TheInfoList

Neptune is the eighth and farthest-known Solar
planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and ...

from the
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

. In the
Solar System The Solar SystemCapitalization of the name varies. The International Astronomical Union, the authoritative body regarding astronomical nomenclature, specifies capitalizing the names of all individual astronomical objects but uses mixed "Sola ...

, it is the fourth-largest planet by diameter, the third-most-massive planet, and the densest
giant planet The giant planets constitute a diverse type of planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, ...
. It is 17 times the mass of
Earth Earth is the third planet from the Sun and the only astronomical object known to harbour and support life. 29.2% of Earth's surface is land consisting of continents and islands. The remaining 70.8% is Water distribution on Earth, covered wi ...

, slightly more massive than its near-twin
Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus, who, according to Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and ...

. Neptune is denser and physically smaller than Uranus because its greater mass causes more gravitational compression of its atmosphere. The planet orbits the Sun once every 164.8
years A year is the orbital period of a planetary body, for example, the Earth, moving in its orbit around the Sun. Due to the Earth's axial tilt, the course of a year sees the passing of the season A season is a division of the year based on ...
at an average distance of . It is named after the Roman god of the sea and has the
astronomical symbol Astronomical symbols are abstract pictorial symbols used to represent astronomical objects, theoretical constructs and observational events in Western culture, European astronomy. The earliest forms of these symbols appear in Greek papyrus tex ...
, representing Neptune's
trident A trident is a three-tine (structural), pronged spear. It is used for spear fishing and historically as a polearm. The trident is the weapon of Poseidon, or Neptune (mythology), Neptune, the God of the Sea in classical mythology. The trident ma ...

. A second symbol, an L-V monogram 16px, ⯉ for 'Le Verrier', analogous to the H monogram 16px, ♅ for Uranus, was never much used outside of France and is now archaic. Neptune is not visible to the unaided eye and is the only planet in the Solar System found by mathematical prediction rather than by
empirical observation Empirical evidence is the information received by means of the senses, particularly by observation and documentation of patterns and behavior through experimentation. The term comes from the Ancient Greek, Greek word for experience, ἐμπει ...
. Unexpected changes in the orbit of Uranus led
Alexis Bouvard thumbnail, Alexis Bouvard's native farm in Contamines-Montjoie Alexis Bouvard (27 June 1767 – 7 June 1843) was a French astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or ...

to deduce that its orbit was subject to gravitational
perturbation Perturbation or perturb may refer to: * Perturbation theory, mathematical methods that give approximate solutions to problems that cannot be solved exactly * Perturbation (geology), changes in the nature of alluvial deposits over time * Perturbation ...
by an unknown planet. After Bouvard's death, the position of Neptune was predicted from his observations, independently, by
John Couch Adams Prof John Couch Adams FRSE Royal Astronomical Society, FRAS LLD (; 5 June 1819 – 21 January 1892) was a British mathematician and astronomer. He was born in Laneast, near Launceston, Cornwall, and died in Cambridge. His most famous achievement ...

and
Urbain Le Verrier Urbain Jean Joseph Le Verrier FRS (FOR) HFRSE Fellowship of the Royal Society of Edinburgh (FRSE) is an award granted to individuals that the Royal Society of Edinburgh, Scotland's national academy of science and Literature, letters, judged ...

. Neptune was subsequently observed with a telescope on 23 September 1846 by Johann Galle within a
degree Degree may refer to: As a unit of measurement * Degree symbol (°), a notation used in science, engineering, and mathematics * Degree (angle), a unit of angle measurement * Degree (temperature), any of various units of temperature measurement ...
of the position predicted by Le Verrier. Its largest moon,
Triton Triton commonly refers to: * Triton (mythology), a Greek god * Triton (moon), a satellite of Neptune Triton may also refer to: Biology * Triton cockatoo, a parrot * Triton (gastropod), a group of sea snails * ''Triton'', a synonym of ''Triturus'', ...

, was discovered shortly thereafter, though none of the planet's remaining 13 known
moons A natural satellite, or moon, is, in the most common usage, an astronomical body that orbit In physics, an orbit is the gravitationally curved trajectory of an physical body, object, such as the trajectory of a planet around a star or ...
were located telescopically until the 20th century. The planet's distance from Earth gives it a very small apparent size, making it challenging to study with Earth-based telescopes. Neptune was visited by ''
Voyager 2 ''Voyager 2'' is a space probe A space probe or a spaceprobe is a robotic spacecraft that doesn't Earth orbit, orbit the Earth (planet), Earth, but instead explores farther into outer space. A space probe may approach the Moon; travel thro ...
'', when it flew by the planet on 25 August 1989; ''Voyager 2'' remains the only spacecraft to visit Neptune. The advent of the
Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope A space telescope or space observatory is a telescope in outer space used to observe astronomical objects. Suggested by Lyman Spitzer in 1946, the first ...

and large
ground-based telescope The following are lists of devices categorized as types of telescopes or devices associated with telescopes. They are broken into major classifications with many variations due to professional, amateur, and commercial sub-types. Telescopes can be ...
s with
adaptive optics Adaptive optics (AO) is a technology used to improve the performance of optical systems by reducing the effect of incoming wavefront In physics, the wavefront of a time-varying field is the set (locus Locus (plural loci) is Latin for "place" ...

has recently allowed for additional detailed observations from afar. Like Jupiter and Saturn, Neptune's atmosphere is composed primarily of
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

and
helium Helium (from el, ἥλιος, helios Helios; Homeric Greek: ), Latinized as Helius; Hyperion and Phaethon are also the names of his father and son respectively. often given the epithets Hyperion ("the one above") and Phaethon ("the shining" ...

, along with traces of
hydrocarbon In organic chemistry Organic chemistry is a branch of chemistry Chemistry is the study of the properties and behavior of . It is a that covers the that make up matter to the composed of s, s and s: their composition, structure, prop ...
s and possibly
nitrogen Nitrogen is the chemical element upright=1.0, 500px, The chemical elements ordered by link=Periodic table In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science ...

, though it contains a higher proportion of "ices" such as water,
ammonia Ammonia is a chemical compound, compound of nitrogen and hydrogen with the chemical formula, formula NH3. A Binary compounds of hydrogen, stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct ch ...

and
methane Methane (, ) is a chemical compound with the chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes a ...
. However, similar to Uranus, its interior is primarily composed of ices and rock; Uranus and Neptune are normally considered "
ice giant An ice giant is a giant planet The giant planets constitute a diverse type of planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equili ...
s" to emphasise this distinction. Traces of methane in the outermost regions in part account for the planet's blue appearance, though an unknown component is believed to color Neptune a deeper blue compared to Uranus. In contrast to the hazy, relatively featureless atmosphere of Uranus, Neptune's atmosphere has active and visible weather patterns. For example, at the time of the ''Voyager 2'' flyby in 1989, the planet's southern hemisphere had a Great Dark Spot comparable to the
Great Red Spot The Great Red Spot is a persistent high-pressure region in the atmosphere of Jupiter The atmosphere of Jupiter is the largest planetary atmosphere in the Solar System. It is mostly made of molecular hydrogen and helium in roughly Sun#Compositi ...
on Jupiter. More recently, in 2018, a newer main dark spot and smaller dark spot were identified and studied. In addition, these weather patterns are driven by the strongest sustained winds of any planet in the Solar System, with recorded wind speeds as high as . 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 . Temperatures at the planet's centre are approximately . Neptune has a faint and fragmented
ring system A ring system is a disc or ring, orbiting an astronomical object In , an astronomical object or celestial object is a naturally occurring , association, or structure that exists in the . In , the terms ''object'' and ''body'' are often u ...
(labelled "arcs"), which was discovered in 1984, then later confirmed by ''Voyager 2''.

# History

## Discovery

Some of the earliest recorded observations ever made through a
telescope A telescope is an optical instrument An optical instrument (or "optic" for short) is a device that processes light waves (or photons), either to enhance an image for viewing or to analyze and determine their characteristic properties. Common ...

,
Galileo Galileo di Vincenzo Bonaiuti de' Galilei ( , ; 15 February 1564 – 8 January 1642), commonly referred to as Galileo, was an astronomer An astronomer is a in the field of who focuses their studies on a specific question or field o ...

's drawings on 28 December 1612 and 27 January 1613 contain plotted points that match up with what is now known to be the position of Neptune. On both occasions, Galileo seems to have mistaken Neptune for a
fixed star The fixed stars ( la, stellae fixae) compose the background of astronomical objects that appear not to move relative to each other in the night sky compared to the foreground of Solar System objects that do. Generally, the fixed stars are taken to ...
when it appeared close—in
conjunction Conjunction may refer to: * Conjunction (astronomy), in which two astronomical bodies appear close together in the sky * Conjunction (astrology), astrological aspect in horoscopic astrology * Conjunction (grammar), a part of speech * Logical conjun ...
—to Jupiter in the
night sky The term night sky, usually associated with astronomy from Earth, refers to the nighttime appearance of astronomical object, celestial objects like stars, planets, and the Moon, which are visible in a clear sky between sunset and sunrise, when ...

. Hence, he is not credited with Neptune's discovery. At his first observation in December 1612, Neptune was almost stationary in the sky because it had just turned that day. This apparent backward motion is created when Earth's orbit takes it past an outer planet. Because 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 2009, a study suggested that Galileo was at least aware that the "star" he had observed had moved relative to the
fixed stars The fixed stars ( la, stellae fixae) compose the background of astronomical object In astronomy, an astronomical object or celestial object is a naturally occurring physical object, physical entity, association, or structure that exists i ...
. In 1821,
Alexis Bouvard thumbnail, Alexis Bouvard's native farm in Contamines-Montjoie Alexis Bouvard (27 June 1767 – 7 June 1843) was a French astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or ...

published astronomical tables of the
orbit In celestial mechanics, an orbit is the curved trajectory of an physical body, object such as the trajectory of a planet around a star, or of a natural satellite around a planet, or of an satellite, artificial satellite around an object or po ...

of Neptune's neighbour
Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus, who, according to Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and ...

. Subsequent observations revealed substantial deviations from the tables, leading Bouvard to hypothesise that an unknown body was perturbing the orbit through
gravitation Gravity (), or gravitation, is a natural phenomenon Types of natural phenomena include: Weather, fog, thunder, tornadoes; biological processes, decomposition, germination seedlings, three days after germination. Germination is t ...

al interaction. In 1843,
John Couch Adams Prof John Couch Adams FRSE Royal Astronomical Society, FRAS LLD (; 5 June 1819 – 21 January 1892) was a British mathematician and astronomer. He was born in Laneast, near Launceston, Cornwall, and died in Cambridge. His most famous achievement ...

began work on the orbit of Uranus using the data he had. He requested extra data from Sir
George Airy Sir George Biddell Airy (; 27 July 18012 January 1892) was an English mathematician and astronomer, and the seventh Astronomer Royal from 1835 to 1881. His many achievements include work on planetary orbits, measuring the mean density of the Ea ...

, the
Astronomer Royal Astronomer Royal is a senior post in the Royal Households of the United Kingdom The Royal Households of the United Kingdom are the collective departments that support members of the British Royal Family The British royal family compri ...
, who supplied it in February 1844. Adams continued to work in 1845–46 and produced several different estimates of a new planet. In 1845–46,
Urbain Le Verrier Urbain Jean Joseph Le Verrier FRS (FOR) HFRSE Fellowship of the Royal Society of Edinburgh (FRSE) is an award granted to individuals that the Royal Society of Edinburgh, Scotland's national academy of science and Literature, letters, judged ...

, independently of Adams, developed his own calculations but aroused no 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
James Challis James Challis Fellow of the Royal Society, FRS (12 December 1803 – 3 December 1882) was an English clergyman, physicist and astronomer. Plumian Professor of Astronomy and Experimental Philosophy and the director of the Cambridge Observatory ...

to search for the planet. Challis vainly scoured the sky throughout August and September. Challis had, in fact, observed Neptune a year before the planet's subsequent discoverer,
Johann Gottfried Galle 200px, Memorial plaque in Wittenberg Johann Gottfried Galle (9 June 1812 – 10 July 1910) was a German astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a specific question or field outside the ...
, and on two occasions, 4 and 12 August 1845. However, his out-of-date star maps and poor observing techniques meant that he failed to recognise the observations as such until he carried out later analysis. Challis was full of remorse but blamed his neglect on his maps and the fact that he was distracted by his concurrent work on comet observations. Meanwhile, Le Verrier sent a letter and urged
Berlin Observatory The Berlin Observatory (Berliner Sternwarte) is a German astronomical institution with a series of observatories and related organizations in and around the city of Berlin in Germany, starting from the 18th century. It has its origins in 1700 wh ...
astronomer Galle to search with the observatory's
refractor A refracting telescope (also called a refractor) is a type of optical telescope An optical telescope is a telescope that gathers and Focus (optics), focuses light, mainly from the Visible spectrum, visible part of the electromagnetic spectrum, ...

. , 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 A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, is not massive enough to cause thermonuclear fusion, and ...

, as opposed to a fixed star. On the evening of 23 September 1846, the day Galle received the letter, he discovered Neptune just northeast of Iota Aquarii, 1° from the "''five degrees east of Delta Capricorn''" position Le Verrier had predicted it to be, about 12° from Adams's prediction, and on the border of Aquarius and Capricornus according to the modern IAU constellation boundaries. In the wake of the discovery, there was a heated nationalistic rivalry between the French and the British over who deserved credit for the discovery. Eventually, an international consensus emerged that Le Verrier and Adams deserved joint 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 The Royal Observatory, Greenwich (ROG; known as the Old Royal Observatory from 1957 to 1998, when the working Royal Greenwich Observatory, RGO, temporarily moved south from Greenwich to Herstmonceux Herstmonceux ( , ) is a village and ci ...

.

## Naming

Shortly after its discovery, Neptune was referred to simply as "the planet exterior to Uranus" or as "Le Verrier's planet". The first suggestion for a name came from Galle, who proposed the name ''
Janus In ancient Roman religion Religion in ancient Rome includes the ethnic religion of Ancient Rome that the ancient Romans, Romans used to define themselves as a people, as well as the religious practices of peoples brought under Roman rule ...
''. In England, Challis put forward the name ''
Oceanus In Greek mythology Greek mythology is the body of myth Myth is a folklore genre Folklore is the expressive body of culture shared by a particular group of people; it encompasses the tradition A tradition is a belief A bel ...

''. Claiming the right to name his discovery, Le Verrier quickly proposed the name ''Neptune'' for this new planet, though falsely stating that this had been officially approved by the French
Bureau des Longitudes The ''Bureau des Longitudes'' () is a France, 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 th ...
. 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 Dominique François Jean Arago ( ca, Domènec Francesc Joan Aragó), known simply as François Arago (; Catalan: ''Francesc Aragó'', ; 26 February 17862 October 1853), was a French mathematician A mathematician is someone who uses an extensive ...

. 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 Sir Frederick William Herschel (; german: Friedrich Wilhelm Herschel; 15 November 1738 – 25 August 1822) was a German-born British astronomer An astronomer is a scientist in the field of astronomy who focuses their studies on a spe ...

, and ''Leverrier'' for the new planet. Struve came out in favour of the name ''Neptune'' on 29 December 1846, to the
Saint Petersburg Academy of Sciences The Russian Academy of Sciences (RAS; russian: Росси́йская акаде́мия нау́к (РАН) ''Rossíiskaya akadémiya naúk'') consists of the national academy of Russia Russia (russian: link=no, Россия, , ), or the ...
. Soon, ''Neptune'' became the internationally accepted name. In
Roman mythology Roman mythology is the body of myths of ancient Rome as represented in the Latin literature, literature and Roman art, visual arts of the Romans. One of a wide variety of genres of Roman folklore, ''Roman mythology'' may also refer to the moder ...
,
Neptune Neptune is the eighth and farthest-known Solar planet from the Sun. In the Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, and the densest giant planet. It is 17 times the mass of Earth, slightly mo ...
was the god of the sea, identified with the Greek
Poseidon Poseidon (; grc-gre, Ποσειδῶν, ) was one of the Twelve Olympians upright=1.8, Fragment of a relief Relief is a sculptural technique where the sculpted elements remain attached to a solid background of the same material. The ...

. 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 deities in
Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as of ...
and Roman mythology. Most languages today use some variant of the name "Neptune" for the planet; indeed, in Chinese, Vietnamese, Japanese, and Korean, the planet's name was translated as "sea king star" (). In
Mongolian Mongolian may refer to: * Something of, from, or related to Mongolia, a country in Asia * Mongolian people, or Mongols * Mongolia (1911–24), the government of Mongolia, 1911–1919 and 1921–1924 * Mongolian language * Mongolian alphabet * Mongo ...

, Neptune is called (), reflecting its namesake god's role as the ruler of the sea. In modern
Greek#REDIRECT Greek Greek may refer to: Greece Anything of, from, or related to Greece Greece ( el, Ελλάδα, , ), officially the Hellenic Republic, is a country located in Southeast Europe. Its population is approximately 10.7 million as of ...
the planet is called ''Poseidon'' (, ), the Greek counterpart of Neptune. In
Hebrew Hebrew (, , or ) is a Northwest Semitic languages, Northwest Semitic language of the Afroasiatic languages, Afroasiatic language family. Historically, it is regarded as one of the spoken languages of the Israelites and their longest-survivi ...
, (), from a Biblical sea monster mentioned in the
Book of Psalms The Book of Psalms ( or ; he, תְּהִלִּים, , lit. "praises"), commonly referred to simply as Psalms, the Psalter or "the Psalms", is the first book of the ("Writings"), the third section of the Tanakh The Hebrew Bible or Tanakh ...
, was selected in a vote managed by the
Academy of the Hebrew Language The Academy of the Hebrew Language ( he, הָאָקָדֶמְיָה לַלָּשׁוֹן הָעִבְרִית, ''ha-akademya la-lashon ha-ivrit'') was established by the Israel Israel (; he, יִשְׂרָאֵל; ar, إِسْرَائِ ...

in 2009 as the official name for the planet, even though the existing Latin term () is commonly used. In Māori, the planet is called , named after the Māori god of the sea."Appendix 5: Planetary Linguistics"
, Nineplanets.org
In
Nahuatl Nahuatl (; ),The Classical Nahuatl word (noun stem ''nāhua'', + absolutive ''-tl'' ) is thought to mean "a good, clear sound" This language name has several spellings, among them náhuatl (the standard spelling in the Spanish language),() Nao ...
, the planet is called , named after the rain god
Tlāloc Tlaloc ( nci-IPA, Tlāloc, ˈtɬaːlok) is a member of the Aztec mythology, pantheon of gods in Aztec religion, Mexica religion. As supreme god of the rain, Tlaloc is also a god of earthly fertility and of water. He was widely worshipped as a bene ...
. In
Thai Thai or THAI may refer to: * Of or from Thailand, a country in Southeast Asia ** Thai people, the dominant ethnic group of Thailand ** Thai language, a Tai-Kadai language spoken mainly in and around Thailand *** Thai script *** Thai (Unicode block) ...

, Neptune is referred to by its Westernised name (), but is also called (, ), after
Ketu Ketu or KETU may refer to: Places * Ketu (Benin), a historical location in present-day Benin * Ketu, Nigeria * Ketu Municipal District Ketu South Municipal Assembly formerly Ketu South District, is one of the eighteen districts in Volta Region, ...
(), the descending
lunar node A lunar node is either of the two orbital nodes of the Moon, that is, the two points at which the orbit of the Moon intersection (Euclidean geometry), intersects the ecliptic. The ''ascending'' (or ''north'') node is where the Moon moves into the ...
, who plays a role in
Hindu astrology Jyotisha or Jyotishya (from Sanskrit ', from ' "light, heavenly body") is the traditional Hindu system of astrology, also known as Hindu astrology, Indian astrology and more recently Vedic astrology. The term ''Hindu astrology'' has been in us ...
. In
Malay Malay may refer to: Languages * Malay language or Bahasa Melayu, a major Austronesian language spoken in Indonesia, Malaysia, Brunei and Singapore ** History of the Malay language#Old Malay, the Malay language from the 4th to the 14th century ** ...
, the name , after the , is attested as far back as the 1970s, but was eventually superseded by the Latinate equivalents (in Malaysian) or (in
Indonesian Indonesian is anything of, from, or related to Indonesia, an archipelagic country in Southeast Asia. It may refer to: * Indonesians, citizens of Indonesia ** Native Indonesians, diverse groups of local inhabitants of the archipelago ** Indonesian ...

). The usual adjectival form is ''Neptunian''. The nonce form ''Poseidean'' (), from
Poseidon Poseidon (; grc-gre, Ποσειδῶν, ) was one of the Twelve Olympians upright=1.8, Fragment of a relief Relief is a sculptural technique where the sculpted elements remain attached to a solid background of the same material. The ...

, has also been used, though the usual adjectival form of Poseidon is ''Poseidonian'' ().

## Status

From its discovery in 1846 until the discovery of Pluto in 1930, Neptune was the farthest-known planet. When Pluto was discovered, it was considered a planet, and Neptune thus became the second-farthest-known planet, except for a 20-year period between 1979 and 1999 when Pluto's elliptical orbit brought it closer than Neptune to the Sun. The discovery of the
Kuiper belt The Kuiper belt () is a circumstellar disc A circumstellar disc (or circumstellar disk) is a torus, pancake or ring-shaped accumulation of matter composed of gas, Cosmic dust, dust, planetesimals, asteroids, or collision fragments in orbit ar ...
in 1992 led many astronomers to debate whether Pluto should be considered a planet or as part of the Kuiper belt. In 2006, the
International Astronomical Union The International Astronomical Union (IAU; french: link=yes, Union astronomique internationale, UAI) is a Non-governmental organization, nongovernmental organisation with the objective of advancing astronomy in all aspects, including promoting ...
defined the word "planet" for the first time, reclassifying Pluto as a "
dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun – something smaller than any of the eight classical planets, but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of d ...
" and making Neptune once again the outermost-known planet in the Solar System.

# Physical characteristics

Neptune's mass of 1.0243 kg is intermediate between Earth and the larger
gas giant A gas giant is a giant planet The giant planets constitute a diverse type of planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilib ...
s: it is 17 times that of Earth but just 1/19th that of
Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...

.The mass of Earth is 5.9736 kg, giving a mass ratio :$\tfrac = \tfrac = 17.09.$ The mass of Uranus is 8.6810 kg, giving a mass ratio :$\tfrac = \tfrac = 14.54.$ The mass of Jupiter is 1.8986 kg, giving a mass ratio :$\tfrac = \tfrac = 18.63.$ Mass values from Its
gravity Gravity (), or gravitation, is a by which all things with or —including s, s, , and even —are attracted to (or ''gravitate'' toward) one another. , gravity gives to s, and the causes the s of the oceans. The gravitational attracti ...

at 1 bar is 11.15 m/s2, 1.14 times the
surface gravity The surface gravity, ''g'', of an astronomical object is the gravitational acceleration experienced at its surface at the equator, including the effects of rotation. The surface gravity may be thought of as the acceleration In mechanics ...
of Earth, and surpassed only by Jupiter. Neptune's
equator The Equator is a circle of latitude, about in circumference, that divides Earth into the Northern Hemisphere, Northern and Southern Hemisphere, Southern hemispheres. It is an imaginary line located at 0 degrees latitude, halfway between the N ...

ial radius of 24,764 km is nearly four times that of Earth. Neptune, like
Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus, who, according to Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and ...

, is an
ice giant An ice giant is a giant planet The giant planets constitute a diverse type of planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equili ...
, a subclass of
giant planet The giant planets constitute a diverse type of planet A planet is an astronomical body orbiting a star or Stellar evolution#Stellar remnants, stellar remnant that is massive enough to be Hydrostatic equilibrium, rounded by its own gravity, ...
, because they are smaller and have higher concentrations of
volatiles Volatiles are the group of chemical elements and chemical compounds that can be readily Volatility (chemistry), vaporized. In contrast with volatiles, elements and compounds that are not readily vaporized are known as Refractory (planetary scien ...
than Jupiter and
Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It only has one-eighth the average density of Earth; how ...

. In the search for
extrasolar planet An exoplanet or extrasolar planet is a planet outside the Solar System. The first possible evidence of an exoplanet was noted in 1917, but was not recognized as such. The first confirmation of detection occurred in 1992. This was followed by the ...
s, Neptune has been used as a
metonym Metonymy () is a figure of speech A figure of speech or rhetorical figure is a word or phrase that entails an intentional deviation from ordinary language use in order to produce a rhetoric Rhetoric () is the Art (skill), art of p ...
: discovered bodies of similar mass are often referred to as "Neptunes", just as scientists refer to various extrasolar bodies as "Jupiters".

## Internal structure

Neptune's internal structure resembles that of
Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus, who, according to Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and ...
. Its atmosphere forms about 5 to 10% of its mass and extends perhaps 10 to 20% of the way towards the core, where it reaches pressures of about 10
GPa The pascal (symbol: Pa) is the SI derived unit SI derived units are units of measurement ' Measurement is the number, numerical quantification (science), quantification of the variable and attribute (research), attributes of an object or event ...
, or about 100,000 times that of Earth's atmosphere. Increasing concentrations of
methane Methane (, ) is a chemical compound with the chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes a ...
,
ammonia Ammonia is a chemical compound, compound of nitrogen and hydrogen with the chemical formula, formula NH3. A Binary compounds of hydrogen, stable binary hydride, and the simplest pnictogen hydride, ammonia is a colourless gas with a distinct ch ...

and water are found in the lower regions of the atmosphere. The mantle is equivalent to 10 to 15 Earth masses and is rich in water, ammonia and methane. As is customary in planetary science, this mixture is referred to as icy even though it is a hot, dense fluid. This fluid, which has a high electrical conductivity, is sometimes called a water–ammonia ocean. The mantle may consist of a layer of ionic water in which the water molecules break down into a soup of hydrogen and oxygen ions, and deeper down
superionic water Superionic water, also called superionic ice or ice XVIII is a phase of water Water is an Inorganic compound, inorganic, Transparency and translucency, transparent, tasteless, odorless, and Color of water, nearly colorless chemical su ...
in which the oxygen crystallises but the
hydrogen ion A hydrogen ion is created when a hydrogen Hydrogen is the chemical element Image:Simple Periodic Table Chart-blocks.svg, 400px, Periodic table, The periodic table of the chemical elements In chemistry, an element is a pure substance cons ...
s float around freely within the oxygen lattice. At a depth of 7,000 km, the conditions may be such that methane decomposes into diamond crystals that rain downwards like hailstones. Scientists also believe that this kind of diamond rain occurs on
Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...

,
Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It only has one-eighth the average density of Earth; how ...

, and
Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus, who, according to Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and ...

. Very-high-pressure experiments at the
Lawrence Livermore National Laboratory Lawrence Livermore National Laboratory (LLNL) is a federal research facility in Livermore, California Livermore (formerly Livermores, Livermore Ranch, and Nottingham) is a city in Alameda County, California, in the United States. With an es ...
suggest that the top of the mantle may be an ocean of liquid carbon with floating solid 'diamonds'. The
core Core or cores may refer to: Science and technology * Core (anatomy) In common parlance, the core of the body is broadly considered to be the torso. Functional movements are highly dependent on this part of the body, and lack of core muscular dev ...
of Neptune is likely composed of iron, nickel and
silicate In chemistry Chemistry is the science, scientific study of the properties and behavior of matter. It is a natural science that covers the Chemical element, elements that make up matter to the chemical compound, compounds composed of atoms, ...
s, with an interior model giving a mass about 1.2 times that of Earth. The pressure at the centre is 7
Mbar The bar is a metric unit of pressure, but not part of the International System of Units International is an adjective (also used as a noun) meaning "between nations". International may also refer to: Music Albums * International (Kevin Michae ...
(700 GPa), about twice as high as that at the centre of Earth, and the temperature may be 5,400 K.

## Atmosphere

At high altitudes, Neptune's atmosphere is 80%
hydrogen Hydrogen is the chemical element with the Symbol (chemistry), symbol H and atomic number 1. Hydrogen is the lightest element. At standard temperature and pressure, standard conditions hydrogen is a gas of diatomic molecules having the che ...

and 19%
helium Helium (from el, ἥλιος, helios Helios; Homeric Greek: ), Latinized as Helius; Hyperion and Phaethon are also the names of his father and son respectively. often given the epithets Hyperion ("the one above") and Phaethon ("the shining" ...

. A trace amount of methane is also present. Prominent absorption bands of methane exist 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 is the methane Methane (, ) is a chemical compound with the chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, u ...
is part of what gives Neptune its blue hue, although Neptune's vivid azure differs from Uranus's milder
cyan Cyan () is the color between green Green is the between and on the . It is evoked by light which has a of roughly 495570 . In systems, used in painting and color printing, it is created by a combination of yellow and ; in the , u ...

. Because 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 subdivided into two main regions: the lower
troposphere The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere Planetary means relating to a planet A planet is an astronomical body orbiting a star or Stellar evo ...
, where temperature decreases with altitude, and the
stratosphere The stratosphere () is the second layer of the atmosphere of the Earth, located above the troposphere The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphe ...

, where temperature increases with altitude. The boundary between the two, the
tropopause The tropopause is the atmospheric boundary that demarcates the troposphere The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of w ...
, lies at a pressure of . The stratosphere then gives way to the
thermosphere The thermosphere is the layer in the Earth's atmosphere directly above the mesosphere and below the exosphere. Within this layer of the atmosphere, ultraviolet radiation causes photoionization/photodissociation of molecules, creating ions; the th ...
at a pressure lower than 10−5 to 10−4 bars (1 to 10 Pa). The thermosphere gradually transitions to the
exosphere The exosphere ( grc, ἔξω "outside, external, beyond", grc, σφαῖρα "sphere") is a thin, atmosphere-like volume surrounding a planet or natural satellite where molecules are gravitationally bound to that body, but where the density ...
. Models suggest that Neptune's troposphere is banded by clouds of varying compositions depending on altitude. The upper-level clouds lie 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 Hydrogen sulfide is a chemical compound A chemical compound is a chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is any substance that has mass and takes up space by havi ...

are thought to form. Above a pressure of five bars, the clouds may consist of ammonia,
ammonium sulfide Ammonium hydrosulfide is the chemical compound with the Chemical formula, formula (NH4)HS. Composition It is the Salt (chemistry), salt derived from the ammonium cation and the hydrosulfide anion. The salt exists as colourless, water-soluble, mica ...
, hydrogen sulfide and water. Deeper clouds of water ice should be found at pressures of about , where the temperature reaches . 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. These altitudes are in the layer where weather occurs, the troposphere. Weather does not occur in the higher stratosphere or thermosphere. Neptune's spectra suggest that its lower stratosphere is hazy due to condensation of products of ultraviolet
photolysis Photodissociation, photolysis, or photodecomposition is a chemical reaction in which a chemical compound is broken down by photons. It is defined as the interaction of one or more photons with one target molecule. Photodissociation is not limited ...
of methane, such as
ethane Ethane ( or ) is an organic Organic may refer to: * Organic, of or relating to an organism, a living entity * Organic, of or relating to an anatomical organ (anatomy), organ Chemistry * Organic matter, matter that has come from a once-living ...
and
ethyne Acetylene (systematic nameA systematic name is a name given in a systematic way to one unique group, organism, object or chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is a ...

. The stratosphere is also home to trace amounts of
carbon monoxide Carbon monoxide (chemical formula CO) is a colorless, odorless, tasteless, flammable gas that is slightly less dense than air. Carbon monoxide consists of one carbon atom and one oxygen atom. It is the simplest molecule of the oxocarbon family. In ...

and
hydrogen cyanide Hydrogen cyanide, sometimes called prussic acid, is a chemical compound A chemical compound is a chemical substance composed of many identical molecules (or molecular entity, molecular entities) composed of atoms from more than one chemical ...

. 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 (UV) is a form of electromagnetic radiation In physics Physics is the natural science that studies matter, its Elementary particle, fundamental constituents, its Motion (physics), motion and behavior through Spacetime, ...

radiation. One candidate for a heating mechanism is atmospheric interaction with ions in the planet's
magnetic field A magnetic field is a vector field In vector calculus and physics, a vector field is an assignment of a vector to each point in a subset of space. For instance, a vector field in the plane can be visualised as a collection of arrows with ...

. Other candidates are
gravity wave In , gravity waves are waves generated in a medium or at the between two media when the of or tries to restore equilibrium. An example of such an interface is that between the and the , which gives rise to s. A gravity wave results when ...
s from the interior that dissipate in the atmosphere. The thermosphere contains traces of
carbon dioxide Carbon dioxide (chemical formula A chemical formula is a way of presenting information about the chemical proportions of s that constitute a particular or molecule, using symbols, numbers, and sometimes also other symbols, such as pare ...

and water, which may have been deposited from external sources such as
meteorite A meteorite is a solid piece of debris from an object, such as a comet A comet is an icy, small Solar System body A small Solar System body (SSSB) is an object in the Solar System The Solar SystemCapitalization of the name varie ...
s and dust.

## Magnetosphere

Neptune resembles Uranus in its
magnetosphere In astronomy Astronomy (from el, ἀστρονομία, literally meaning the science that studies the laws of the stars) is a natural science that studies astronomical object, celestial objects and celestial event, phenomena. It uses m ...

, with a
magnetic field A magnetic field is a vector field In vector calculus and physics, a vector field is an assignment of a vector to each point in a subset of space. For instance, a vector field in the plane can be visualised as a collection of arrows with ...

strongly tilted relative to its
rotation A rotation is a circular movement of an object around a center (or point) of rotation. The plane (geometry), geometric plane along which the rotation occurs is called the ''rotation plane'', and the imaginary line extending from the center an ...

al axis at 47° and offset at least 0.55 radius, or about 13,500 km from the planet's physical centre. Before ''Voyager 2'' 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 is single or multiphase fluid flow that occurs spontaneously due to the combined effects of material property heterogeneity Homogeneity and heterogeneity are concepts often used in the sciences and statistics Statistics ...

fluid motions in a thin spherical shell of
electrically conducting Electrical resistivity (also called specific electrical resistance or volume resistivity) is a fundamental property of a material that quantifies how strongly it resists electric current. Its inverse, called electrical conductivity, quantifies ...
liquids (probably a combination of ammonia, methane and water) resulting in a
dynamo A dynamo is an that creates using a . Dynamos were the first electrical generators capable of delivering power for industry, and the foundation upon which many other later devices were based, including the , the , and the . Today, the simple ...

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 The magnetic moment is the magnetic strength and orientation of a or other object that produces a . Examples of objects that have magnetic moments include: loops of (such as s), permanent magnets, s (such as s), various s, and many astronomical ...

of Neptune is about 2.2 T·m3 (14 μT·''R''''N''3, where ''R''''N'' 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 A quadrupole or quadrapole is one of a sequence of configurations of things like electric charge or current, or gravitational mass that can exist in ideal form, but it is usually just part of a multipole expansionA multipole expansion is a mathe ...

moment that may exceed the
dipole momentDipole moment may refer to: *Electric dipole moment, the measure of the electrical polarity of a system of charges **Transition dipole moment, the electrical dipole moment in quantum mechanics **Molecular dipole moment, the electric dipole moment o ...

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 centre and geometrical constraints of the field's dynamo generator. Neptune's
bow shock In astrophysics, a bow shock occurs when the magnetosphere of an astrophysical object interacts with the nearby flowing ambient plasma (physics), plasma such as the solar wind. For Earth and other magnetized planets, it is the boundary at whi ...
, where the magnetosphere begins to slow the
solar wind The solar wind is a stream of charged particle In physics Physics (from grc, φυσική (ἐπιστήμη), physikḗ (epistḗmē), knowledge of nature, from ''phýsis'' 'nature'), , is the natural science that studies matter, i ...

, occurs at a distance of 34.9 times the radius of the planet. The
magnetopause The magnetopause is the abrupt boundary between a magnetosphere and the surrounding Plasma (physics), plasma. For planetary science, the magnetopause is the boundary between the planet's magnetic field and the solar wind. The location of the magn ...

, 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 likely much farther.

# Climate

Neptune's weather is characterised by extremely dynamic storm systems, with winds reaching speeds of almost —nearly reaching
supersonic Supersonic speed is the speed of an object that exceeds the speed of sound The speed of sound is the distance travelled per unit of time by a sound wave as it propagates through an elasticity (solid mechanics), elastic medium. At , the spe ...
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. Burgess (1991):64–70. The general pattern of winds showed prograde rotation at high latitudes vs. retrograde rotation at lower latitudes. The difference in flow direction is thought 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. Neptune differs from Uranus in its typical level of meteorological activity. ''Voyager 2'' observed weather phenomena on Neptune during its 1989 flyby, but no comparable phenomena on Uranus during its 1986 fly-by. The abundance of methane, ethane and
acetylene Acetylene (systematic nameA systematic name is a name given in a systematic way to one unique group, organism, object or chemical substance A chemical substance is a form of matter In classical physics and general chemistry, matter is ...

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 because photochemistry cannot account for the distribution without meridional circulation. In 2007, it was discovered that the upper troposphere of Neptune's south pole was about 10 K warmer than the rest of its atmosphere, which averages approximately . The temperature differential is enough to let methane, which elsewhere is frozen in the troposphere, escape into the stratosphere near the pole. The relative "hot spot" is due to Neptune's
axial tilt In , axial tilt, also known as obliquity, is the angle between an object's and its al axis, or, equivalently, the angle between its ial plane and . It differs from . At an obliquity of 0 degrees, the two axes point in the same direction; i.e., ...
, which has exposed the south pole to the
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

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 Albedo (prounounced ; la, albedo, meaning 'whiteness') is the measure of the diffuse reflection Diffuse reflection is the reflectionReflection or reflexion may refer to: Philosophy * Self-reflection Science * Reflection (physics), a comm ...

. 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 Spot, an
anticyclonic File:HadleyCross-sec.jpg, 230px, Hadley cell circulation tends to create anticyclonic patterns in the Horse latitudes, depositing drier air and contributing to the world's great deserts. An anticyclone is a weather meteorological phenomenon, pheno ...
storm system spanning was discovered by
NASA The National Aeronautics and Space Administration (NASA; ) is an independent agency A regulatory agency or regulatory authority, is a Public benefit corporation Public-benefit corporation is a term that has different meanings in differen ...

's ''Voyager 2'' spacecraft. The storm resembled the
Great Red Spot The Great Red Spot is a persistent high-pressure region in the atmosphere of Jupiter The atmosphere of Jupiter is the largest planetary atmosphere in the Solar System. It is mostly made of molecular hydrogen and helium in roughly Sun#Compositi ...
of Jupiter. Some five years later, on 2 November 1994, the
Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope A space telescope or space observatory is a telescope in outer space used to observe astronomical objects. Suggested by Lyman Spitzer in 1946, the first ...

did not see the Great Dark Spot on the planet. Instead, a new storm similar to the Great Dark Spot was found in Neptune's northern hemisphere. The is another storm, a white cloud group farther south than the Great Dark Spot. This nickname first arose during the months leading up to the ''Voyager 2'' encounter in 1989, when they were observed moving at speeds faster than the Great Dark Spot (and images acquired later would subsequently reveal the presence of clouds moving even faster than those that had initially been detected by ''Voyager 2''). The Small Dark Spot is a southern cyclonic storm, the second-most-intense storm observed during the 1989 encounter. It was initially completely dark, but as ''Voyager 2'' approached the planet, a bright core developed and can be seen in most of the highest-resolution images. More recently, in 2018, a newer main dark spot and smaller dark spot were identified and studied. Neptune's dark spots are thought to occur in the
troposphere The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere Planetary means relating to a planet A planet is an astronomical body orbiting a star or Stellar evo ...
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 The tropopause is the atmospheric boundary that demarcates the troposphere The troposphere is the first and lowest layer of the atmosphere of the Earth, and contains 75% of the total mass of the planetary atmosphere, 99% of the total mass of w ...
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. File:A storm is coming Neptune.tif, The appearance of a Northern Great Dark Spot in 2018 is evidence of a huge storm brewing. File:Neptune Dark Spot Jr. Hubble.png, The Northern Great Dark Spot and a smaller companion storm imaged by Hubble in 2020 File:Neptune's Great Dark Spot.jpg, The Great Dark Spot, as imaged by ''Voyager 2'' File:Neptune’s shrinking vortex.jpg, Neptune's shrinking vortex

## Internal heating

Neptune's more varied weather when compared to Uranus is due in part to its higher internal heating. The upper regions of Neptune's troposphere reach a low temperature of . At a depth where the atmospheric pressure equals , the temperature is . 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; whereas Neptune radiates about 2.61 times as much energy as it receives from the Sun. Neptune is the farthest planet from the Sun, and lies over 50% farther from the Sun than Uranus, and receives only 40% its amount of sunlight, yet its internal energy is sufficient to drive the fastest planetary winds seen in the Solar System. Depending on the thermal properties of its interior, the heat left over from Neptune's formation may be sufficient to explain its current heat flow, though it is more difficult to simultaneously explain
Uranus Uranus is the seventh planet from the Sun. Its name is a reference to the Greek god of the sky, Uranus, who, according to Greek mythology Greek mythology is the body of myths originally told by the Ancient Greece, ancient Greeks, and ...

's lack of internal heat while preserving the apparent similarity between the two planets.

# Orbit and rotation

The average distance between Neptune and the Sun is (about 30.1 astronomical units (AU)), and it completes an orbit on average every 164.79 years, subject to a variability of around ±0.1 years. The perihelion distance is 29.81 AU; the aphelion distance is 30.33 AU.Jean Meeus, ''Astronomical Algorithms'' (Richmond, VA: Willmann-Bell, 1998) 273. Supplemented by further use of VSOP87. The last three aphelia were 30.33 AU, the next is 30.34 AU. The perihelia are even more stable at 29.81 AU On 11 July 2011, Neptune completed its first full Barycentric coordinates (astronomy), barycentric orbit since its discovery in 1846, although it did not appear at its exact discovery position in the sky, because Earth was in a different location in its 365.26-day orbit. Because of the motion of the Sun in relation to the Barycenter#Barycenter in astronomy, 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 12 July 2011.(Bill Folkner at JPL)
—Numbers generated using the Solar System Dynamics Group, Horizons On-Line Ephemeris System. The elliptical orbit of Neptune is inclined 1.77° compared to that of Earth. The axial tilt of Neptune is 28.32°, which is similar to the tilts of Earth (23°) and Mars (25°). As a result, Neptune experiences similar seasonal changes to Earth. 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. Because its axial tilt is comparable to 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.

## Orbital resonances

Neptune's orbit has a profound impact on the region directly beyond it, known as the
Kuiper belt The Kuiper belt () is a circumstellar disc A circumstellar disc (or circumstellar disk) is a torus, pancake or ring-shaped accumulation of matter composed of gas, Cosmic dust, dust, planetesimals, asteroids, or collision fragments in orbit ar ...
. 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 became destabilised 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 Orbital resonance, 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 plutinos 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 has a number of known Trojan (astronomy), trojan objects occupying both the
Sun The Sun is the star A star is an astronomical object consisting of a luminous spheroid of plasma (physics), plasma held together by its own gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many othe ...

–Neptune and Lagrangian points—gravitationally stable regions leading and trailing Neptune in its orbit, respectively. Neptune trojans can be viewed as being in a 1:1 resonance with Neptune. Some Neptune trojans are remarkably stable in their orbits, and are likely to have formed alongside Neptune rather than being captured. The first object identified as associated with Neptune's trailing Lagrangian point was . Neptune also has a temporary quasi-satellite, . The object has been a quasi-satellite of Neptune for about 12,500 years and it will remain in that dynamical state for another 12,500 years.

# Formation and migration

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 (astrophysics), accretion, and various hypotheses have been advanced to explain their formation. One is that the ice giants were not formed by core accretion but from instabilities within the original protoplanetary disk, 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 Planetary migration, migrated to their current orbits after the removal of the gaseous protoplanetary disc. This hypothesis of migration after formation is 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.

# Moons

Neptune has 14 known natural satellite, moons.
Triton Triton commonly refers to: * Triton (mythology), a Greek god * Triton (moon), a satellite of Neptune Triton may also refer to: Biology * Triton cockatoo, a parrot * Triton (gastropod), a group of sea snails * ''Triton'', a synonym of ''Triturus'', ...

is the largest Neptunian moon, comprising more than 99.5% of the mass in orbit around Neptune,Mass of Triton: 2.14 kg. Combined mass of 12 other known moons of Neptune: 7.53 kg, or 0.35%. The mass of the rings is negligible. and it is the only one massive enough to be spheroidal.
Triton Triton commonly refers to: * Triton (mythology), a Greek god * Triton (moon), a satellite of Neptune Triton may also refer to: Biology * Triton cockatoo, a parrot * Triton (gastropod), a group of sea snails * ''Triton'', a synonym of ''Triturus'', ...

was 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 was probably once a
dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun – something smaller than any of the eight classical planets, but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of d ...
in the Kuiper belt. It is close enough to Neptune to be locked into a synchronous rotation, and it is slowly spiralling inward because of tidal acceleration. It will eventually 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 . Neptune's second-known satellite (by order of discovery), the irregular moon Nereid (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.$\tfrac = \tfrac - 1 = 2/0.2488 - 1 \approx 7.039.$ From July to September 1989, ''Voyager 2'' discovered six moons of Neptune. Of these, the irregularly shaped Proteus (moon), 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 0.25% the mass of Triton. Neptune's innermost four moons—Naiad (moon), Naiad, Thalassa (moon), Thalassa, Despina (moon), Despina and Galatea (moon), Galatea—orbit close enough to be within Neptune's rings. The next-farthest out, Larissa (moon), 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, Larissa was found to have caused it. Five new irregular moons discovered between 2002 and 2003 were announced in 2004. A new moon and the smallest yet, Hippocamp (moon), Hippocamp, was found in 2013 by combining multiple Hubble images. Because Neptune was the Roman god of the sea, Neptune's moons have been named after lesser sea gods.

## Planetary rings

Neptune has a planetary ring system, though one much less substantial than that of Rings of Saturn, 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, 63,000 km from the centre of Neptune, the Le Verrier Ring, at 53,000 km, and the broader, fainter Galle Ring, at 42,000 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 57,000 km. The first of these planetary rings was detected in 1968 by a team led by Edward Guinan. In the early 1980s, analysis of this data along with newer observations led to the hypothesis 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 from ''Voyager 2'' in 1989 settled the issue by showing several faint rings. 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 short timescales. Astronomers now estimate that the arcs are corralled into their current form by the gravitational effects of Galatea (moon), 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.

# Observation

Neptune brightened about 10% between 1980 and 2000 mostly due to the changing of the seasons. Neptune may continue to brighten as it approaches perihelion in 2042. The apparent magnitude currently ranges from 7.67 to 7.89 with a mean of 7.78 and a standard deviation of 0.06. Prior to 1980 the planet was as faint as magnitude 8.0. Neptune is too faint to be visible to the naked eye. It can be outshone by Jupiter's Galilean moons, the
dwarf planet A dwarf planet is a small planetary-mass object that is in direct orbit of the Sun – something smaller than any of the eight classical planets, but still a world in its own right. The prototypical dwarf planet is Pluto. The interest of d ...
Ceres (dwarf planet), Ceres and the asteroids 4 Vesta, 2 Pallas, 7 Iris, 3 Juno, and 6 Hebe. A telescope or strong binoculars will resolve Neptune as a small blue disk, similar in appearance to Uranus. Because of the distance of Neptune from Earth, its angular diameter only ranges from 2.2 to 2.4 arcseconds, the smallest of the Solar System planets. Its small apparent size makes it challenging to study visually. Most telescopic data was fairly limited until the advent of the
Hubble Space Telescope The Hubble Space Telescope (often referred to as HST or Hubble) is a space telescope A space telescope or space observatory is a telescope in outer space used to observe astronomical objects. Suggested by Lyman Spitzer in 1946, the first ...

and large ground-based telescopes with
adaptive optics Adaptive optics (AO) is a technology used to improve the performance of optical systems by reducing the effect of incoming wavefront In physics, the wavefront of a time-varying field is the set (locus Locus (plural loci) is Latin for "place" ...

(AO). The first scientifically useful observation of Neptune from ground-based telescopes using adaptive optics was commenced in 1997 from Hawaii. Neptune is currently entering its spring and summer season and has been shown to be heating up, with increased atmospheric activity and brightness as a consequence. Combined with technological advancements, ground-based telescopes with adaptive optics are recording increasingly more detailed images of it. Both ''Hubble'' and the adaptive-optics telescopes on Earth have made many new discoveries within the Solar System since the mid-1990s, with a large increase in the number of known satellites and moons around the outer planet, among others. In 2004 and 2005, five new small satellites of Neptune with diameters between 38 and 61 kilometres were discovered. From Earth, Neptune goes through apparent retrograde motion every 367 days, resulting in a looping motion against the background stars during each opposition (astronomy), opposition. These loops carried it close to the 1846 discovery coordinates in April and July 2010 and again in October and November 2011. Neptune's 164-year orbital period means that the planet takes an average of 13 years to move through each constellation of the zodiac. In 2011, it completed its first full orbit of the Sun since being discovered and returned to where it was first spotted northeast of Iota Aquarii. Observation of Neptune in the radio-frequency band shows that it is a source of both continuous emission and irregular bursts. Both sources are thought to originate from its 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.

# Exploration

''
Voyager 2 ''Voyager 2'' is a space probe A space probe or a spaceprobe is a robotic spacecraft that doesn't Earth orbit, orbit the Earth (planet), Earth, but instead explores farther into outer space. A space probe may approach the Moon; travel thro ...
'' is the only spacecraft that has visited Neptune. The spacecraft closest approach to the planet occurred on 25 August 1989. Because 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 Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It only has one-eighth the average density of Earth; how ...

and its moon Titan (moon), Titan. The images relayed back to Earth from ''Voyager 2'' became the basis of a 1989 PBS all-night program, ''Neptune All Night''. During the encounter, signals from the spacecraft required 246 minutes to reach Earth. Hence, for the most part, ''Voyager 2'' mission relied on preloaded commands for the Neptune encounter. The spacecraft performed a near-encounter with the moon Nereid (moon), Nereid before it came within 4,400 km of Neptune's atmosphere on 25 August, then passed close to the planet's largest moon
Triton Triton commonly refers to: * Triton (mythology), a Greek god * Triton (moon), a satellite of Neptune Triton may also refer to: Biology * Triton cockatoo, a parrot * Triton (gastropod), a group of sea snails * ''Triton'', a synonym of ''Triturus'', ...

later the same day. Burgess (1991):46–55. 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. Neptune's rotation period was determined using measurements of radio emissions and ''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. The flyby also provided the first accurate measurement of Neptune's mass which was found to be 0.5 percent less than previously calculated. The new figure disproved the hypothesis that an undiscovered Planet X acted upon the orbits of Neptune and Uranus. Since 2018, the China National Space Administration has been studying a concept for a pair of ''Voyager''-like interstellar probes tentatively known as ''Interstellar Express'' or ''Interstellar Heliosphere Probe''. Both probes will be launched at the same time in 2024 and take differing paths to explore opposing ends of the heliosphere; the second probe, ''IHP-2'', will fly by Neptune in January 2038, passing only 1,000 km above the cloud tops, and potentially carry an atmospheric impactor to be released during its approach. Afterward, it will continue on its mission throughout the Kuiper belt toward the tail of the heliosphere, so far unexplored. After ''Voyager 2'' and ''IHP-2''s flybys, the next step in scientific exploration of the Neptunian system is considered to be an orbital mission; most proposals have been by
NASA The National Aeronautics and Space Administration (NASA; ) is an independent agency A regulatory agency or regulatory authority, is a Public benefit corporation Public-benefit corporation is a term that has different meanings in differen ...

, most often for a Flagship Program, Flagship orbiter. Such a hypothetical mission is envisioned to be possible in the late 2020s or early 2030s. However, there have been discussions to launch Neptune missions sooner. In 2003, there was a proposal in NASA's "Vision Missions Studies" for a "Neptune Odyssey, Neptune Orbiter with Probes" mission that does ''Cassini–Huygens, Cassini''-level science. Another, more recent proposal was for ''Argo (NASA spacecraft), Argo'', a flyby spacecraft to be launched in 2019, that would visit
Jupiter Jupiter is the fifth planet from the Sun and the List of Solar System objects by size, largest in the Solar System. It is a gas giant with a mass more than two and a half times that of all the other planets in the Solar System combined, but ...

,
Saturn Saturn is the sixth planet from the Sun and the second-largest in the Solar System, after Jupiter. It is a gas giant with an average radius of about nine and a half times that of Earth. It only has one-eighth the average density of Earth; how ...

, Neptune, and a Kuiper belt object. The focus would be on Neptune and its largest moon
Triton Triton commonly refers to: * Triton (mythology), a Greek god * Triton (moon), a satellite of Neptune Triton may also refer to: Biology * Triton cockatoo, a parrot * Triton (gastropod), a group of sea snails * ''Triton'', a synonym of ''Triturus'', ...

to be investigated around 2029. The proposed ''New Horizons 2'' mission (which was later scrapped) might also have done a close flyby of the Neptunian system. Currently a pending proposal for the Discovery program, Trident (spacecraft), ''Trident'' would conduct a flyby of Neptune and Triton; however, the mission was not selected for Discovery 15 or 16. ''Neptune Odyssey'' is the current mission concept for a Neptune orbiter and atmospheric probe being studied as a possible large strategic science mission by NASA that would launch between 2031 and 2033, and arrive at Neptune by 2049.

* Outline of Neptune * Hot Neptune * Planets in astrology#Neptune, Neptune in astrology * Neptunium * ''Neptune, the Mystic'' – one of the seven movements in Gustav Holst's ''Planets'' suite * Timeline of the far future * List_of_gravitationally_rounded_objects_of_the_Solar_System#Planets, Stats of planets in the Solar System

# Bibliography

* *

* *

NASA's Neptune fact sheet

from Bill Arnett's nineplanets.org
Neptune
Astronomy Cast episode No. 63, includes full transcript.
Neptune Profile
a
NASA's Solar System Exploration site

A children's guide to Neptune. *

(The Planetary Society)
Interactive 3D gravity simulation of Neptune and its inner moons

{{Authority control Neptune, Astronomical objects discovered in 1846, 18460923 Gas giants Ice giants Outer planets