating from the planet's core, conversion of methane under high pressure into hydrogen, diamond
and longer hydrocarbon
s (the hydrogen and diamond would then rise and sink, respectively, releasing gravitational potential energy), and convection
in the lower atmosphere that causes gravity wave
s to break above the tropopause.
On July 9, 2011, Neptune completed its first full barycentric
orbit since its discovery in 1846, although it did not appear at its exact discovery position in our sky because the Earth was in a different location in its 365.25-day orbit. Because of the motion of the Sun in relation to the barycentre of the Solar System, on 11 July Neptune was also not at its exact discovery position in relation to the Sun; if the more common heliocentric
coordinate system is used, the discovery longitude was reached on July 9, 2011.
The elliptical orbit of Neptune is inclined 1.77° compared to the Earth. Because of an eccentricity
of 0.011, the distance between Neptune and the Sun varies by 101 million km between perihelion and aphelion, the nearest and most distant points of the planet from the Sun along the orbital path, respectively.
The axial tilt of Neptune is 28.32°, which is similar to the tilts of Earth (23°) and Mars (25°). As a result, this planet experiences similar seasonal changes. The long orbital period of Neptune means that the seasons last for forty Earth years. Its sidereal rotation period (day) is roughly 16.11 hours. Since its axial tilt is comparable to the Earth's, the variation in the length of its day over the course of its long year is not any more extreme.
Because Neptune is not a solid body, its atmosphere undergoes differential rotation
. The wide equatorial zone rotates with a period of about 18 hours, which is slower than the 16.1-hour rotation of the planet's magnetic field. By contrast, the reverse is true for the polar regions where the rotation period is 12 hours. This differential rotation is the most pronounced of any planet in the Solar System, and it results in strong latitudinal wind shear.
Neptune's orbit has a profound impact on the region directly beyond it, known as the Kuiper belt. The Kuiper belt is a ring of small icy worlds, similar to the asteroid belt
but far larger, extending from Neptune's orbit at 30 AU out to about 55 AU from the Sun. Much in the same way that Jupiter's gravity dominates the asteroid belt
, shaping its structure, so Neptune's gravity dominates the Kuiper belt
. Over the age of the Solar System, certain regions of the Kuiper belt become destabilized by Neptune's gravity, creating gaps in the Kuiper belt's structure. The region between 40 and 42 AU is an example.
There do exist orbits within these empty regions where objects can survive for the age of the Solar System. These resonances
occur when Neptune's orbital period is a precise fraction of that of the object, such as 1:2, or 3:4. If, say, an object orbits the Sun once for every two Neptune orbits, it will only complete half an orbit by the time Neptune returns to its original position. The most heavily populated resonance in the Kuiper belt, with over 200 known objects, is the 2:3 resonance. Objects in this resonance complete 2 orbits for every 3 of Neptune, and are known as plutino
s because the largest of the known Kuiper belt objects, Pluto
, is among them. Although Pluto crosses Neptune's orbit regularly, the 2:3 resonance ensures they can never collide. The 3:4, 3:5, 4:7 and 2:5 resonances are less populated.
Neptune possesses a number of trojan objects
occupying the Sun
-Neptune Lagrangian point
— a gravitationally stable region leading it in its orbit. Neptune trojan
s can be viewed as being in a 1:1 resonance with Neptune. Neptune trojans are remarkably stable in their orbits, and are likely to have formed alongside Neptune rather than being captured. The first and so far only object identified as associated with Neptune's trailing Lagrangian point
is 2008 LC18
.
The formation of the ice giants, Neptune and Uranus, has proven difficult to model precisely. Current models suggest that the matter density in the outer regions of the Solar System was too low to account for the formation of such large bodies from the traditionally accepted method of core accretion
, and various hypotheses have been advanced to explain their creation. One is that the ice giants were not created by core accretion but from instabilities within the original protoplanetary disc, and later had their atmospheres blasted away by radiation from a nearby massive OB star
.
An alternative concept is that they formed closer to the Sun, where the matter density was higher, and then subsequently migrated
to their current orbits after the removal of the gaseous protoplanetary disc. This hypothesis of migration after formation is currently favoured, due to its ability to better explain the occupancy of the populations of small objects observed in the trans-Neptunian region. The current most widely accepted explanation of the details of this hypothesis is known as the Nice model
, which explores the effect of a migrating Neptune and the other giant planets on the structure of the Kuiper belt.
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 spheroid
al, 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, Triton was the coldest object that had yet been measured in the solar system, with estimated temperatures of −235 °C.
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.
.jpg)
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 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 occultation 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. As Neptune was the Roman god of the sea, the planet's moons have been named after lesser sea gods.
, 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 asteroid
s 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 to 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 carried it close to the 1846 discovery coordinates in April and July 2010 and will again 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.
Voyager 2
' s closest approach 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 images relayed back to Earth from Voyager 2 became the basis of a 1989 PBS
all-night program, Neptune All Night.
.jpg)
During the encounter, signals from the spacecraft required 246 minutes to reach the Earth. Hence, for the most part, the Voyager 2 mission relied on pre-loaded commands for the Neptune encounter. The spacecraft performed a near-encounter with the moon Nereid
before it came within 4400 km of Neptune's atmosphere on August 25, then passed close to the planet's largest moon Triton
later the same day.
The spacecraft verified the existence of a magnetic field surrounding the planet and discovered that the field was offset from the centre and tilted in a manner similar to the field around Uranus. The question of the planet's rotation period was settled using measurements of radio emissions. Voyager 2 also showed that Neptune had a surprisingly active weather system. Six new moons were discovered, and the planet was shown to have more than one ring.
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
.
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
from the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
in the Solar System
Solar System
The Solar System consists of the Sun and the astronomical objects gravitationally bound in orbit around it, all of which formed from the collapse of a giant molecular cloud approximately 4.6 billion years ago. The vast majority of the system's mass is in the Sun...
. Named for the Roman god of the sea
Neptune (mythology)
Neptune was the god of water and the sea in Roman mythology and religion. He is analogous with, but not identical to, the Greek god Poseidon. In the Greek-influenced tradition, Neptune was the brother of Jupiter and Pluto, each of them presiding over one of the three realms of the universe,...
, it is the fourth-largest planet by diameter and the third largest by mass. Neptune is 17 times the mass of Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
and is slightly more massive than its near-twin Uranus
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...
, which is 15 times the mass of Earth but not as dense. On average, Neptune orbits the Sun at a distance of 30.1 AU
Astronomical unit
An astronomical unit is a unit of length equal to about or approximately the mean Earth–Sun distance....
, approximately 30 times the Earth–Sun distance. Its astronomical symbol is ♆, a stylized version of the god Neptune's trident
Trident
A trident , also called a trishul or leister or gig, is a three-pronged spear. It is used for spear fishing and was also a military weapon. Tridents are featured widely in mythical, historical and modern culture. The major Hindu god, Shiva the Destroyer and the sea god Poseidon or Neptune are...
.
Discovered on September 23, 1846, Neptune was the first planet found by mathematical prediction rather than by empirical observation. Unexpected changes in the orbit of Uranus
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...
led Alexis Bouvard
Alexis Bouvard
Alexis Bouvard was a French astronomer. He is particularly noted for his careful observations of the irregularities in the motion of Uranus and his hypothesis of the existence of an eighth planet in the solar system.-Life:...
to deduce that its orbit was subject to gravitation
Gravitation
Gravitation, or gravity, is a natural phenomenon by which physical bodies attract with a force proportional to their mass. Gravitation is most familiar as the agent that gives weight to objects with mass and causes them to fall to the ground when dropped...
al perturbation
Perturbation (astronomy)
Perturbation is a term used in astronomy in connection with descriptions of the complex motion of a massive body which is subject to appreciable gravitational effects from more than one other massive body....
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
Triton (moon)
Triton is the largest moon of the planet Neptune, discovered on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation. At 2,700 km in diameter, it is...
, 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
Voyager 2
The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...
, which flew by the planet on August 25, 1989.
Neptune is similar in composition to Uranus
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...
, and both have compositions which differ from those of the larger gas giant
Gas giant
A gas giant is a large planet that is not primarily composed of rock or other solid matter. There are four gas giants in the Solar System: Jupiter, Saturn, Uranus, and Neptune...
s, Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
and Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...
. Neptune's atmosphere, while similar to Jupiter's and Saturn's in that it is composed primarily of hydrogen
Hydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
and helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
, along with traces of hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
s and possibly nitrogen
Nitrogen
Nitrogen is a chemical element that has the symbol N, atomic number of 7 and atomic mass 14.00674 u. Elemental nitrogen is a colorless, odorless, tasteless, and mostly inert diatomic gas at standard conditions, constituting 78.08% by volume of Earth's atmosphere...
, contains a higher proportion of "ices" such as water, ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
and methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
. Astronomers sometimes categorize Uranus and Neptune as "ice giant
Ice giant
Ice Giant or Ice giants may refer to:* A type of gas giant composed largely of materials less volatile than hydrogen and helium* The frost giants of Norse mythology, see also Hrimthurs...
s" 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. For example, at the time of the 1989 Voyager 2 flyby, the planet's southern hemisphere possessed a Great Dark Spot
Great Dark Spot
The Great Dark Spot is the name given to a series of dark spots on Neptune similar in appearance to Jupiter's Great Red Spot. The first one was observed in 1989 by NASA's Voyager 2 probe. Like Jupiter's spot, they are anticyclonic storms...
comparable to the Great Red Spot on Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
. 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. 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. Temperatures at the planet's centre are approximately 5400 kelvins (5,126.9 °C). Neptune has a faint and fragmented ring system
Planetary ring
A planetary ring is a ring of cosmic dust and other small particles orbiting around a planet in a flat disc-shaped region.The most notable planetary rings known in Earth's solar system are those around Saturn, but the other three gas giants of the solar system possess ring systems of their...
, which may have been detected during the 1960s but was only indisputably confirmed in 1989 by Voyager 2.
Discovery
Galileo'sGalileo Galilei
Galileo Galilei , was an Italian physicist, mathematician, astronomer, and philosopher who played a major role in the Scientific Revolution. His achievements include improvements to the telescope and consequent astronomical observations and support for Copernicanism...
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
Fixed star
The fixed stars are celestial objects that do not seem to move in relation to the other stars of the night sky. Hence, a fixed star is any star except for the Sun. A nebula or other starlike object may also be called a fixed star. People in many cultures have imagined that the stars form pictures...
when it appeared very close—in conjunction—to Jupiter in the night sky
Night sky
The term night sky refers to the sky as seen at night. The term is usually associated with astronomy, with reference to views of celestial bodies such as stars, the Moon, and planets that become visible on a clear night after the Sun has set. Natural light sources in a night sky include moonlight,...
; 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
Retrograde and direct motion
Apparent retrograde motion is the motion of a planetary body in a direction opposite to that of other bodies within its system as observed from a particular vantage point...
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
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...
. In July 2009 University of Melbourne
University of Melbourne
The University of Melbourne is a public university located in Melbourne, Victoria. Founded in 1853, it is the second oldest university in Australia and the oldest in Victoria...
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
Alexis Bouvard
Alexis Bouvard was a French astronomer. He is particularly noted for his careful observations of the irregularities in the motion of Uranus and his hypothesis of the existence of an eighth planet in the solar system.-Life:...
published astronomical tables of the orbit
Orbit
In physics, an orbit is the gravitationally curved path of an object around a point in space, for example the orbit of a planet around the center of a star system, such as the Solar System...
of Neptune's neighbor Uranus. Subsequent observations revealed substantial deviations from the tables, leading Bouvard to hypothesize that an unknown body was perturbing
Perturbation (astronomy)
Perturbation is a term used in astronomy in connection with descriptions of the complex motion of a massive body which is subject to appreciable gravitational effects from more than one other massive body....
the orbit through gravitation
Gravitation
Gravitation, or gravity, is a natural phenomenon by which physical bodies attract with a force proportional to their mass. Gravitation is most familiar as the agent that gives weight to objects with mass and causes them to fall to the ground when dropped...
al interaction. In 1843, John Couch Adams
John Couch Adams
John Couch Adams was a British mathematician and astronomer. Adams was born in Laneast, near Launceston, Cornwall, and died in Cambridge. The Cornish name Couch is pronounced "cooch"....
began work on the orbit of Uranus using the data he had. Via James Challis, he requested from Sir George Airy
George Biddell Airy
Sir George Biddell Airy PRS KCB was an English mathematician and astronomer, Astronomer Royal from 1835 to 1881...
, the Astronomer Royal
Astronomer Royal
Astronomer Royal is a senior post in the Royal Household of the Sovereign of the United Kingdom. There are two officers, the senior being the Astronomer Royal dating from 22 June 1675; the second is the Astronomer Royal for Scotland dating from 1834....
, who sent the data in February 1844. Adams continued to work on this in 1845–46 and produced several different estimates of a new planet, but did not respond to a request from Airy about the orbit of Uranus.
Cambridge Observatory
Cambridge Observatory is an astronomical observatory at the University of Cambridge in the East of England. It was first established in 1823 and is now part of the site of the Institute of Astronomy...
director James Challis
James Challis
James Challis FRS was an English clergyman, physicist and astronomer. Plumian Professor and director of the Cambridge Observatory, he investigated a wide range of physical phenomena though made few lasting contributions outside astronomy...
to search for the planet. Challis vainly scoured the sky throughout August and September.
Meantime, Le Verrier by letter urged Berlin Observatory
Berlin Observatory
The Berlin Observatory is a series of observatories and related organizations in and around the city of Berlin in Germany, starting from the 18th century...
astronomer Johann Gottfried Galle
Johann Gottfried Galle
Johann Gottfried Galle was a German astronomer at the Berlin Observatory who, on 23 September 1846, with the assistance of student Heinrich Louis d'Arrest, was the first person to view the planet Neptune, and know what he was looking at...
to search with the observatory's refractor. Heinrich d'Arrest
Heinrich Louis d'Arrest
Heinrich Louis d'Arrest was a German astronomer, born in Berlin. His name is sometimes given as Heinrich Ludwig 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
Planet
A planet is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and has cleared its neighbouring region of planetesimals.The term planet is ancient, with ties to history, science,...
, 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.
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. Since 1966 Dennis Rawlins
Dennis Rawlins
Dennis Rawlins is an American astronomer, historian, and publisher.-Polar controversies:While studying historical magnetic declination data in polar regions, Rawlins was surprised to find that there were no such data from the 1909 expedition of Robert E. Peary, eventually leading him to become...
has questioned the credibility of Adams's claim to co-discovery and the issue was re-evaluated by historians with the return in 1998 of the "Neptune papers" (historical documents) to the Royal Observatory, Greenwich
Royal Observatory, Greenwich
The Royal Observatory, Greenwich , in London, England played a major role in the history of astronomy and navigation, and is best known as the location of the prime meridian...
. After reviewing the documents, they suggest that "Adams does not deserve equal credit with Le Verrier for the discovery of Neptune. That credit belongs only to the person who succeeded both in predicting the planet's place and in convincing astronomers to search for it."
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 JanusJanus (mythology)
In ancient Roman religion and mythology, Janus is the god of beginnings and transitions, thence also of gates, doors, doorways, endings and time. He is usually a two-faced god since he looks to the future and the past...
. In England, Challis put forward the name Oceanus
Oceanus
Oceanus ; , Ōkeanós) was a pseudo-geographical feature in classical antiquity, believed by the ancient Greeks and Romans to be the world-ocean, an enormous river encircling the world....
.
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
Bureau des Longitudes
The Bureau des Longitudes is a French scientific institution, founded by decree of 25 June 1795 and charged with the improvement of nautical navigation, standardisation of time-keeping, geodesy and astronomical observation. During the 19th century, it was responsible for synchronizing clocks...
. 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
François Arago
François Jean Dominique Arago , known simply as François Arago , was a French mathematician, physicist, astronomer and politician.-Early life and work:...
. This suggestion met with stiff resistance outside France
France
The French Republic , The French Republic , The French Republic , (commonly known as France , is a unitary semi-presidential republic in Western Europe with several overseas territories and islands located on other continents and in the Indian, Pacific, and Atlantic oceans. Metropolitan France...
. French almanacs quickly reintroduced the name Herschel for Uranus, after that planet's discoverer Sir William Herschel
William Herschel
Sir Frederick William Herschel, KH, FRS, German: Friedrich Wilhelm Herschel was a German-born British astronomer, technical expert, and composer. Born in Hanover, Wilhelm first followed his father into the Military Band of Hanover, but emigrated to Britain at age 19...
, and Leverrier for the new planet.
Struve
Friedrich Georg Wilhelm von Struve
Friedrich Georg Wilhelm von Struve was a Danish-Baltic German astronomer from a famous dynasty.-Life:...
came out in favour of the name Neptune on December 29, 1846, to the Saint Petersburg Academy of Sciences
Russian Academy of Sciences
The Russian Academy of Sciences consists of the national academy of Russia and a network of scientific research institutes from across the Russian Federation as well as auxiliary scientific and social units like libraries, publishers and hospitals....
. Soon Neptune became the internationally accepted name. In Roman mythology
Roman mythology
Roman mythology is the body of traditional stories pertaining to ancient Rome's legendary origins and religious system, as represented in the literature and visual arts of the Romans...
, Neptune
Neptune (mythology)
Neptune was the god of water and the sea in Roman mythology and religion. He is analogous with, but not identical to, the Greek god Poseidon. In the Greek-influenced tradition, Neptune was the brother of Jupiter and Pluto, each of them presiding over one of the three realms of the universe,...
was the god of the sea, identified with the Greek Poseidon
Poseidon
Poseidon was the god of the sea, and, as "Earth-Shaker," of the earthquakes in Greek mythology. The name of the sea-god Nethuns in Etruscan was adopted in Latin for Neptune in Roman mythology: both were sea gods analogous to 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
Greek mythology
Greek mythology is the body of myths and legends belonging to the ancient Greeks, concerning their gods and heroes, the nature of the world, and the origins and significance of their own cult and ritual practices. They were a part of religion in ancient Greece...
and Roman mythology
Roman mythology
Roman mythology is the body of traditional stories pertaining to ancient Rome's legendary origins and religious system, as represented in the literature and visual arts of the Romans...
.
Most languages today, even in countries that have no direct link to Graeco-Roman culture, use some variant of the name "Neptune" for the planet; in Chinese
Chinese language
The Chinese language is a language or language family consisting of varieties which are mutually intelligible to varying degrees. Originally the indigenous languages spoken by the Han Chinese in China, it forms one of the branches of Sino-Tibetan family of languages...
, Japanese
Japanese language
is a language spoken by over 130 million people in Japan and in Japanese emigrant communities. It is a member of the Japonic language family, which has a number of proposed relationships with other languages, none of which has gained wide acceptance among historical linguists .Japanese is an...
and Korean
Korean language
Korean is the official language of the country Korea, in both South and North. It is also one of the two official languages in the Yanbian Korean Autonomous Prefecture in People's Republic of China. There are about 78 million Korean speakers worldwide. In the 15th century, a national writing...
, the planet's name was literally translated as "sea king star" (海王星), since Neptune was the god of the sea.
Status
From its discovery in 1846 until the subsequent discovery of PlutoPluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...
in 1930, Neptune was the farthest known planet. Upon Pluto's discovery Neptune became the penultimate planet, save for a 20-year period between 1979 and 1999 when Pluto's elliptical orbit brought it closer to the sun than Neptune. The discovery of the Kuiper belt
Kuiper belt
The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...
in 1992 led many astronomers to debate whether Pluto should be considered a planet in its own right or part of the belt's larger structure. In 2006, the International Astronomical Union
International Astronomical Union
The International Astronomical Union IAU is a collection of professional astronomers, at the Ph.D. level and beyond, active in professional research and education in astronomy...
defined the word "planet" for the first time, reclassifying Pluto as a "dwarf planet
Dwarf planet
A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...
" and making Neptune once again the last planet in the Solar System.
Composition and structure
Kilogram
The kilogram or kilogramme , also known as the kilo, is the base unit of mass in the International System of Units and is defined as being equal to the mass of the International Prototype Kilogram , which is almost exactly equal to the mass of one liter of water...
, Neptune is an intermediate body between Earth
Earth
Earth is the third planet from the Sun, and the densest and fifth-largest of the eight planets in the Solar System. It is also the largest of the Solar System's four terrestrial planets...
and the larger gas giants
Gas Giants
Gas Giants were a pop rock band from Tempe, Arizona, formed as a successor project to the Gin Blossoms. The group was known as The Pharaohs when they formed in 1997, but changed their name after their label, A&M Records, merged with Universal Records and the band changed hands, re-signing with...
: its mass is seventeen times that of the Earth but just 1/19th that of Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
. The planet's surface gravity
Surface gravity
The surface gravity, g, of an astronomical or other object is the gravitational acceleration experienced at its surface. The surface gravity may be thought of as the acceleration due to gravity experienced by a hypothetical test particle which is very close to the object's surface and which, in...
is only surpassed by Jupiter
Jupiter
Jupiter is the fifth planet from the Sun and the largest planet within the Solar System. It is a gas giant with mass one-thousandth that of the Sun but is two and a half times the mass of all the other planets in our Solar System combined. Jupiter is classified as a gas giant along with Saturn,...
. Neptune's equator
Equator
An equator is the intersection of a sphere's surface with the plane perpendicular to the sphere's axis of rotation and containing the sphere's center of mass....
ial radius of 24764 km is nearly four times that of the Earth. Neptune and Uranus
Uranus
Uranus is the seventh planet from the Sun. It has the third-largest planetary radius and fourth-largest planetary mass in the Solar System. It is named after the ancient Greek deity of the sky Uranus , the father of Cronus and grandfather of Zeus...
are often considered a sub-class of gas giant termed "ice giants", due to their smaller size and higher concentrations of volatiles
Volatiles
In planetary science, volatiles are that group of chemical elements and chemical compounds with low boiling points that are associated with a planet's or moon's crust and/or atmosphere. Examples include nitrogen, water, carbon dioxide, ammonia, hydrogen, and methane, all compounds of C, H, O...
relative to Jupiter and Saturn
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...
. In the search for extrasolar planet
Extrasolar planet
An extrasolar planet, or exoplanet, is a planet outside the Solar System. A total of such planets have been identified as of . It is now known that a substantial fraction of stars have planets, including perhaps half of all Sun-like stars...
s Neptune has been used as a metonym
Metonymy
Metonymy is a figure of speech used in rhetoric in which a thing or concept is not called by its own name, but by the name of something intimately associated with that thing or concept...
: discovered bodies of similar mass are often referred to as "Neptunes", just as astronomers refer to various extra-solar bodies as "Jupiters".
Internal structure
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 GPaPascal (unit)
The pascal is the SI derived unit of pressure, internal pressure, stress, Young's modulus and tensile strength, named after the French mathematician, physicist, inventor, writer, and philosopher Blaise Pascal. It is a measure of force per unit area, defined as one newton per square metre...
. Increasing concentrations of methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
, ammonia
Ammonia
Ammonia is a compound of nitrogen and hydrogen with the formula . It is a colourless gas with a characteristic pungent odour. Ammonia contributes significantly to the nutritional needs of terrestrial organisms by serving as a precursor to food and fertilizers. Ammonia, either directly or...
and water are found in the lower regions of the atmosphere.
Mantle (geology)
The mantle is a part of a terrestrial planet or other rocky body large enough to have differentiation by density. The interior of the Earth, similar to the other terrestrial planets, is chemically divided into layers. The mantle is a highly viscous layer between the crust and the outer core....
reaches temperatures of 2,000 K to 5,000 K. It 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
Volatiles
In planetary science, volatiles are that group of chemical elements and chemical compounds with low boiling points that are associated with a planet's or moon's crust and/or atmosphere. Examples include nitrogen, water, carbon dioxide, ammonia, hydrogen, and methane, all compounds of C, H, O...
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 mantle may consist of a layer of ionic water where the water molecules break down into a soup of hydrogen and oxygen ions, and deeper down superionic water
Superionic Water
Superionic water is a theoretical phase of water under extreme heat and pressure which has properties of both a solid and a liquid.At high temperatures and pressures, such as in the interior of giant planets, it is argued that water exists as ionic water in which the molecules break down into a...
in which the oxygen crystallises but the hydrogen ions float around freely within the oxygen lattice.
The core
Planetary core
The planetary core consists of the innermost layer of a planet.The core may be composed of solid and liquid layers, while the cores of Mars and Venus are thought to be completely solid as they lack an internally generated magnetic field. In our solar system, core size can range from about 20% to...
of Neptune is composed of iron
Iron
Iron is a chemical element with the symbol Fe and atomic number 26. It is a metal in the first transition series. It is the most common element forming the planet Earth as a whole, forming much of Earth's outer and inner core. It is the fourth most common element in the Earth's crust...
, nickel
Nickel
Nickel is a chemical element with the chemical symbol Ni and atomic number 28. It is a silvery-white lustrous metal with a slight golden tinge. Nickel belongs to the transition metals and is hard and ductile...
and silicate
Silicate
A silicate is a compound containing a silicon bearing anion. The great majority of silicates are oxides, but hexafluorosilicate and other anions are also included. This article focuses mainly on the Si-O anions. Silicates comprise the majority of the earth's crust, as well as the other...
s, with an interior model giving a mass about 1.2 times that of the Earth. The pressure at the centre is 7 Mbar
Bar (unit)
The bar is a unit of pressure equal to 100 kilopascals, and roughly equal to the atmospheric pressure on Earth at sea level. Other units derived from the bar are the megabar , kilobar , decibar , centibar , and millibar...
(700 GPa), millions of times more than that on the surface of the Earth, and the temperature may be 5,400 K.
Atmosphere
At high altitudes, Neptune's atmosphere is 80% hydrogenHydrogen
Hydrogen is the chemical element with atomic number 1. It is represented by the symbol H. With an average atomic weight of , hydrogen is the lightest and most abundant chemical element, constituting roughly 75% of the Universe's chemical elemental mass. Stars in the main sequence are mainly...
and 19% helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
. A trace amount of methane is also present. Prominent absorption bands of methane occur at wavelengths above 600 nm, in the red and infrared portion of the spectrum. As with Uranus, this absorption of red light by the atmospheric methane
Atmospheric methane
Atmospheric methane levels are of interest due to its impact on climate change. Atmospheric methane is one of the most potent and influential greenhouse gases on Earth. The 100-year global warming potential of methane is 25, i.e...
is part of what gives Neptune its blue hue,
although Neptune's vivid azure
Azure (color)
The color bleu de France is displayed at right.Bleu de France is a color that has been associated in heraldry with the Kings of France since the 12th century.-Brandeis blue:...
differs from Uranus's milder cyan
Cyan
Cyan from , transliterated: kýanos, meaning "dark blue substance") may be used as the name of any of a number of colors in the blue/green range of the spectrum. In reference to the visible spectrum cyan is used to refer to the color obtained by mixing equal amounts of green and blue light or the...
. Since Neptune's atmospheric methane content is similar to that of Uranus, some unknown atmospheric constituent is thought to contribute to Neptune's colour.
Neptune's atmosphere is sub-divided into two main regions; the lower troposphere
Troposphere
The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....
, where temperature decreases with altitude, and the stratosphere
Stratosphere
The stratosphere is the second major layer of Earth's atmosphere, just above the troposphere, and below the mesosphere. It is stratified in temperature, with warmer layers higher up and cooler layers farther down. This is in contrast to the troposphere near the Earth's surface, which is cooler...
, where temperature increases with altitude. The boundary between the two, the tropopause
Tropopause
The tropopause is the atmospheric boundary between the troposphere and the stratosphere.-Definition:Going upward from the surface, it is the point where air ceases to cool with height, and becomes almost completely dry...
, occurs at a pressure of 0.1 bars (10 kPa). The stratosphere then gives way to the thermosphere
Thermosphere
The thermosphere is the biggest of all the layers of the Earth's atmosphere directly above the mesosphere and directly below the exosphere. Within this layer, ultraviolet radiation causes ionization. The International Space Station has a stable orbit within the middle of the thermosphere, between...
at a pressure lower than 10−5 to 10−4 microbars (1 to 10 Pa). The thermosphere gradually transitions to the exosphere
Exosphere
The exosphere is the uppermost layer of Earth's atmosphere. In the exosphere, an upward travelling molecule moving fast enough to attain escape velocity can escape to space with a low chance of collisions; if it is moving below escape velocity it will be prevented from escaping from the celestial...
.
Hydrogen sulfide
Hydrogen sulfide is the chemical compound with the formula . It is a colorless, very poisonous, flammable gas with the characteristic foul odor of expired eggs perceptible at concentrations as low as 0.00047 parts per million...
are believed to form. Above a pressure of five bars, the clouds may consist of ammonia, ammonium sulfide
Ammonium sulfide
Ammonium hydrosulfide is the chemical compound with the formula SH. It is the salt derived from the ammonium cation and the hydrosulfide anion. The salt exists as colourless, water soluble, micaceous crystals. The compound is encountered mainly as a solution, not as the solid...
, hydrogen sulfide and water. Deeper clouds of water ice should be found at pressures of about 50 bars (5 MPa), where the temperature reaches 0 °C. Underneath, clouds of ammonia and hydrogen sulfide may be found.
High-altitude clouds on Neptune have been observed casting shadows on the opaque cloud deck below. There are also high-altitude cloud bands that wrap around the planet at constant latitude. These circumferential bands have widths of 50–150 km and lie about 50–110 km above the cloud deck.
Neptune's spectra
Visible spectrum
The visible spectrum is the portion of the electromagnetic spectrum that is visible to the human eye. Electromagnetic radiation in this range of wavelengths is called visible light or simply light. A typical human eye will respond to wavelengths from about 390 to 750 nm. In terms of...
suggest that its lower stratosphere is hazy due to condensation of products of ultraviolet photolysis of methane, such as ethane and acetylene. The stratosphere is also home to trace amounts of carbon monoxide
Carbon monoxide
Carbon monoxide , also called carbonous oxide, is a colorless, odorless, and tasteless gas that is slightly lighter than air. It is highly toxic to humans and animals in higher quantities, although it is also produced in normal animal metabolism in low quantities, and is thought to have some normal...
and hydrogen cyanide. The stratosphere of Neptune is warmer than that of Uranus due to the elevated concentration of hydrocarbons.
For reasons that remain obscure, the planet's thermosphere is at an anomalously high temperature of about 750 K. The planet is too far from the Sun for this heat to be generated by ultraviolet
Ultraviolet
Ultraviolet light is electromagnetic radiation with a wavelength shorter than that of visible light, but longer than X-rays, in the range 10 nm to 400 nm, and energies from 3 eV to 124 eV...
radiation. One candidate for a heating mechanism is atmospheric interaction with ions in the planet's magnetic field
Magnetosphere
A magnetosphere is formed when a stream of charged particles, such as the solar wind, interacts with and is deflected by the intrinsic magnetic field of a planet or similar body. Earth is surrounded by a magnetosphere, as are the other planets with intrinsic magnetic fields: Mercury, Jupiter,...
. Other candidates are gravity wave
Gravity wave
In fluid dynamics, gravity waves are waves generated in a fluid medium or at the interface between two media which has the restoring force of gravity or buoyancy....
s from the interior that dissipate in the atmosphere. The thermosphere contains traces of carbon dioxide
Carbon dioxide
Carbon dioxide is a naturally occurring chemical compound composed of two oxygen atoms covalently bonded to a single carbon atom...
and water, which may have been deposited from external sources such as meteorite
Meteorite
A meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Meteorites can be big or small. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids...
s and dust.
Magnetosphere
Neptune also resembles Uranus in its magnetosphereMagnetosphere
A magnetosphere is formed when a stream of charged particles, such as the solar wind, interacts with and is deflected by the intrinsic magnetic field of a planet or similar body. Earth is surrounded by a magnetosphere, as are the other planets with intrinsic magnetic fields: Mercury, Jupiter,...
, with a magnetic field
Magnetic field
A magnetic field is a mathematical description of the magnetic influence of electric currents and magnetic materials. The magnetic field at any given point is specified by both a direction and a magnitude ; as such it is a vector field.Technically, a magnetic field is a pseudo vector;...
strongly tilted relative to its rotation
Rotation
A rotation is a circular movement of an object around a center of rotation. A three-dimensional object rotates always around an imaginary line called a rotation axis. If the axis is within the body, and passes through its center of mass the body is said to rotate upon itself, or spin. A rotation...
al axis at 47° and offset at least 0.55 radii, or about 13500 km from the planet's physical centre. Before Voyager 2s arrival at Neptune, it was hypothesised that Uranus's tilted magnetosphere was the result of its sideways rotation. In comparing the magnetic fields of the two planets, scientists now think the extreme orientation may be characteristic of flows in the planets' interiors. This field may be generated by convective
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....
fluid motions in a thin spherical shell of
electrically conducting
Electrical conductor
In physics and electrical engineering, a conductor is a material which contains movable electric charges. In metallic conductors such as copper or aluminum, the movable charged particles are electrons...
liquids (probably a combination of ammonia, methane and water) resulting in a dynamo
Dynamo
- Engineering :* Dynamo, a magnetic device originally used as an electric generator* Dynamo theory, a theory relating to magnetic fields of celestial bodies* Solar dynamo, the physical process that generates the Sun's magnetic field- Software :...
action.
The dipole component of the magnetic field at the magnetic equator of Neptune is about 14 microteslas
Tesla (unit)
The tesla is the SI derived unit of magnetic field B . One tesla is equal to one weber per square meter, and it was defined in 1960 in honour of the inventor, physicist, and electrical engineer Nikola Tesla...
(0.14 G
Gauss (unit)
The gauss, abbreviated as G, is the cgs unit of measurement of a magnetic field B , named after the German mathematician and physicist Carl Friedrich Gauss. One gauss is defined as one maxwell per square centimeter; it equals 1 tesla...
). The dipole magnetic moment
Magnetic moment
The magnetic moment of a magnet is a quantity that determines the force that the magnet can exert on electric currents and the torque that a magnetic field will exert on it...
of Neptune is about 2.2 T·m3 (14 μT·RN3, where RN is the radius of Neptune). Neptune's magnetic field has a complex geometry that includes relatively large contributions from non-dipolar components, including a strong quadrupole
Quadrupole
A quadrupole or quadrapole is one of a sequence of configurations of—for example—electric charge or current, or gravitational mass that can exist in ideal form, but it is usually just part of a multipole expansion of a more complex structure reflecting various orders of complexity.-Mathematical...
moment that may exceed the dipole moment in strength. By contrast, Earth, Jupiter and Saturn have only relatively small quadrupole moments, and their fields are less tilted from the polar axis. The large quadrupole moment of Neptune may be the result of offset from the planet's center and geometrical constraints of the field's dynamo generator.
Neptune's bow shock
Bow shock
A bow shock is the area between a magnetosphere and an ambient medium. For stars, this is typically the boundary between their stellar wind and the interstellar medium....
, where the magnetosphere begins to slow the solar wind
Solar wind
The solar wind is a stream of charged particles ejected from the upper atmosphere of the Sun. It mostly consists of electrons and protons with energies usually between 1.5 and 10 keV. The stream of particles varies in temperature and speed over time...
, occurs at a distance of 34.9 times the radius of the planet. The magnetopause
Magnetopause
The magnetopause is the abrupt boundary between a magnetosphere and the surrounding plasma. For planetary science, the magnetopause is the boundary between the planet’s magnetic field and the solar wind. The location of the magnetopause is determined by the balance between the pressure of the...
, 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 farther.
Planetary rings
Planetary ring
A planetary ring is a ring of cosmic dust and other small particles orbiting around a planet in a flat disc-shaped region.The most notable planetary rings known in Earth's solar system are those around Saturn, but the other three gas giants of the solar system possess ring systems of their...
system, though one much less substantial than that of Saturn
Rings of Saturn
The rings of Saturn are the most extensive planetary ring system of any planet in the Solar System. They consist of countless small particles, ranging in size from micrometres to metres, that form clumps that in turn orbit about Saturn...
. The rings may consist of ice particles coated with silicates or carbon-based material, which most likely gives them a reddish hue. The three main rings are the narrow Adams Ring, 63000 km from the centre of Neptune, the Le Verrier Ring, at 53000 km, and the broader, fainter Galle Ring, at 42000 km. A faint outward extension to the Le Verrier Ring has been named Lassell; it is bounded at its outer edge by the Arago Ring at 57000 km.
The first of these planetary rings was discovered in 1968 by a team led by Edward Guinan
Edward Guinan
Edward F. Guinan is currently a professor in Villanova University's Department of Astronomy and Astrophysics. He and two colleagues discovered Neptune's ring system in 1968. He was also involved in building Iran's first high-powered telescope in the 1970s...
, but it was later thought that this ring might be incomplete. Evidence that the rings might have gaps first arose during a stellar occultation
Occultation
An occultation is an event that occurs when one object is hidden by another object that passes between it and the observer. The word is used in astronomy . It can also refer to any situation wherein an object in the foreground blocks from view an object in the background...
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
Galatea (moon)
Galatea , also known as Neptune VI, is the fourth closest inner satellite of Neptune. It is named after Galatea, one of the Nereids of Greek legend.Galatea was discovered in late July 1989 from the images taken by the Voyager 2 probe...
, 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.
Climate
One difference between Neptune and Uranus is the typical level of meteorological activity. When the Voyager 2 spacecraft flew by Uranus in 1986, that planet was visually quite bland. In contrast Neptune exhibited notable weather phenomena during the 1989 Voyager 2 fly-by.Supersonic
Supersonic speed is a rate of travel of an object that exceeds the speed of sound . For objects traveling in dry air of a temperature of 20 °C this speed is approximately 343 m/s, 1,125 ft/s, 768 mph or 1,235 km/h. Speeds greater than five times the speed of sound are often...
flow. More typically, by tracking the motion of persistent clouds, wind speeds have been shown to vary from 20 m/s in the easterly direction to 325 m/s westward. At the cloud tops, the prevailing winds range in speed from 400 m/s along the equator to 250 m/s at the poles. Most of the winds on Neptune move in a direction opposite the planet's rotation. The general pattern of winds showed prograde rotation at high latitudes vs. retrograde rotation at lower latitudes. The difference in flow direction is believed to be a "skin effect" and not due to any deeper atmospheric processes. At 70° S latitude, a high-speed jet travels at a speed of 300 m/s.
The abundance of methane, ethane and ethyne at Neptune's equator is 10–100 times greater than at the poles. This is interpreted as evidence for upwelling at the equator and subsidence near the poles.
In 2007 it was discovered that the upper troposphere of 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, which elsewhere lies frozen in Neptune's upper atmosphere, leak out as gas through the south pole and into space. The relative "hot spot" is due to Neptune's axial tilt
Axial tilt
In astronomy, axial tilt is the angle between an object's rotational axis, and a line perpendicular to its orbital plane...
, which has exposed the south pole to the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
for the last quarter of Neptune's year, or roughly 40 Earth years. As Neptune slowly moves towards the opposite side of the Sun, the south pole will be darkened and the north pole illuminated, causing the methane release to shift to the north pole.
Because of seasonal changes, the cloud bands in the southern hemisphere of Neptune have been observed to increase in size and albedo. This trend was first seen in 1980 and is expected to last until about 2020. The long orbital period of Neptune results in seasons lasting forty years.
Storms
In 1989, the Great Dark SpotGreat Dark Spot
The Great Dark Spot is the name given to a series of dark spots on Neptune similar in appearance to Jupiter's Great Red Spot. The first one was observed in 1989 by NASA's Voyager 2 probe. Like Jupiter's spot, they are anticyclonic storms...
, an anti-cyclonic
Anticyclone
An anticyclone is a weather phenomenon defined by the United States' National Weather Service's glossary as "[a] large-scale circulation of winds around a central region of high atmospheric pressure, clockwise in the Northern Hemisphere, counterclockwise in the Southern Hemisphere"...
storm system spanning 13000×6600 km, was discovered by NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
's Voyager 2 spacecraft. The storm resembled the Great Red Spot of Jupiter. Some five years later, on November 2, 1994, the Hubble Space Telescope
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
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
Troposphere
The troposphere is the lowest portion of Earth's atmosphere. It contains approximately 80% of the atmosphere's mass and 99% of its water vapor and aerosols....
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
Vortex
A vortex is a spinning, often turbulent,flow of fluid. Any spiral motion with closed streamlines is vortex flow. The motion of the fluid swirling rapidly around a center is called a vortex...
structures. Often associated with dark spots are brighter, persistent methane clouds that form around the tropopause
Tropopause
The tropopause is the atmospheric boundary between the troposphere and the stratosphere.-Definition:Going upward from the surface, it is the point where air ceases to cool with height, and becomes almost completely dry...
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.
Internal heat
Neptune's more varied weather when compared to Uranus is believed to be due in part to its higher internal heat. Although Neptune lies half again as far from the Sun as Uranus, and receives only 40% its amount of sunlight, the two planets' surface temperatures are roughly equal. The upper regions of Neptune's troposphere reach a low temperatureTemperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
of -221.4 °C. At a depth where the atmospheric pressure
Pressure
Pressure is the force per unit area applied in a direction perpendicular to the surface of an object. Gauge pressure is the pressure relative to the local atmospheric or ambient pressure.- Definition :...
equals 1 bars (100 kPa), the temperature is -201.15 °C. Deeper inside the layers of gas, the temperature rises steadily. As with Uranus, the source of this heating is unknown, but the discrepancy is larger: Uranus only radiates 1.1 times as much energy as it receives from the Sun; while Neptune radiates about 2.61 times as much energy as it receives from the Sun. 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. Several possible explanations have been suggested, including radiogenic heating from the planet's core, conversion of methane under high pressure into hydrogen, diamond
Diamond
In mineralogy, diamond is an allotrope of carbon, where the carbon atoms are arranged in a variation of the face-centered cubic crystal structure called a diamond lattice. Diamond is less stable than graphite, but the conversion rate from diamond to graphite is negligible at ambient conditions...
and longer hydrocarbon
Hydrocarbon
In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons from which one hydrogen atom has been removed are functional groups, called hydrocarbyls....
s (the hydrogen and diamond would then rise and sink, respectively, releasing gravitational potential energy), and convection
Convection
Convection is the movement of molecules within fluids and rheids. It cannot take place in solids, since neither bulk current flows nor significant diffusion can take place in solids....
in the lower atmosphere that causes gravity wave
Gravity wave
In fluid dynamics, gravity waves are waves generated in a fluid medium or at the interface between two media which has the restoring force of gravity or buoyancy....
s to break above the tropopause.
Orbit and rotation
The average distance between Neptune and the Sun is 4.50 billion km (about 30.1 AU), and it completes an orbit on average every 164.79 years, subject to a variability of around ±0.1 years.On July 9, 2011, Neptune completed its first full barycentric
Barycentric coordinates (astronomy)
In astronomy, barycentric coordinates are non-rotating coordinates with origin at the center of mass of two or more bodies.The barycenter is the point between two objects where they balance each other. For example, it is the center of mass where two or more celestial bodies orbit each other...
orbit since its discovery in 1846, although it did not appear at its exact discovery position in our sky because the Earth was in a different location in its 365.25-day orbit. Because of the motion of the Sun in relation to the barycentre of the Solar System, on 11 July Neptune was also not at its exact discovery position in relation to the Sun; if the more common heliocentric
Heliocentrism
Heliocentrism, or heliocentricism, is the astronomical model in which the Earth and planets revolve around a stationary Sun at the center of the universe. The word comes from the Greek . Historically, heliocentrism was opposed to geocentrism, which placed the Earth at the center...
coordinate system is used, the discovery longitude was reached on July 9, 2011.
The elliptical orbit of Neptune is inclined 1.77° compared to the Earth. Because of an eccentricity
Orbital eccentricity
The orbital eccentricity of an astronomical body is the amount by which its orbit deviates from a perfect circle, where 0 is perfectly circular, and 1.0 is a parabola, and no longer a closed orbit...
of 0.011, the distance between Neptune and the Sun varies by 101 million km between perihelion and aphelion, the nearest and most distant points of the planet from the Sun along the orbital path, respectively.
The axial tilt of Neptune is 28.32°, which is similar to the tilts of Earth (23°) and Mars (25°). As a result, this planet experiences similar seasonal changes. The long orbital period of Neptune means that the seasons last for forty Earth years. Its sidereal rotation period (day) is roughly 16.11 hours. Since its axial tilt is comparable to the Earth's, the variation in the length of its day over the course of its long year is not any more extreme.
Because Neptune is not a solid body, its atmosphere undergoes differential rotation
Differential rotation
Differential rotation is seen when different parts of a rotating object move with different angular velocities at different latitudes and/or depths of the body and/or in time. This indicates that the object is not solid. In fluid objects, such as accretion disks, this leads to shearing...
. The wide equatorial zone rotates with a period of about 18 hours, which is slower than the 16.1-hour rotation of the planet's magnetic field. By contrast, the reverse is true for the polar regions where the rotation period is 12 hours. This differential rotation is the most pronounced of any planet in the Solar System, and it results in strong latitudinal wind shear.
Orbital resonances
Asteroid belt
The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets...
but far larger, extending from Neptune's orbit at 30 AU out to about 55 AU from the Sun. Much in the same way that Jupiter's gravity dominates the asteroid belt
Asteroid belt
The asteroid belt is the region of the Solar System located roughly between the orbits of the planets Mars and Jupiter. It is occupied by numerous irregularly shaped bodies called asteroids or minor planets...
, shaping its structure, so Neptune's gravity dominates the Kuiper belt
Kuiper belt
The Kuiper belt , sometimes called the Edgeworth–Kuiper belt, is a region of the Solar System beyond the planets extending from the orbit of Neptune to approximately 50 AU from the Sun. It is similar to the asteroid belt, although it is far larger—20 times as wide and 20 to 200 times as massive...
. Over the age of the Solar System, certain regions of the Kuiper belt become destabilized by Neptune's gravity, creating gaps in the Kuiper belt's structure. The region between 40 and 42 AU is an example.
There do exist orbits within these empty regions where objects can survive for the age of the Solar System. These resonances
Orbital resonance
In celestial mechanics, an orbital resonance occurs when two orbiting bodies exert a regular, periodic gravitational influence on each other, usually due to their orbital periods being related by a ratio of two small integers. Orbital resonances greatly enhance the mutual gravitational influence of...
occur when Neptune's orbital period is a precise fraction of that of the object, such as 1:2, or 3:4. If, say, an object orbits the Sun once for every two Neptune orbits, it will only complete half an orbit by the time Neptune returns to its original position. The most heavily populated resonance in the Kuiper belt, with over 200 known objects, is the 2:3 resonance. Objects in this resonance complete 2 orbits for every 3 of Neptune, and are known as plutino
Plutino
In astronomy, a plutino is a trans-Neptunian object in 2:3 mean motion resonance with Neptune. For every 2 orbits that a plutino makes, Neptune orbits 3 times. Plutinos are named after Pluto, which follows an orbit trapped in the same resonance, with the Italian diminutive suffix -ino...
s because the largest of the known Kuiper belt objects, Pluto
Pluto
Pluto, formal designation 134340 Pluto, is the second-most-massive known dwarf planet in the Solar System and the tenth-most-massive body observed directly orbiting the Sun...
, is among them. Although Pluto crosses Neptune's orbit regularly, the 2:3 resonance ensures they can never collide. The 3:4, 3:5, 4:7 and 2:5 resonances are less populated.
Neptune possesses a number of trojan objects
Trojan (astronomy)
In astronomy, a Trojan is a minor planet or natural satellite that shares an orbit with a larger planet or moon, but does not collide with it because it orbits around one of the two Lagrangian points of stability , and , which lie approximately 60° ahead of and behind the larger body,...
occupying the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
-Neptune Lagrangian point
Lagrangian point
The Lagrangian points are the five positions in an orbital configuration where a small object affected only by gravity can theoretically be stationary relative to two larger objects...
— a gravitationally stable region leading it in its orbit. Neptune trojan
Neptune Trojan
Neptune trojans are Kuiper belt object-like bodies in solar orbit that have the same orbital period as Neptune and follow roughly the same orbital path...
s can be viewed as being in a 1:1 resonance with Neptune. Neptune trojans are remarkably stable in their orbits, and are likely to have formed alongside Neptune rather than being captured. The first and so far only object identified as associated with Neptune's trailing Lagrangian point
Lagrangian point
The Lagrangian points are the five positions in an orbital configuration where a small object affected only by gravity can theoretically be stationary relative to two larger objects...
is 2008 LC18
2008 LC18
' is a Neptune trojan discovered on June 7, 2008, by Scott S. Sheppard and Chad Trujillo using the Subaru telescope. It was the seventh such object to be discovered, as well as the first to be found in the region of the trailing Lagrangian point about 60 degrees behind Neptune. It has the same...
.
Formation and migration
Accretion (astrophysics)
In astrophysics, the term accretion is used for at least two distinct processes.The first and most common is the growth of a massive object by gravitationally attracting more matter, typically gaseous matter in an accretion disc. Accretion discs are common around smaller stars or stellar remnants...
, and various hypotheses have been advanced to explain their creation. One is that the ice giants were not created by core accretion but from instabilities within the original protoplanetary disc, and later had their atmospheres blasted away by radiation from a nearby massive OB star
OB star
OB stars are hot, massive stars of spectral types O or B which form in loosely organized groups called OB associations. They are short lived, and thus don't move very far from where they were formed within their life. During their lifetime, they will emit copious amounts of ultraviolet radiation...
.
An alternative concept is that they formed closer to the Sun, where the matter density was higher, and then subsequently migrated
Planetary migration
Planetary migration occurs when a planet or other stellar satellite interacts with a disk of gas or planetesimals, resulting in the alteration of the satellite's orbital parameters, especially its semi-major axis...
to their current orbits after the removal of the gaseous protoplanetary disc. This hypothesis of migration after formation is currently favoured, due to its ability to better explain the occupancy of the populations of small objects observed in the trans-Neptunian region. The current most widely accepted explanation of the details of this hypothesis is known as the Nice model
Nice model
The Nice model is a scenario for the dynamical evolution of the Solar System. It is named for the location of the Observatoire de la Côte d'Azur, where it was initially developed, in Nice, France. It proposes the migration of the giant planets from an initial compact configuration into their...
, which explores the effect of a migrating Neptune and the other giant planets on the structure of the Kuiper belt.
Moons
Natural satellite
A natural satellite or moon is a celestial body that orbits a planet or smaller body, which is called its primary. The two terms are used synonymously for non-artificial satellites of planets, of dwarf planets, and of minor planets....
. 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 spheroid
Spheroid
A spheroid, or ellipsoid of revolution is a quadric surface obtained by rotating an ellipse about one of its principal axes; in other words, an ellipsoid with two equal semi-diameters....
al, is Triton
Triton (moon)
Triton is the largest moon of the planet Neptune, discovered on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation. At 2,700 km in diameter, it is...
, discovered by William Lassell
William Lassell
William Lassell FRS was an English merchant and astronomer.Born in Bolton and educated in Rochdale after the death of his father, he was apprenticed from 1814 to 1821 to a merchant in Liverpool. He then made his fortune as a beer brewer, which enabled him to indulge his interest in astronomy...
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
Dwarf planet
A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...
in the Kuiper belt. It is close enough to Neptune to be locked into a synchronous rotation
Synchronous rotation
In astronomy, synchronous rotation is a planetological term describing a body orbiting another, where the orbiting body takes as long to rotate on its axis as it does to make one orbit; and therefore always keeps the same hemisphere pointed at the body it is orbiting...
, and it is slowly spiraling inward because of tidal acceleration
Tidal acceleration
Tidal acceleration is an effect of the tidal forces between an orbiting natural satellite , and the primary planet that it orbits . The "acceleration" is usually negative, as it causes a gradual slowing and recession of a satellite in a prograde orbit away from the primary, and a corresponding...
and eventually will be torn apart, in about 3.6 billion years, when it reaches the Roche limit
Roche limit
The Roche limit , sometimes referred to as the Roche radius, is the distance within which a celestial body, held together only by its own gravity, will disintegrate due to a second celestial body's tidal forces exceeding the first body's gravitational self-attraction...
. In 1989, Triton was the coldest object that had yet been measured in the solar system, with estimated temperatures of −235 °C.
Neptune's second known satellite (by order of discovery), the irregular moon Nereid
Nereid (moon)
Nereid , also known as Neptune II, is the third-largest moon of Neptune. It has a highly eccentric orbit. Nereid was discovered by Gerard Kuiper in 1949 and was the second moon of Neptune to be discovered.- Discovery and naming :...
, 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.
Proteus (moon)
Proteus , also known as Neptune VIII, is the second largest Neptunian moon, and Neptune's largest inner satellite. Discovered by Voyager 2 spacecraft in 1989, it is named after Proteus, the shape-changing sea god of Greek mythology...
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 the mass of Triton. Neptune's innermost four moons—Naiad
Naiad (moon)
Naiad , also known as Neptune III, is the innermost satellite of Neptune, named after the Naiads of Greek legend.Naiad was discovered sometime before mid-September 1989 from the images taken by the Voyager 2 probe. The last moon to be discovered during the flyby, it was designated...
, Thalassa
Thalassa (moon)
Thalassa , also known as Neptune IV, is the second innermost satellite of Neptune. Thalassa was named after sea goddesss Thalassa, a daughter of Aether and Hemera from Greek mythology. "Thalassa" is also the Greek word for "sea"....
, Despina
Despina (moon)
Despina , also known as Neptune V, is the third closest inner satellite of Neptune. It is named after Despoina, a nymph who was a daughter of Poseidon and Demeter....
and Galatea
Galatea (moon)
Galatea , also known as Neptune VI, is the fourth closest inner satellite of Neptune. It is named after Galatea, one of the Nereids of Greek legend.Galatea was discovered in late July 1989 from the images taken by the Voyager 2 probe...
—orbit close enough to be within Neptune's rings. The next-farthest out, Larissa
Larissa (moon)
Larissa , also known as Neptune VII, is the fifth-closest inner satellite of Neptune. It is named after Larissa, a lover of Poseidon in Greek mythology and eponymous nymph of the city in Thessaly.- Discovery :...
, was originally discovered in 1981 when it had occulted a star. This occultation 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. As Neptune was the Roman god of the sea, the planet's moons have been named after lesser sea gods.
Observation
Neptune is never visible to the naked eyeNaked eye
The naked eye is a figure of speech referring to human visual perception unaided by a magnifying or light-collecting optical device, such as a telescope or microscope. Vision corrected to normal acuity using corrective lenses is considered "naked"...
, having a brightness between magnitudes
Apparent magnitude
The apparent magnitude of a celestial body is a measure of its brightness as seen by an observer on Earth, adjusted to the value it would have in the absence of the atmosphere...
+7.7 and +8.0, which can be outshone by Jupiter's Galilean moons
Galilean moons
The Galilean moons are the four moons of Jupiter discovered by Galileo Galilei in January 1610. They are the largest of the many moons of Jupiter and derive their names from the lovers of Zeus: Io, Europa, Ganymede and Callisto. Ganymede, Europa and Io participate in a 1:2:4 orbital resonance...
, the dwarf planet
Dwarf planet
A dwarf planet, as defined by the International Astronomical Union , is a celestial body orbiting the Sun that is massive enough to be spherical as a result of its own gravity but has not cleared its neighboring region of planetesimals and is not a satellite...
Ceres and the asteroid
Asteroid
Asteroids are a class of small Solar System bodies in orbit around the Sun. They have also been called planetoids, especially the larger ones...
s 4 Vesta
4 Vesta
Vesta, formally designated 4 Vesta, is one of the largest asteroids, with a mean diameter of about . It was discovered by Heinrich Wilhelm Olbers on March 29, 1807, and is named after the Roman virgin goddess of home and hearth, Vesta....
, 2 Pallas
2 Pallas
Pallas, formally designated 2 Pallas, is the second asteroid to have been discovered , and one of the largest. It is estimated to constitute 7% of the mass of the asteroid belt, and its diameter of 530–565 km is comparable to, or slightly larger than, that of 4 Vesta. It is however 20%...
, 7 Iris
7 Iris
7 Iris is a large main-belt asteroid. Among the S-type asteroids, it ranks fifth in geometric mean diameter after Eunomia, Juno, Amphitrite and Herculina....
, 3 Juno
3 Juno
Juno , formal designation 3 Juno in the Minor Planet Center catalogue system, was the third asteroid to be discovered and is one of the larger main-belt asteroids, being one of the two largest stony asteroids, along with 15 Eunomia. Juno is estimated to contain 1% of the total mass of the asteroid...
and 6 Hebe
6 Hebe
6 Hebe is a large main-belt asteroid, containing around half a percent of the mass of the belt. Its apparently high bulk density , however, means that by volume it does not rank among the top twenty asteroids...
. 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
Angular diameter
The angular diameter or apparent size of an object as seen from a given position is the “visual diameter” of the object measured as an angle. In the vision sciences it is called the visual angle. The visual diameter is the diameter of the perspective projection of the object on a plane through its...
of the planet only ranges from 2.2 to 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
Hubble Space Telescope
The Hubble Space Telescope is a space telescope that was carried into orbit by a Space Shuttle in 1990 and remains in operation. A 2.4 meter aperture telescope in low Earth orbit, Hubble's four main instruments observe in the near ultraviolet, visible, and near infrared...
and large ground-based telescopes with adaptive optics
Adaptive optics
Adaptive optics is a technology used to improve the performance of optical systems by reducing the effect of wavefront distortions. It is used in astronomical telescopes and laser communication systems to remove the effects of atmospheric distortion, and in retinal imaging systems to reduce the...
.
From the Earth, Neptune goes through apparent retrograde motion
Retrograde and direct motion
Apparent retrograde motion is the motion of a planetary body in a direction opposite to that of other bodies within its system as observed from a particular vantage point...
every 367 days, resulting in a looping motion against the background stars during each opposition. These loops carried it close to the 1846 discovery coordinates in April and July 2010 and will again in October and November 2011.
Observation of Neptune in the radio
Radio
Radio is the transmission of signals through free space by modulation of electromagnetic waves with frequencies below those of visible light. Electromagnetic radiation travels by means of oscillating electromagnetic fields that pass through the air and the vacuum of space...
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
Infrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
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.
Exploration
Voyager 2
The Voyager 2 spacecraft is a 722-kilogram space probe launched by NASA on August 20, 1977 to study the outer Solar System and eventually interstellar space...
Voyager 1
The Voyager 1 spacecraft is a 722-kilogram space probe launched by NASA in 1977, to study the outer Solar System and eventually interstellar space. Operating for as of today , the spacecraft receives routine commands and transmits data back to the Deep Space Network. At a distance of as of...
Saturn
Saturn is the sixth planet from the Sun and the second largest planet in the Solar System, after Jupiter. Saturn is named after the Roman god Saturn, equated to the Greek Cronus , the Babylonian Ninurta and the Hindu Shani. Saturn's astronomical symbol represents the Roman god's sickle.Saturn,...
and its moon Titan
Titan (moon)
Titan , or Saturn VI, is the largest moon of Saturn, the only natural satellite known to have a dense atmosphere, and the only object other than Earth for which clear evidence of stable bodies of surface liquid has been found....
. The images relayed back to Earth from Voyager 2 became the basis of a 1989 PBS
Public Broadcasting Service
The Public Broadcasting Service is an American non-profit public broadcasting television network with 354 member TV stations in the United States which hold collective ownership. Its headquarters is in Arlington, Virginia....
all-night program, Neptune All Night.
Nereid (moon)
Nereid , also known as Neptune II, is the third-largest moon of Neptune. It has a highly eccentric orbit. Nereid was discovered by Gerard Kuiper in 1949 and was the second moon of Neptune to be discovered.- Discovery and naming :...
before it came within 4400 km of Neptune's atmosphere on August 25, then passed close to the planet's largest moon Triton
Triton (moon)
Triton is the largest moon of the planet Neptune, discovered on October 10, 1846, by English astronomer William Lassell. It is the only large moon in the Solar System with a retrograde orbit, which is an orbit in the opposite direction to its planet's rotation. At 2,700 km in diameter, it is...
later the same day.
The spacecraft verified the existence of a magnetic field surrounding the planet and discovered that the field was offset from the centre and tilted in a manner similar to the field around Uranus. The question of the planet's rotation period was settled using measurements of radio emissions. Voyager 2 also showed that Neptune had a surprisingly active weather system. Six new moons were discovered, and the planet was shown to have more than one ring.
In 2003, there was a proposal to NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
's "Vision Missions Studies" to implement a "Neptune Orbiter with Probes
Neptune Orbiter
Neptune Orbiter was a proposed NASA unmanned planetary spacecraft to explore the planet Neptune. It was envisioned that it would be launched sometime around 2016 and take 8 to 12 years to reach the planet; however, no longer lists any possible launch date. The Neptune Orbiter was designed to...
" mission that does Cassini-level science without fission-based electric power or propulsion. The work is being done in conjunction with JPL
Jet Propulsion Laboratory
Jet Propulsion Laboratory is a federally funded research and development center and NASA field center located in the San Gabriel Valley area of Los Angeles County, California, United States. The facility is headquartered in the city of Pasadena on the border of La Cañada Flintridge and Pasadena...
and the California Institute of Technology
California Institute of Technology
The California Institute of Technology is a private research university located in Pasadena, California, United States. Caltech has six academic divisions with strong emphases on science and engineering...
.
See also
- Hot NeptuneHot NeptuneA hot Neptune is an extrasolar planet in an orbit close to its star , with a mass similar to that of Uranus or Neptune. Recent observations have revealed a larger potential population of hot Neptunes than previously thought...
- Neptune in astrology
- NeptuniumNeptuniumNeptunium is a chemical element with the symbol Np and atomic number 93. A radioactive metal, neptunium is the first transuranic element and belongs to the actinide series. Its most stable isotope, 237Np, is a by-product of nuclear reactors and plutonium production and it can be used as a...
- Neptune in fictionNeptune in fiction-On Neptune:The planet Neptune has been used as a reference and setting in various films and works of fiction:* The first fictional visit of Neptune, portrayed as glacial but nevertheless inhabited, occurred in Spirito gentil .* In H. G...
- Colonization of Neptune
- Neptune OrbiterNeptune OrbiterNeptune Orbiter was a proposed NASA unmanned planetary spacecraft to explore the planet Neptune. It was envisioned that it would be launched sometime around 2016 and take 8 to 12 years to reach the planet; however, no longer lists any possible launch date. The Neptune Orbiter was designed to...
—proposed space probe to Neptune (not before 2035—may have been indefinitely postponed, as NASA's Strategic Exploration Plan no longer includes it) - Neptune TrojanNeptune TrojanNeptune trojans are Kuiper belt object-like bodies in solar orbit that have the same orbital period as Neptune and follow roughly the same orbital path...
—asteroids orbiting in Neptune's Lagrangian points - The PlanetsThe PlanetsThe Planets, Op. 32, is a seven-movement orchestral suite by the English composer Gustav Holst, written between 1914 and 1916. Each movement of the suite is named after a planet of the Solar System and its corresponding astrological character as defined by Holst...
—Neptune is one of the seven movements in Gustav HolstGustav HolstGustav Theodore Holst was an English composer. He is most famous for his orchestral suite The Planets....
's orchestral suite, The Planets
External links
- NASA's Neptune fact sheet
- "Neptune." Smith, Bradford A. World Book Online Reference Center. 2004. World Book, Inc. (NASA.gov)
- Neptune from Bill Arnett's nineplanets.org
- Neptune Astronomy CastAstronomy CastAstronomy Cast is an educational nonprofit podcast discussing various topics in the field of astronomy. The specific subject matter of each episode shifts from week to week, ranging from planets and stars to cosmology and mythbusting...
episode #63, includes full transcript. - Neptune Profile at NASA's Solar System Exploration site
- Planets – Neptune A children's guide to Neptune.