Neptune is not visible to the unaided eye and is the only planet in the Solar System that was not initially observed by directempirical observation. Rather, unexpected changes in the orbit of Uranus ledAlexis Bouvard to hypothesise that its orbit was subject to gravitationalperturbation by an unknown planet. After Bouvard's death, the position of Neptune was mathematically predicted from his observations, independently, byJohn Couch Adams andUrbain Le Verrier. Neptune was subsequently directly observed with a telescope on 23 September 1846[2] byJohann Gottfried Galle within adegree of the position predicted by Le Verrier. Its largest moon,Triton, was discovered shortly thereafter, though none of the planet'sremaining moons were located telescopically until the 20th century.
The planet's distance fromEarth gives it a smallapparent size, and its distance from the Sun renders it very dim, making it challenging to study with Earth-based telescopes. Only the advent of theHubble Space Telescope and of large ground-based telescopes withadaptive optics allowed for detailed observations.Voyager 2, whichflew by Neptune on 25 August 1989, remains the only spacecraft to ever visit the planet.[22][23] Like thegas giants (Jupiter andSaturn), Neptune's atmosphere is composed primarily ofhydrogen andhelium, along with traces ofhydrocarbons and possiblynitrogen, but contains a higher proportion ofices such as water,ammonia andmethane. Similar to Uranus, its interior is primarily composed of ices and rock;[24] both planets are normally considered "ice giants" to distinguish them.[25] Along withRayleigh scattering, traces of methane in the outermost regions make Neptune appear faintly blue.[26][27]
In contrast to the strongly seasonal atmosphere of Uranus, which can be featureless for long periods of time, Neptune's atmosphere has active and consistently visible weather patterns. At the time of theVoyager 2 flyby in 1989, the planet's southern hemisphere had aGreat Dark Spot comparable to theGreat Red Spot on Jupiter. In 2018, a newer main dark spot and smaller dark spot were identified and studied.[28] These weather patterns are driven by the strongest sustained winds of any planet in the Solar System, as high as 2,100 km/h (580 m/s; 1,300 mph).[29] 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 55 K (−218 °C; −361 °F). Temperatures at the planet's centre are approximately 5,400 K (5,100 °C; 9,300 °F).[30][31] Neptune hasa faint and fragmented ring system (labelled "arcs"), discovered in 1984 and confirmed byVoyager 2.[32]
Some of the earliest knowntelescopic observations ever, Galileo's drawings on 28 December 1612 and 27 January 1613 (New Style) contain plotted points that match what is now known to have been the positions of Neptune on those dates. Both times, Galileo seems to have mistaken Neptune for afixed star when it appeared close—inconjunction—to Jupiter in thenight sky.[33] 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 turnedretrograde 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.[34] In 2009, a study suggested that Galileo was at least aware that the "star" he had observed had moved relative to fixed stars.[35]
In 1821,Alexis Bouvard published astronomical tables of theorbit ofUranus.[36] Subsequent observations revealed substantial deviations from the tables, leading Bouvard to hypothesize that an unknown body wasperturbing the orbit throughgravitational interaction.[37] In 1843,John Couch Adams began work on the orbit of Uranus using the data he had. He requested extra data from SirGeorge Airy, theAstronomer Royal, who supplied it in February 1844. Adams continued to work in 1845–1846 and produced several different estimates for the position of an undiscovered planet beyond Uranus.[38][39]
Independently from Adams,Urbain Le Verrier developed his own calculations in 1845–1846 that pointed to an undiscovered planet, but aroused no enthusiasm among his compatriots. In June 1846, upon seeing Le Verrier's first published estimate of a suspected undiscovered planet's longitude and its similarity to Adams's estimate, Airy persuadedJames Challis to search for it. Challis vainly scoured the sky throughout August and September.[37][40] Challis had, in fact, observed Neptune a year before the planet's subsequent discoverer,Johann Gottfried Galle, 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 recognize 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.[41][37][42]
Meanwhile, Le Verrier sent a letter and urgedBerlin Observatory astronomer Galle to search with the observatory'srefractor.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 aplanet, as opposed to a fixed star. On the evening of 23 September 1846, the day Galle received the letter, he discovered Neptune just northeast ofIota Aquarii, 1° from the "five degrees east ofDelta Capricorn" position Le Verrier had predicted it to be,[43][44] about 12° from Adams's prediction, and on the border ofAquarius andCapricornus according to the modernIAU constellation boundaries.
The 9" refractor used by Galle to discover Neptune
In the wake of the discovery, there was a 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.[45] 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 theRoyal Observatory, Greenwich.[46][47]
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 nameJanus. In England, Challis put forward the nameOceanus.[48]
Claiming the right to name his discovery, Le Verrier quickly proposed the nameNeptune for this new planet, though falsely stating that this had been officially approved by the FrenchBureau des Longitudes.[49] In October, he sought to name the planetLe Verrier, after himself, and he had loyal support in this from the observatory director,François Arago. This suggestion met with stiff resistance outside France.[50] French almanacs quickly reintroduced the nameHerschel for Uranus, after that planet's discoverer Sir William Herschel, andLeverrier for the new planet.[51]
Struve came out in favour of the nameNeptune on 29 December 1846, to theSaint Petersburg Academy of Sciences,[52] after the colour of the planet as viewed through a telescope.[53] Soon,Neptune became the internationally accepted name. InRoman mythology,Neptune was the god of the sea, identified with the GreekPoseidon. The demand for a mythological name seemed to be in keeping with the nomenclature of the other planets, all of which were named for deities inGreek and Roman mythology.[f][54]
Most languages today use some variant of the name "Neptune" for the planet. In Chinese, Vietnamese, Japanese, and Korean, the planet's name was translated as "sea king star" (海王星).[55][56] InMongolian, Neptune is calledDalain van (Далайн ван), reflecting its namesake god's role as the ruler of the sea. In modernGreek, the planet is calledPoseidon (Ποσειδώνας,Poseidonas), the Greek counterpart of Neptune.[57]In Hebrew,Rahab (רהב), froma Biblical sea monster mentioned in theBook of Psalms, was selected in a vote managed by theAcademy of the Hebrew Language in 2009 as the official name for the planet, even though the existing Latin termNeptun (נפטון) is commonly used.[58][59] InMāori, the planet is calledTangaroa, named after theMāori god of the sea.[60] InNahuatl, the planet is calledTlāloccītlalli, named after the rain godTlāloc.[60] InThai, Neptune is referred to by the Westernised nameDao Nepchun/Nepjun (ดาวเนปจูน) but is also calledDao Ket (ดาวเกตุ,lit.'star of Ketu'), afterKetu (केतु), the descendinglunar node, who plays a role inHindu astrology. InMalay, the nameWaruna, after theHindu god of seas, is attested as far back as the 1970s,[61] but was eventually superseded by the Latinate equivalentsNeptun (inMalaysian[62]) orNeptunus (inIndonesian[63]).
The usual adjectival form isNeptunian. Thenonce formPoseidean (/pəˈsaɪdiən/), fromPoseidon, has also been used,[5] though the usual adjectival form ofPoseidon isPoseidonian (/ˌpɒsaɪˈdoʊniən/).[64]
Status
From its discovery in 1846 until thediscovery of Pluto in 1930, Neptune was the farthest known planet. WhenPluto 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, making Neptune the ninth planet from the Sun during this period.[65][66] The increasingly accurate estimations of Pluto's mass from ten timesthat of Earth's to far less thanthat of the Moon[67] and the discovery of theKuiper belt in 1992 led many astronomers to debate whether Pluto should be considered a planet or as part of the Kuiper belt.[68][69] In 2006, theInternational Astronomical Uniondefined the word "planet" for the first time, reclassifying Pluto as a "dwarf planet" and making Neptune once again the outermost-known planet in the Solar System.[70]
Physical characteristics
A size comparison of Neptune and Earth
Neptune's mass of 1.024×1026 kg[8] is intermediate between Earth and the largergas giants: it is 17.15 times that of Earth but just 1/19ththat of Jupiter.[g] Itsgravity at 1 bar is 11.27 m/s2, 1.15 times thesurface gravity of Earth,[8] and surpassed only by Jupiter.[71] Neptune'sequatorial radius of 24,764 km[11] is nearly four timesthat of Earth. Neptune, likeUranus, is anice giant, a subclass ofgiant planet, because they are smaller and have higher concentrations ofvolatiles than Jupiter and Saturn.[72] In the search forexoplanets, Neptune has been used as ametonym: discovered bodies of similar mass are often referred to as "Neptunes",[73] just as scientists refer to various extrasolar bodies as "Jupiters".
Neptune's internal structure resemblesthat of Uranus. Its atmosphere forms about 5 to 10% of its mass and extends perhaps 10 to 20% of the way towards the core. Pressure in the atmosphere reaches about 10 GPa, or about 105 atmospheres. Increasing concentrations ofmethane,ammonia and water are found in the lower regions of the atmosphere.[30]
Illustration of the physical component of Neptune's interior and its surroundings infalse colours
The mantle is equivalent to 10 to 15 Earth masses and is rich in water, ammonia and methane.[2] As is customary in planetary science, this mixture is calledicy even though it is a hot, densesupercritical fluid. This fluid, which has a high electrical conductivity, is sometimes called a water–ammonia ocean.[74] The mantle may consist of a layer of ionic water in which the water molecules break down into a soup of hydrogen andoxygenions, and deeper downsuperionic water in which the oxygen crystallizes but thehydrogen ions float around freely within the oxygen lattice.[75] At a depth of 7,000 km, the conditions may be such that methane decomposes into diamond crystals that rain downwards like hailstones.[76][77][78] Scientists believe that this kind of diamond rain occurs on Jupiter, Saturn, and Uranus.[79][77] Very-high-pressure experiments atLawrence Livermore National Laboratory suggest that the top of the mantle may be an ocean of liquid carbon with floating solid 'diamonds'.[80][81][82]
Thecore of Neptune is likely composed of iron, nickel andsilicates, with an interiormodel giving a mass about 1.2x that of Earth.[24] The pressure at the centre is 7 Mbar (700 GPa), about twice as high as that at the centre of Earth, and the temperature may be 5,400 K (5,100 °C; 9,300 °F).[30][31]
Bands of high-altitude clouds cast shadows on Neptune's lower cloud deck. The colour is exaggerated to show the clouds more clearly.
At high altitudes, Neptune's atmosphere is 80%hydrogen and 19%helium.[30] A trace amount ofmethane is 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 atmospheric methane is part of what gives Neptune its faint blue hue,[84] which is more pronounced for Neptune's due to concentrated haze in Uranus's atmosphere.[85][86]
Neptune's atmosphere is subdivided into two main regions: the lowertroposphere, where temperature decreases with altitude, and thestratosphere, where temperature increases with altitude. The boundary between the two, thetropopause, lies at a pressure of 0.1 bars (10 kPa).[25] The stratosphere then gives way to thethermosphere at a pressure lower than 10−5 to 10−4 bars (1 to 10 Pa).[25] The thermosphere gradually transitions to theexosphere.[87]
Models suggest that Neptune's troposphere is banded by clouds of varying compositions depending on altitude.[83] 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 ofammonia andhydrogen sulfide are thought 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 (5.0 MPa), where the temperature reaches 273 K (0 °C; 32 °F). Underneath, clouds of ammonia and hydrogen sulfide may be found.[88]
High-altitude clouds on Neptune have been observed casting shadows on the opaque cloud deck below. There are high-altitude cloud bands that wrap around the planet at constant latitudes. These circumferential bands have widths of 50–150 km and lie about 50–110 km above the cloud deck.[89] These altitudes are in the layer where weather occurs, the troposphere. Weather does not occur in the higher stratosphere or thermosphere. In August 2023, the high-altitude clouds of Neptune vanished, prompting a study spanning thirty years of observations by the Hubble Space Telescope and ground-based telescopes. The study found that Neptune's high-altitude cloud activity is bound toSolar cycles, and not to the planet's seasons.[83][90][91]
Neptune'sspectra suggest that its lower stratosphere is hazy due to condensation of products of ultravioletphotolysis of methane, such asethane andethyne.[25][30] The stratosphere is home to trace amounts ofcarbon monoxide andhydrogen cyanide.[25][92] The stratosphere of Neptune is warmer than that of Uranus due to the elevated concentration of hydrocarbons.[25]
For reasons that remain obscure, the planet's thermosphere is at an anomalously high temperature of about 750 K (477 °C; 890 °F).[93][94] The planet is too far from the Sun for this heat to be generated byultraviolet radiation. One candidate for a heating mechanism is atmospheric interaction with ions in the planet'smagnetic field. Other candidates aregravity waves from the interior that dissipate in the atmosphere. The thermosphere contains traces ofcarbon dioxide and water, which may have been deposited from external sources such asmeteorites and dust.[88][92]
Colour
Neptune's atmosphere is faintly blue in theoptical spectrum, only slightly more saturated than the blue of Uranus's atmosphere. Early renderings of the two planets greatly exaggerated Neptune's colour contrast "to better reveal the clouds, bands and winds", making it seem deep blue compared to Uranus's off-white. The two planets had been imaged with different systems, making it hard to directly compare the resultingcomposite images. This was revisited with the colour normalised over time, most comprehensively in late 2023.[86][95]
Original 2-colour (orange-green) NASA/JPL image fromVoyager 2, with exaggerated colour[96]
Colour recalibrated in 2016 (Justin Cowart), preserving some enhancement for contrast[97]
Colour recalibrated in 2023 (Patrick Irwin), approximating the true colour[98]
Magnetosphere
Neptune'smagnetosphere consists of amagnetic field that is strongly tilted relative to itsrotational axis at 47° and offset of at least 0.55 radius (~13,500 km) from the planet's physical centre—resembling Uranus's magnetosphere. Before the arrival ofVoyager 2 to Neptune, it was hypothesised that Uranus's sideways rotation caused its tilted magnetosphere. 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 byconvective fluid motions in a thin spherical shell ofelectrically conducting liquids (probably a combination of ammonia, methane and water),[88] resulting in adynamo action.[99]
The dipole component of the magnetic field at the magnetic equator of Neptune is about 14 microteslas (0.14 G).[100] The dipolemagnetic moment of Neptune is about 2.2 × 1017 T·m3 (14 μT·RN3, whereRN is the radius of Neptune). Neptune's magnetic field has a complex geometry that includes relatively large contributions from non-dipolar components, including a strongquadrupole moment that may exceed thedipole 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 an offset from the planet's centre and geometrical constraints of the field's dynamo generator.[101][102]
Neptune'sbow shock, where the magnetosphere begins to slow thesolar wind, occurs at a distance of 34.9 times the radius of the planet. Themagnetopause, 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.[101]
Left: a visible light image of Neptune taken by the Hubble Space Telescope.Right: the Hubble image composited with a near-infrared image taken by the James Webb Telescope. As the aurorae cannot be observed in the visible band, their image in near-infrared has been rendered as shades of cyan.
Measurements byVoyager 2 in extreme-ultraviolet and radio frequencies revealed that Neptune has faint and weak but complex and uniqueaurorae; however, these observations were limited in time and did not contain infrared. Subsequent astronomers using the Hubble Space Telescope have not glimpsed the aurorae, in contrast to the more well-defined aurorae of Uranus.[103][104] In March 2025, aurorae on Neptune were pictured for the first time by combiningvisible light images from the Hubble Space Telescope withnear-infrared (NIR) images from theJames Webb Space Telescope. The relevant data were taken in June 2023. The James Webb Space Telescope attempted to learn thespectrography of Neptune's atmosphere and it was able to findtrihydrogen cations (H+ 3) which is generated during an aurora and is considered as a clear indicator of auroral activity on both gas giants and ice giants. The nature of Neptune's aurorae is greatly influenced by the peculiar nature of its magnetic field. Unlike theEarth,Jupiter orSaturn, Neptune's magnetic poles are not aligned with the planet's rotational poles which is why Neptune's aurorae mostly occur around its mid-latitude areas instead of its poles like on Earth or Jupiter.[105]
Neptune's weather is characterized by extremely dynamic storm systems, with winds reaching speeds of almost 600 m/s (2,200 km/h; 1,300 mph)—exceedingsupersonic flow.[29] 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.[107] At the cloud tops, the prevailing winds range in speed from 400 m/s along the equator to 250 m/s at the poles.[88] Most of the winds on Neptune move in a direction opposite the planet's rotation.[108] 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.[25] At 70°S latitude, a high-speed jet travels at a speed of 300 m/s.[25] Due to seasonal changes, the cloud bands in the southern hemisphere of Neptune have been observed to increase in size andalbedo. This trend was first seen in 1980. The long orbital period of Neptune results in seasons lasting 40 Earth years.[109]
Neptune differs from Uranus in its typical level ofmeteorological activity.Voyager 2 observed weather phenomena on Neptune during its 1989 flyby,[110] but no comparable phenomena on Uranus during its 1986 flyby.
The abundance of methane, ethane andacetylene 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, as photochemistry cannot account for the distribution without meridional circulation.[25]
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 about 73 K (−200 °C). The temperature differential is enough to let methane, which elsewhere is frozen in the troposphere, escape into the stratosphere near the pole.[111] The relative "hot spot" is due to Neptune'saxial tilt, which has exposed the south pole to theSun 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.[112]
Storms
The Great Dark Spot imaged byVoyager 2 in an enhanced colour image
In 1989, theGreat Dark Spot, ananticyclonic storm system spanning 13,000 km × 6,600 km (8,100 mi × 4,100 mi),[110] was discovered byNASA'sVoyager 2 spacecraft. The storm resembled theGreat Red Spot of Jupiter. Some five years later, on 2 November 1994, theHubble 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 Neptune's northern hemisphere.[113]
TheScooter is another storm, a white cloud group farther south than the Great Dark Spot. This nickname first arose during the months leading up to theVoyager 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 byVoyager 2).[108] TheSmall Dark Spot is a southern cyclonic storm, the second-most-intense storm observed during the 1989 encounter. It was initially completely dark, but asVoyager 2 approached the planet, a bright core developed, which can be seen in most of the highest-resolution images.[114] In 2018, a newer main dark spot and smaller dark spot were identified and studied.[28] In 2023, the first ground-based observation of a dark spot on Neptune was announced.[115]
The Northern Great Dark Spot imaged by Hubble in 2018
The Northern Great Dark Spot and a smaller companion storm imaged by Hubble in 2020
Neptune's SDS-2015[116] vortex in around two years, which has appeared to be shrinking and disappearing
Neptune's dark spots are thought to occur in thetroposphere at lower altitudes than the brighter cloud features,[117] 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 bevortex structures.[89] Often associated with dark spots are brighter, persistent methane clouds that form around thetropopause layer.[118] 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.[119]
Four images taken a few hours apart with the NASA/ESAHubble Space Telescope'sWide Field Camera 3. Near-infrared radiation data has been used as a red channel.[120]
In 1989,Voyager 2'sPlanetary Radio Astronomy (PRA) experiment observed around 60 lightning flashes, or Neptunian electrostatic discharges emitting energies over7×108J.[121] A plasma wave system (PWS) detected 16 electromagnetic wave events with a frequency range of50–12 kHz at magnetic latitudes 7–33˚.[122][123] These plasma wave detections were possibly triggered by lightning over 20 minutes in the ammonia clouds of the magnetosphere.[123]
DuringVoyager 2's closest approach to Neptune, the PWS instrument provided Neptune's first plasma wave detections at a sample rate of 28,800 samples per second.[123] The measured plasma densities range from10−3 – 10−1 cm3.[123][124] Neptunian lightning may occur in three cloud layers,[125] with microphysical modelling suggesting that most of these occurrences happen in the water clouds of the troposphere or the shallow ammonia clouds of the magnetosphere.[122][126] Neptune is predicted to have 1/19 the lightning flash rate of Jupiter and to display most of its lightning activity at high latitudes. However, lightning on Neptune seems to resemble lightning on Earth rather than Jovian lightning.[121]
Internal heating
Neptune's more varied weather when compared to Uranus is due in part to its higherinternal heating. The upper regions of Neptune's troposphere reach a low temperature of 51.8 K (−221.3 °C). At a depth where theatmospheric pressure equals 1 bar (100 kPa), the temperature is 72.00 K (−201.15 °C).[127] 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;[128] whereas Neptune radiates about 2.61 times as much energy as it receives from the Sun.[16]
Neptune is over 50% farther from the Sun than Uranus and receives only ~40% of Uranus's amount of sunlight;[25] however, its internal energy is still enough for the fastest planetary winds 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 harder to explain Uranus's lack of internal heat while preserving the apparent similarity between the two planets.[129]
Orbit and rotation
Neptune (red arc) completes one orbit around the Sun (centre) for every 164.79 orbits of Earth. The light blue dot represents Uranus.
The average distance between Neptune and the Sun is4.5 billion km (about 30.1 astronomical units (AU), the mean distance from the Earth to the Sun), 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, and the aphelion distance is 30.33 AU.[h] Neptune'sorbital eccentricity is only 0.008678, making it the planet in the Solar System with the second most circular orbit afterVenus.[131] The orbit of Neptune isinclined 1.77° compared to that of Earth.
On 11 July 2011, Neptune completed its first fullbarycentric orbit since its discovery in 1846;[132] 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 barycentre of the Solar System, on 11 July, Neptune was not at its exact discovery position in relation to the Sun—if the more commonheliocentric coordinate system is used, the discovery longitude was reached on 12 July 2011.[12][133][134][135]
Theaxial tilt of Neptune is 28.32°,[136] which is similar to the tilts of Earth (23°) and Mars (25°). As a result, Neptune experiences seasonal changes similar to those on Earth. The long orbital period of Neptune means that the seasons last for forty Earth years.[109] Its sidereal rotation period (day) is roughly 16.11 hours.[12] 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 undergoesdifferential 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,[137] and it results in strong latitudinal wind shear.[89]
A simulation showing the outer planets and Kuiper belt: a) before Jupiter and Saturn reached a 2:1 resonance; b) after inward scattering of Kuiper belt objects following the orbital shift of Neptune; c) after ejection of scattered Kuiper belt bodies by Jupiter
The formation of the ice giants, Neptune and Uranus, has been 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 coreaccretion, 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 originalprotoplanetary disc and later had their atmospheres blasted away by radiation from a nearby massiveOB star.[72]
An alternative concept is that they formed closer to the Sun, where the matter density was higher, and then subsequentlymigrated to their current orbits after the removal of the gaseous protoplanetary disc.[138] 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.[139] The current most widely accepted[140][141][142] explanation of the details of this hypothesis is known as theNice model, which is adynamical evolution scenario that explores the potential effect of a migrating Neptune and the other giant planets on the structure of the Kuiper belt.
A diagram showing the major orbital resonances in theKuiper belt caused by Neptune: the highlighted regions are the 2:3 resonance (plutinos), the nonresonant"classical belt" (cubewanos), and the 1:2 resonance (twotinos).
Neptune's orbit has a profound impact on the region directly beyond it, known as theKuiper belt. The Kuiper belt is a ring of small icy worlds, similar to theasteroid belt but far larger, extending from Neptune's orbit at 30 AU out to about 55 AU from the Sun.[143] Much in the same way thatJupiter's gravity dominates the asteroid belt, 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 its structure. The region between 40 and 42 AU is an example.[144]
There do exist orbits within these empty regions where objects can survive for the age of the Solar System. Theseresonances 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,[145] is the 2:3 resonance. Objects in this resonance complete 2 orbits for every 3 of Neptune, and are known asplutinos because the largest of the known Kuiper belt objects,Pluto, is among them.[146] Although Pluto crosses Neptune's orbit regularly, the 2:3 resonance makes it so that they can never collide.[147] The 3:4, 3:5, 4:7 and 2:5 resonances are less populated.[148]
Neptune has a number of knowntrojan objects occupying both theSun–NeptuneL4 andL5Lagrangian points—gravitationally stable regions leading and trailing Neptune in its orbit, respectively.[149]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 beingcaptured. The first object identified as associated with Neptune's trailingL5 Lagrangian point was2008 LC18.[150] Neptune has a temporaryquasi-satellite,(309239) 2007 RW10.[151] 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.[151]
Neptune has 16 knownmoons.[152]Triton is the largest Neptunian moon, accounting for more than 99.5% of the mass in orbit around Neptune,[i] and is the only one massive enough to bespheroidal. Triton was discovered byWilliam Lassell just 17 days after the discovery of Neptune itself. Unlike all other large planetary moons in the Solar System, Triton has aretrograde orbit, indicating that it was captured rather than forming in place; it was probably once adwarf planet in the Kuiper belt.[153] It is close enough to Neptune to be locked into asynchronous rotation, and it is slowly spiralling inward because oftidal acceleration. It will eventually be torn apart, in about 3.6 billion years, when it reaches theRoche limit.[154] In 1989, Triton was the coldest object that had yet been measured in the Solar System,[155] with estimated temperatures of 38 K (−235 °C).[156][157] This very low temperature is due to Triton's very high albedo which causes it to reflect a lot of sunlight instead of absorbing it.[158][159]
Neptune's second-known satellite (by order of discovery), the irregular moonNereid, has one of the most eccentric orbits of any satellite in the Solar System. The eccentricity of 0.7512 gives it anapoapsis that is seven times itsperiapsis distance from Neptune.[j]
From July to September 1989,Voyager 2 discovered six moons of Neptune.[160] Of these, the irregularly shapedProteus 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.[161] Although the second-most-massive Neptunian moon, it is only 0.25% the mass of Triton. Neptune's innermost four moons—Naiad,Thalassa,Despina andGalatea—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 whenVoyager 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.[162][163] A new moon and the smallest yet,Hippocamp, was found in 2013 by combining multiple Hubble images.[164] Because Neptune was the Roman god of the sea, Neptune's moons have been named after lesser sea gods.[54]
Neptune has aplanetary ring system, though one much less substantial than that ofSaturn andUranus.[165] The rings may consist of ice particles coated with silicates or carbon-based material, which most likely gives them a reddish hue.[166] 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.[167]
The first of these planetary rings was detected in 1968 by a team led byEdward Guinan.[32][168] In the early 1980s, analysis of this data along with newer observations led to the hypothesis that this ring might be incomplete.[169] Evidence that the rings might have gaps first arose during a stellaroccultation in 1984 when the rings obscured a star on immersion but not on emersion.[170] Images fromVoyager 2 in 1989 settled the issue by showing several faint rings.
The outermost ring, Adams, contains five prominent arcs now namedCourage,Liberté,Egalité 1,Egalité 2 andFraternité (Courage, Liberty, Equality and Fraternity).[171] 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 ofGalatea, a moon just inward from the ring.[172][173]
Earth-based observations announced in 2005 appeared to show that Neptune's rings were much more unstable than previously thought. Images taken from theW. M. Keck Observatory in 2002 and 2003 show considerable decay in the rings when compared to images byVoyager 2. In particular, it seems that theLiberté arc might disappear in as little as one century.[174]
Observation
Movement of Neptune in front of the stars ofAquarius in 2022
Neptune brightened about 10% between 1980 and 2000 mostly due to the changing of the seasons.[175] Neptune may continue to brighten as it approaches perihelion in 2042. Theapparent magnitude currently ranges from 7.67 to 7.89 with a mean of 7.78 and a standard deviation of 0.06.[18] Prior to 1980, the planet was as faint as magnitude 8.0.[18] Neptune is too faint to be visible to thenaked eye. It can be outshone by Jupiter'sGalilean moons, thedwarf planetCeres and theasteroids4 Vesta,2 Pallas,7 Iris,3 Juno, and6 Hebe.[176] A telescope or strong binoculars will resolve Neptune as a small blue disk, similar in appearance to Uranus.[177]
Because of the distance of Neptune from Earth, itsangular diameter only ranges from 2.2 to 2.4 arcseconds,[8][20] 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 theHubble Space Telescope and large ground-based telescopes withadaptive optics (AO).[178][179][180] The first scientifically useful observation of Neptune from ground-based telescopes using adaptive optics was commenced in 1997 from Hawaii.[181] Neptune is currently approaching perihelion (closest approach to the Sun) 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. BothHubble 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.[182]
From Earth, Neptune goes throughapparent retrograde motion every 367 days, resulting in a looping motion against the background stars during eachopposition. These loops carried it close to the 1846 discovery coordinates in April and July 2010 and again in October and November 2011.[135]
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 ofIota Aquarii.[43]
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.[88] In theinfrared 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.[183]
An animation ofVoyager 2's trajectory from 20 August 1977 to 30 December 2000Voyager 2·Earth·Jupiter·Saturn·Uranus· Neptune·Sun
Voyager 2 is the only spacecraft that has visited Neptune. The spacecraft's 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 moonTriton, regardless of the consequences to the trajectory, similarly to what was done forVoyager 1's encounter withSaturn and its moonTitan. The images relayed back to Earth fromVoyager 2 became the basis of a 1989PBS all-night program,Neptune All Night.[184]
During the encounter, signals from the spacecraft required 246 minutes to reach Earth. Hence, for the most part,Voyager 2's mission relied on preloaded commands for the Neptune encounter. The spacecraft performed a near-encounter with the moonNereid before it came within 4,400 km of Neptune's atmosphere on 25 August, then passed close to the planet's largest moon Triton later the same day.[185]
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 andVoyager 2 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.[160][185] The flyby provided the first accurate measurement of Neptune's mass which was found to be 0.5 per cent less than previously calculated. The new figure disproved the hypothesis that an undiscoveredPlanet X acted upon the orbits of Neptune and Uranus.[186][187]
Since 2018, theChina National Space Administration has been studying a concept for a pair ofVoyager-like interstellar probes tentatively known asShensuo.[188] Both probes would be launched in the 2020s and take differing paths to explore opposing ends of theheliosphere; the second probe,IHP-2, would 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.[189] Afterward, it will continue its mission throughout theKuiper belt toward the heliosphere tail, which is so far unexplored.
AfterVoyager 2 andIHP-2's flybys, the next step in scientific exploration of the Neptunian system is considered to be an orbital mission; most proposals have been byNASA, most often for aFlagship orbiter.[190] In 2003, there was a proposal in NASA's "Vision Missions Studies" for a "Neptune Orbiter with Probes" mission that doesCassini-level science.[191] A subsequent proposal, that was not selected, was forArgo, a flyby spacecraft to be launched in 2019, that would visit Jupiter, Saturn, Neptune, and a Kuiper belt object. The focus would have been on Neptune and its largest moon Triton to be investigated around 2029.[192]
The proposedNew Horizons 2 mission might have done a close flyby of the Neptunian system, but it was later scrapped. Currently a pending proposal for theDiscovery Program, theTrident spacecraft would conduct a flyby of Neptune and Triton;[193] however, the mission was not selected for Discovery 15 or 16.Neptune Odyssey is another concept for a Neptune orbiter and atmospheric probe that was studied as a possiblelarge strategic science mission by NASA; it would have launched between 2031 and 2033, and arrive at Neptune by 2049.[194] However, for logistical reasons theUranus Orbiter and Probe mission was selected as theice giant orbiter mission recommendation, with top priority ahead of theEnceladus Orbilander.[195]
Two notable proposals for a Triton-focused Neptune orbiter mission that would be costed right between theTrident andOdyssey missions (under theNew Frontiers program) areTriton Ocean World Surveyor andNautilus, with cruise stages taking place in the 2031–47 and 2041–56 time periods, respectively.[196][197] Neptune is a potential target for China'sTianwen-5, which could arrive in 2058.[198]
^Neptune's dark spots are not permanent features; the large dark spot observed byVoyager 2 was designated GDS-89 for "Great Dark Spot 1989".
^Orbital elements refer to the Neptune barycentre and Solar System barycentre. These are the instantaneousosculating values at the preciseJ2000 epoch. Barycentre quantities are given because, in contrast to the planetary centre, they do not experience appreciable changes on a day-to-day basis from the motion of the moons.
^abcdefgRefers to the level of 1 bar (100 kPa) atmospheric pressure
^Based on the volume within the level of 1 bar atmospheric pressure
^A second symbol, an 'LV' monogram for 'Le Verrier', analogous to the 'H' monogram for Uranus. It was never much used outside of France and is now archaic.
^One could argue that it is true except for the 'Earth', which in the English language is the name of a Germanic deity,Erda. TheIAU policy is that one may call the Earth and the Moon by any name commonly used in the language. According to the IAU, 'Terra' and 'Luna' arenot the official names of planet Earth and its moon:"Naming of Astronomical Objects".International Astronomical Union.Archived from the original on 21 March 2024. Retrieved27 April 2024.
^The mass of Earth is 5.9722×1024 kg, giving a mass ratio
The mass of Uranus is 8.6810×1025 kg, giving a mass ratio
The mass of Jupiter is 1.8986×1027 kg, giving a mass ratio
Mass values fromWilliams, David R. (29 November 2007)."Planetary Fact Sheet – Metric". NASA.Archived from the original on 5 September 2014. Retrieved13 March 2008.
^The last three aphelia were 30.33 AU, the next is 30.34 AU. The perihelia are even more stable at 29.81 AU.[130]
^Mass of Triton: 2.14×1022 kg. Combined mass of 12 other known moons of Neptune: 7.53×1019 kg, or 0.35%. The mass of the rings is negligible.
^"HORIZONS Planet-center Batch call for September 2042 Perihelion".ssd.jpl.nasa.gov (Perihelion for Neptune's planet-center (899) occurs on 2042-Sep-04 at 29.80647406au during a rdot flip from negative to positive). NASA/JPL.Archived from the original on 7 September 2021. Retrieved7 September 2021.
^de Pater, Imke; Lissauer, Jack J. (2015).Planetary Sciences (2nd updated ed.). New York: Cambridge University Press. p. 250.ISBN978-0-521-85371-2.Archived from the original on 26 November 2016. Retrieved17 August 2016.
^Kennett, Carolyn (2022).Uranus and Neptune. Reaktion Books. p. 185.ISBN978-1-78914-642-4.
^"Exploration | Neptune".NASA Solar System Exploration. 10 November 2017.Archived from the original on 17 July 2020. Retrieved3 February 2020.In 1989, NASA's Voyager 2 became the first-and only-spacecraft to study Neptune up close.
^Munsell, Kirk; Smith, Harman; Harvey, Samantha (13 November 2007)."Neptune overview".Solar System Exploration. NASA. Archived fromthe original on 3 March 2008. Retrieved20 February 2008.
^Levenson, Thomas (2015).The Hunt for Vulcan ... and how Albert Einstein Destroyed a Planet, Discovered Relativity, and Deciphered the Universe. Random House. p. 38.
^Baum, Richard; Sheehan, William (2003).In Search of Planet Vulcan: The ghost in Newton's clockwork universe. Basic Books. pp. 109–10.ISBN978-0-7382-0889-3.
^Gingerich, Owen (October 1958). "The naming of Uranus and Neptune".Astronomical Society of the Pacific Leaflets.8 (352):9–15.Bibcode:1958ASPL....8....9G.
^"Greek Names of the Planets". 25 April 2010.Archived from the original on 9 May 2010. Retrieved14 July 2012.Neptune orPoseidon as is its Greek name, was the God of the Seas. It is the eight planet from the sun ...
^Mohamed, Kadir (1975)."Waruna".Kamus Kebangsaan Ejaan Baru, Inggeris-bahasa Malaysia, Bahasa Malaysia-Inggeris. Titiwangsa. pp. 299, 857.Archived from the original on 29 September 2021. Retrieved29 May 2021.
^"Neptun".Kamus Dewan (4th ed.). Dewan Bahasa dan Pustaka Malaysia. 2017.Archived from the original on 7 May 2021. Retrieved5 May 2021.
^"Neptunus".Kamus Besar Bahasa Indonesia (3rd ed.). Badan Pengembangan dan Pembinaan Bahasa Indonesia. 2016.Archived from the original on 29 September 2021. Retrieved5 May 2021.
^Unsöld, Albrecht; Baschek, Bodo (2001).The New Cosmos: An introduction to astronomy and astrophysics (5th ed.). Springer. Table 3.1, page 47.Bibcode:2001ncia.book.....U.ISBN978-3-540-67877-9.
^Chavez, Erandi; de Pater, Imke; Redwing, Erin; Molter, Edward M.; Roman, Michael T.; Zorzi, Andrea; Alvarez, Carlos; Campbell, Randy; de Kleer, Katherine; Hueso, Ricardo; Wong, Michael H.; Gates, Elinor; Lynam, Paul David; Davies, Ashley G.; Aycock, Joel; Mcilroy, Jason; Pelletier, John; Ridenour, Anthony; Stickel, Terry (1 November 2023)."Evolution of Neptune at near-infrared wavelengths from 1994 through 2022".Icarus.404 115667.arXiv:2307.08157.Bibcode:2023Icar..40415667C.doi:10.1016/j.icarus.2023.115667.S2CID259515455.Archived from the original on 24 August 2023. Retrieved24 August 2023.The clear positive correlation we find between cloud activity and Solar Lyman-Alpha (121.56 nm) irradiance lends support to the theory that the periodicity in Neptune's cloud activity results from photochemical cloud/haze production triggered by Solar ultraviolet emissions.
^Connerney, J.E.P.; Acuña, Mario H.; Ness, Norman F. (1991). "The magnetic field of Neptune".Journal of Geophysical Research.96: 19,023–42.Bibcode:1991JGR....9619023C.doi:10.1029/91JA01165.
^SDS-2015 meant that it was a Southern Dark Spot that was discovered in 2015. H. Wong, Michael; Tollefson, Joshua; I. Hsu, Andrew; de Pater, Imke; A. Simon, Amy; Hueso, Ricardo; Sánchez-Lavega, Agustín; Sromovsky, Lawrence; Fry, Patrick; Luszcz-Cook, Statia (15 February 2018)."A New Dark Vortex on Neptune".The American Astronomical Society.155 (3). Abstract section.doi:10.3847/1538-3881 (inactive 1 July 2025).{{cite journal}}: CS1 maint: DOI inactive as of July 2025 (link)
^Sromovsky, L.A.; Fry, P.M.; Dowling, T.E.; Baines, K.H. (2000). "The unusual dynamics of new dark spots on Neptune".Bulletin of the American Astronomical Society.32: 1005.Bibcode:2000DPS....32.0903S.
^Pater, Imke de; Lissauer, Jack J. (6 December 2001).Planetary Sciences. Cambridge University Press. p. 224.ISBN978-0-521-48219-6.Archived from the original on 29 September 2021. Retrieved15 March 2023.
^Meeus, Jean (1998).Astronomical Algorithms. Richmond, VA: Willmann-Bell. p. 273. Supplemented by further use of VSOP87.
^ab"Horizons Output for Neptune 2010–2011". 16 November 2007. Archived fromthe original on 2 May 2013. Retrieved25 February 2008.—Numbers generated using the Solar System Dynamics Group, Horizons On-Line Ephemeris System.
^Williams, David R. (6 January 2005)."Planetary Fact Sheets". NASA.Archived from the original on 25 September 2008. Retrieved28 February 2008.
^"New Uranus and Neptune Moons".Earth & Planetary Laboratory. Carnegie Institution for Science. 23 February 2024.Archived from the original on 23 February 2024. Retrieved23 February 2024.
^Chyba, Christopher F.; Jankowski, D.G.; Nicholson, P.D. (1989). "Tidal evolution in the Neptune-Triton system".Astronomy and Astrophysics.219 (1–2):L23 –L26.Bibcode:1989A&A...219L..23C.
^Guinan, E.F.; Harris, C.C.; Maloney, F.P. (1982). "Evidence for a Ring System of Neptune".Bulletin of the American Astronomical Society.14: 658.Bibcode:1982BAAS...14..658G.
^Munsell, Kirk; Smith, Harman; Harvey, Samantha (13 November 2007)."Planets: Neptune: Rings".Solar System Exploration. NASA.Archived from the original on 4 July 2012. Retrieved29 February 2008.
^In 1977, for example, even the rotation period of Neptune remained uncertain.Cruikshank, D.P. (1 March 1978). "On the rotation period of Neptune".Astrophysical Journal Letters.220:L57 –L59.Bibcode:1978ApJ...220L..57C.doi:10.1086/182636.
^Max, C.; MacIntosh, B.; Gibbard, S.; Roe, H.; et al. (1999). "Adaptive Optics Imaging of Neptune and Titan with the W.M. Keck Telescope".Bulletin of the American Astronomical Society.31: 1512.Bibcode:1999AAS...195.9302M.
^Spilker, T.R.; Ingersoll, A.P. (2004). "Outstanding Science in the Neptune System From an Aerocaptured Vision Mission".Bulletin of the American Astronomical Society.36: 1094.Bibcode:2004DPS....36.1412S.
^Steckel, Amanda; Conrad, Jack William; Dekarske, Jason; Dolan, Sydney; Downey, Brynna Grace; Felton, Ryan; Hanson, Lavender Elle; Giesche, Alena; Horvath, Tyler; Maxwell, Rachel; Shumway, Andrew O; Siddique, Anamika; Strom, Caleb; Teece, Bronwyn; Todd, Jessica; Trinh, Kevin T; Velez, Michael A; Walter, Callum Andrew; Lowes, Leslie L; Hudson, Troy; Scully, Jennifer E. C. (12 December 2023)."The Science Case for Nautilus: A Multi-Flyby Mission Concept to Triton".AGU - Agu23 (3089). AGU.Bibcode:2023AGUFM.P23D3089S.Archived from the original on 11 January 2024. Retrieved11 January 2024.
Miner, Ellis D.; Wessen, Randii R. (2002).Neptune: The Planet, Rings, and Satellites. Springer-Praxis books in astronomy and space sciences. London; New York:Springer.ISBN978-1-85233-216-7.
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