Neptune Neptune symbol.svg
Neptune Full.jpg
Neptune's Great Dark Spot and its companion bright smudge; on the west limb the fast moving bright feature called Scooter and the little dark spot are visible.
Discovery [1]
Discovered by
Discovery date 23 September 1846
Pronunciation n/
n/ [2]
Adjectives Neptunian
Orbital characteristics [6] [a]
Epoch J2000
Aphelion 30.33  AU (4.54 billion km)
Perihelion 29.81  AU (4.46 billion km)
30.110387  AU (4.50 billion km)
Eccentricity 0.009456
367.49 days [4]
5.43 km/s [4]
Inclination 1.767975° to ecliptic
6.43° to Sun's equator
0.72° to invariable plane [5]
Known satellites 14
Physical characteristics
Mean radius
24,622±19 km [7] [b]
Equatorial radius
24,764±15 km [7] [b]
3.883 Earths
Polar radius
24,341±30 km [7] [b]
3.829 Earths
Flattening 0.0171±0.0013
7.6183×109 km2 [8] [b]
14.98 Earths
Volume 6.254×1013 km3 [4] [b]
57.74 Earths
Mass 1.0243×1026 kg [4]
17.147 Earths
5.15×105 Suns
Mean density
1.638 g/cm3 [4] [c]
11.15  m/s2 [4] [b]
1.14  g
0.23 [9] (estimate)
23.5 km/s [4] [b]
0.6713 day [4]
16 h 6 min 36 s
Equatorial rotation velocity
2.68 km/s (9,650 km/h)
28.32° (to orbit) [4]
North pole right ascension
19h 57m 20s [7]
North pole declination
42.950° [7]
Albedo 0.290 ( bond)
0.41 ( geom.) [4]
Surface temp. min mean max
1 bar level 72  K (−201 °C) [4]
0.1 bar (10 kPa) 55  K (−218 °C) [4]
8.02 to 7.78 [4] [10]
2.2–2.4″ [4] [10]
Atmosphere [4]
19.7±0.6 km
Composition by volume

Neptune is the eighth and farthest known planet from the Sun in the Solar System. In the Solar System, it is the fourth-largest planet by diameter, the third-most-massive planet, and the densest giant planet. Neptune is 17 times the mass of Earth and is slightly more massive than its near-twin Uranus, which is 15 times the mass of Earth and slightly larger than Neptune. [d] Neptune orbits the Sun once every 164.8  years at an average distance of 30.1 astronomical units (4.50×109 km). It is named after the Roman god of the sea and has the astronomical symbol ♆, a stylised version of the god Neptune's trident.

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. Unexpected changes in the orbit of Uranus led Alexis Bouvard to deduce that its orbit was subject to gravitational perturbation by an unknown planet. Neptune was subsequently observed with a telescope on 23 September 1846 [1] by Johann Galle within a degree of the position predicted by Urbain Le Verrier. Its largest moon, Triton, was discovered shortly thereafter, though none of the planet's remaining known 13 moons 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, when it flew by the planet on 25 August 1989. [11] The advent of the Hubble Space Telescope and large ground-based telescopes with adaptive optics has recently allowed for additional detailed observations from afar.

Like Jupiter and Saturn, Neptune's atmosphere is composed primarily of hydrogen and helium, along with traces of hydrocarbons and possibly nitrogen, but it contains a higher proportion of "ices" such as water, ammonia, and methane. However, its interior, like that of Uranus, is primarily composed of ices and rock, [12] which is why Uranus and Neptune are normally considered " ice giants" to emphasise this distinction. [13] Traces of methane in the outermost regions in part account for the planet's blue appearance. [14]

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 on Jupiter. These weather patterns are driven by the strongest sustained winds of any planet in the Solar System, with recorded wind speeds as high as 2,100 kilometres per hour (580 m/s; 1,300 mph). [15] 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). Temperatures at the planet's centre are approximately 5,400 K (5,100 °C). [16] [17] Neptune has a faint and fragmented ring system (labelled "arcs"), which was discovered in 1982, then later confirmed by Voyager 2. [18]



Some of the earliest recorded observations ever made through a telescope, Galileo'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 when it appeared close—in conjunction—to Jupiter in the night sky; [19] 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 retrograde 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. [20] In July 2009, University of Melbourne physicist David Jamieson announced new evidence suggesting that Galileo was at least aware that the "star" he had observed had moved relative to the fixed stars. [21]

In 1821, Alexis Bouvard published astronomical tables of the orbit of Neptune's neighbour Uranus. [22] Subsequent observations revealed substantial deviations from the tables, leading Bouvard to hypothesise that an unknown body was perturbing the orbit through gravitational interaction. [23] In 1843, John Couch Adams began work on the orbit of Uranus using the data he had. Via Cambridge Observatory director James Challis, he requested extra data from Sir George Airy, the Astronomer Royal, who supplied it in February 1844. Adams continued to work in 1845–46 and produced several different estimates of a new planet. [24] [25]

In 1845–46, Urbain Le Verrier, 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 Challis to search for the planet. Challis vainly scoured the sky throughout August and September. [23] [26]

Meanwhile, Le Verrier by letter urged Berlin Observatory astronomer Johann Gottfried Galle to search with the observatory's refractor. Heinrich d'Arrest, a student at the observatory, suggested to Galle that they could compare a recently drawn chart of the sky in the region of Le Verrier's predicted location with the current sky to seek the displacement characteristic of a planet, as opposed to a fixed star. On the evening of 23 September 1846, the day Galle received the letter, he discovered Neptune within 1° of where Le Verrier had predicted it to be, about 12° from Adams' prediction. Challis later realised that he had observed the planet twice, on 4 and 12 August, but did not recognise it as a planet because he lacked an up-to-date star map and was distracted by his concurrent work on comet observations. [23] [27]

In the wake of the discovery, there was much nationalistic rivalry between the French and the British over who deserved credit for the discovery. Eventually, an international consensus emerged that both Le Verrier and Adams jointly deserved credit. Since 1966, Dennis Rawlins has questioned the credibility of Adams's claim to co-discovery, and the issue was re-evaluated by historians with the return in 1998 of the "Neptune papers" (historical documents) to the Royal Observatory, Greenwich. [28] After reviewing the documents, they suggest that "Adams does not deserve equal credit with Le Verrier for the discovery of Neptune. That credit belongs only to the person who succeeded both in predicting the planet's place and in convincing astronomers to search for it." [29]


Shortly after its discovery, Neptune was referred to simply as "the planet exterior to Uranus" or as "Le Verrier's planet". The first suggestion for a name came from Galle, who proposed the name Janus. In England, Challis put forward the name Oceanus. [30]

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. [31] In October, he sought to name the planet Le Verrier, after himself, and he had loyal support in this from the observatory director, François Arago. This suggestion met with stiff resistance outside France. [32] French almanacs quickly reintroduced the name Herschel for Uranus, after that planet's discoverer Sir William Herschel, and Leverrier for the new planet. [33]

Struve came out in favour of the name Neptune on 29 December 1846, to the Saint Petersburg Academy of Sciences. [34] Soon, Neptune became the internationally accepted name. In Roman mythology, Neptune was the god of the sea, identified with the Greek Poseidon. The demand for a mythological name seemed to be in keeping with the nomenclature of the other planets, all of which, except for Earth, were named for deities in Greek and Roman mythology. [35]

Most languages today, even in countries that have no direct link to Greco-Roman culture, use some variant of the name "Neptune" for the planet. However, in Chinese, Japanese, and Korean, the planet's name was translated as "sea king star" (海王星), because Neptune was the god of the sea. [36] In Mongolian, Neptune is called Dalain Van (Далайн ван), reflecting its namesake god's role as the ruler of the sea. In modern Greek the planet is called Poseidon (Ποσειδώνας, Poseidonas), the Greek counterpart of Neptune. [37] In Hebrew, "Rahab" (רהב), from a Biblical sea monster mentioned in the Book of Psalms, was selected in a vote managed by the Academy of the Hebrew Language in 2009 as the official name for the planet, even though the existing Latin term "Neptun" (נפטון) is commonly used. [38] [39] In Māori, the planet is called Tangaroa, named after the Māori god of the sea. [40] In Nahuatl, the planet is called Tlāloccītlalli, named after the rain god Tlāloc. [40] In Thai, Neptune is referred both by its Westernised name Dao Nepjun (ดาวเนปจูน), and is also named Dao Ketu (ดาวเกตุ, "Star of Ketu"), after the descending lunar node Ketu (केतु) who plays a role in Hindu astrology.


From its discovery in 1846 until the subsequent 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. [41] The discovery of the Kuiper belt in 1992 led many astronomers to debate whether Pluto should be considered a planet or as part of the Kuiper belt. [42] [43] In 2006, the International Astronomical Union defined the word "planet" for the first time, reclassifying Pluto as a " dwarf planet" and making Neptune once again the outermost known planet in the Solar System. [44]

Other Languages
Alemannisch: Neptun (Planet)
አማርኛ: ነፕቲዩን
العربية: نبتون
armãneashti: Neptun
অসমীয়া: নেপচুন
Avañe'ẽ: Netúno
azərbaycanca: Neptun (planet)
Bân-lâm-gú: Hái-ông-chheⁿ
Basa Banyumasan: Neptunus
башҡортса: Нептун
беларуская: Нептун (планета)
беларуская (тарашкевіца)‎: Нэптун
български: Нептун (планета)
Boarisch: Neptun (Planet)
bosanski: Neptun
буряад: Далайн ван
čeština: Neptun (planeta)
Diné bizaad: Néʼtoon
eesti: Neptuun
emiliàn e rumagnòl: Netón
estremeñu: Netunu (praneta)
فارسی: نپتون
Fiji Hindi: Varungrah
føroyskt: Neptun
Frysk: Neptunus
Gaelg: Neptioon
Gàidhlig: Neaptun
galego: Neptuno
贛語: 海王星
客家語/Hak-kâ-ngî: Hói-vòng-sên
한국어: 해왕성
Hawaiʻi: Nepekune
Հայերեն: Նեպտուն
हिन्दी: वरुण (ग्रह)
hrvatski: Neptun
Ido: Neptuno
Ilokano: Neptuno
Bahasa Indonesia: Neptunus
interlingua: Neptuno
ᐃᓄᒃᑎᑐᑦ/inuktitut: ᓂᑉᑑᓐ
isiZulu: UNephthuni
עברית: נפטון
Basa Jawa: Neptunus
ಕನ್ನಡ: ನೆಪ್ಚೂನ್
Kapampangan: Neptune
ქართული: ნეპტუნი
kaszëbsczi: Neptun
kernowek: Nevyon (planet)
Kiswahili: Neptun
коми: Нептун
Kongo: Neptunus
Kreyòl ayisyen: Neptin (planèt)
Кыргызча: Нептун
лезги: Нептун
Lëtzebuergesch: Neptun (Planéit)
Livvinkarjala: Neptunus
la .lojban.: neptun
македонски: Нептун
مصرى: نيبتون
مازِرونی: نپتون
Bahasa Melayu: Neptun
Mìng-dĕ̤ng-ngṳ̄: Hāi-uòng-sĭng
Mirandés: Netuno (planeta)
монгол: Далайн ван
မြန်မာဘာသာ: နက်ပကျွန်းဂြိုဟ်
Nederlands: Neptunus (planeet)
Nedersaksies: Neptunus (planeet)
नेपाली: वरुणग्रह
नेपाल भाषा: वरुण
日本語: 海王星
Napulitano: Nettuno
нохчийн: Нептун
Nordfriisk: Neptunus
norsk nynorsk: Planeten Neptun
ଓଡ଼ିଆ: ନେପଚୁନ
oʻzbekcha/ўзбекча: Neptun
ਪੰਜਾਬੀ: ਵਰੁਣ (ਗ੍ਰਹਿ)
پنجابی: نیپچون
پښتو: نیپټون
Patois: Neptyuun
ភាសាខ្មែរ: ភពណិបទូន
Piemontèis: Netun (pianeta)
Plattdüütsch: Neptun (Planet)
polski: Neptun
português: Netuno (planeta)
Qaraqalpaqsha: Neptun (planeta)
română: Neptun
rumantsch: Neptun (planet)
русиньскый: Нептун (планета)
русский: Нептун
саха тыла: Нептун
davvisámegiella: Neptun
संस्कृतम्: नेप्चून्-ग्रहः
sardu: Nettunu
Scots: Neptune
Seeltersk: Neptun
shqip: Neptuni
සිංහල: නෙප්චූන්
Simple English: Neptune
slovenčina: Neptún
slovenščina: Neptun
ślůnski: Neptůn
Soomaaliga: Docay
کوردی: نێپتۆن
српски / srpski: Нептун
srpskohrvatski / српскохрватски: Neptun (planeta)
Basa Sunda: Néptunus
suomi: Neptunus
svenska: Neptunus
Tagalog: Neptuno
татарча/tatarça: Нептун (планета)
తెలుగు: నెప్ట్యూన్
тоҷикӣ: Нептун
Türkçe: Neptün
Türkmençe: Neptun
тыва дыл: Нептун
українська: Нептун (планета)
اردو: نیپچون
ئۇيغۇرچە / Uyghurche: نېپتۇن
vepsän kel’: Neptun (planet)
Tiếng Việt: Sao Hải Vương
Volapük: Neptun
Võro: Neptuun
文言: 海王星
Winaray: Neptuno
Wolof: Neptuun
吴语: 海王星
ייִדיש: נעפטון
Yorùbá: Nẹ́ptúnù
粵語: 海王星
Zazaki: Neptun
žemaitėška: Neptūns
中文: 海王星