View source for Neptune

{{about|the planet}}
{{pp-semi-vandalism|small=yes}}
{{Infobox Planet
<!--
 Please note the formatting and layout of this infobox has been matched with the other bodies of the solar system. Please do not arbitrarily change it without discussion.
 Scroll down to edit the contents of this page.
 Additional parameters for this template are available at
 [[Template:Infobox Planet]].
-->
| bgcolour = #97C3F4
| name = Neptune
| symbol = [[Image:Neptune symbol.svg|20px|Astronomical symbol for Neptune.]]
| image = [[Image:neptune.jpg|240px|Neptune from Voyager 2]]
| caption = Neptune from ''Voyager 2''
| discovery = yes
| discoverer = [[Urbain Le Verrier]]<br /> [[John Couch Adams]]<br /> [[Johann Gottfried Galle|Johann Galle]]
| discovered = September 23, 1846<ref name=Hamilton>{{cite web
 | first=Calvin J. | last=Hamilton | date=August 4, 2001
 | url=http://www.solarviews.com/eng/neptune.htm
 | title=Neptune |publisher=Views of the Solar System
 | accessdate=2007-08-13}}</ref>
| orbit_ref =
<ref name=horizons>{{cite web
 | first=Donald K. | last=Yeomans | date =July 13, 2006
 | url = http://ssd.jpl.nasa.gov/?horizons
 | title = HORIZONS System | publisher = NASA JPL
 | accessdate = 2007-08-08 }}—At the site, go to the "web interface" then select "Ephemeris Type: ELEMENTS", "Target Body: Neptune Barycenter" and "Center: Sun".</ref><ref name=barycentre>Orbital elements refer to the barycentre of the Neptune system, and are the instantaneous [[osculating orbit|osculating]] values at the precise [[J2000]] 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 to the motion of the moons.</ref>
| epoch = [[J2000]]
| aphelion = 4,553,946,490&nbsp;[[Kilometre|km]]<br />30.44125206&nbsp;[[Astronomical unit|AU]]
| perihelion = 4,452,940,833&nbsp;km<br />29.76607095&nbsp;AU
| semimajor = 4,503,443,661&nbsp;km<br />30.10366151&nbsp;AU
| eccentricity = 0.011214269
| period = 60,190<ref name="fact2" />&nbsp;[[day]]s<br />164.79&nbsp;[[julian year (astronomy)|years]]<br />89,666 Neptune [[solar day]]s<ref name="planet_years">{{cite web|url = http://cseligman.com/text/sky/rotationvsday.htm|title = Rotation Period and Day Length|last = Seligman |first = Courtney|accessdate = 2009-08-13}}</ref>
| synodic_period = 367.49&nbsp;day<ref name="fact">{{cite web
  | url = http://nssdc.gsfc.nasa.gov/planetary/factsheet/neptunefact.html
  | title = Neptune Fact Sheet
  | date = September 1, 2004
  | publisher = NASA | accessdate = 2007-08-14
  | last = Williams |first = David R. }}</ref>
| avg_speed = 5.43&nbsp;km/s<ref name="fact"/>
| inclination = 1.767975° to [[Ecliptic]]<br />6.43° to [[Ecliptic#Ecliptic_and_planets|Sun’s equator]]<br>0.72° to [[Invariable plane]]<ref name=meanplane>{{cite web
  |date=2009-04-03
  |title=The MeanPlane (Invariable plane) of the Solar System passing through the barycenter
  |url=http://home.comcast.net/~kpheider/MeanPlane.gif
  |accessdate=2009-04-10}} (produced with [http://chemistry.unina.it/~alvitagl/solex/ Solex 10] written by Aldo Vitagliano; see also [[Invariable plane]])</ref>
| asc_node = 131.794310°
| arg_peri = 265.646853°
| mean_anomaly = 267.767281°
| satellites = [[Moons of Neptune|13]]
| physical_characteristics = yes
| flattening = 0.0171 ± 0.0013 <!-- calculated from data in ref name=Seidelmann2007 -->
| equatorial_radius = 24,764 ± 15&nbsp;km<ref name=Seidelmann2007>{{cite journal
 | first=Seidelmann | last=P. Kenneth
 | coauthors=Archinal, B. A.; A’hearn, M. F. et al.
 | title= Report of the IAU/IAGWorking Group on cartographic coordinates and rotational elements
 | journal=Celestial Mechanics and Dynamical Astronomy
 | volume=90 | pages=155–180 | year=2007
 | doi=10.1007/s10569-007-9072-y
 | url=http://adsabs.harvard.edu/doi/10.1007/s10569-007-9072-y
 |publisher=Springer Netherlands | accessdate=2008-03-07 }}</ref><ref name=1bar>Refers to the level of {{convert|1|bar|kPa}} atmospheric pressure</ref><br />3.883 Earths
| polar_radius = 24,341 ± 30&nbsp;km<ref name=Seidelmann2007/><ref name=1bar/><br />3.829 Earths
| surface_area = 7.6408{{e|9}}&nbsp;km²<ref name="fact2"/><ref name=1bar/><br />14.98 Earths
| volume = 6.254{{e|13}}&nbsp;km³<ref name="fact"/><ref name=1bar/><br />57.74 Earths
| mass = 1.0243×10<sup>26</sup>&nbsp;kg<ref name="fact"/><br />17.147 Earths
| density = 1.638&nbsp;g/cm³<ref name="fact"/><ref name=1bar/>
| surface_grav = 11.15&nbsp;[[Acceleration|m/s²]]<ref name="fact"/><ref name=1bar/><br />1.14&nbsp;[[g-force|''g'']]
| escape_velocity = 23.5&nbsp;km/s<ref name="fact"/><ref name=1bar/>
| sidereal_day = 0.6713&nbsp;day<ref name="fact"/><br/>16&nbsp;h 6&nbsp;min 36&nbsp;s
| rot_velocity = 2.68&nbsp;km/s<br />9,660&nbsp;km/h
| axial_tilt = 28.32°<ref name="fact"/>
| right_asc_north_pole = {{RA|19|57|20}}<ref name=Seidelmann2007/>
| declination = 42.950°<ref name=Seidelmann2007/>
| albedo = 0.290 ([[Bond albedo|bond]])<br/>
0.41 ([[Geometric albedo|geom.]])<ref name="fact"/>
| magnitude = 8.0 to 7.78<ref name="fact"/><ref name=ephemeris/>
| angular_size = 2.2[[Minute of arc#Symbols, abbreviations and subdivisions|″]]–2.4″<ref name="fact"/><ref name=ephemeris/>
| temperatures = yes
| temp_name1 = 1 bar level
| min_temp_1 = 
| mean_temp_1 = 72&nbsp;[[Kelvin|K]]<ref name="fact"/>
| max_temp_1 = 
| temp_name2 = {{convert|0.1|bar|kPa|abbr=on}}
| min_temp_2 = 
| mean_temp_2 = 55&nbsp;K<ref name="fact"/>
| max_temp = 
| pronounce = {{IPA-en|ˈnɛptjuːn|}}<ref>{{cite book
 | first= Elizabeth | last=Walter | date=April 21, 2003
 | title=Cambridge Advanced Learner's Dictionary
 | publisher=Cambridge University Press
 | edition=Second | isbn=0521531063 }}</ref>, {{IPA-en|ˈnɛptun|US|en-us-Neptune.ogg}}
| adjectives = Neptunian
| atmosphere = yes
| atmosphere_ref =
<ref name="fact"/>
| scale_height = 19.7&nbsp;±&nbsp;0.6 km
| atmosphere_composition = 
<table>
<tr><td>
80±3.2%</td><td>[[Hydrogen]] (H<sub>2</sub>)
</td></tr><tr><td>
19±3.2%</td><td>[[Helium]]
</td></tr><tr><td>
1.5±0.5%</td><td>[[Methane]]
</td></tr><tr><td>
~0.019%</td><td>[[Hydrogen deuteride]] (HD)
</td></tr><tr><td>
~0.00015%</td><td>[[Ethane]]
</td></tr><tr><td>
'''Ices''':</td><td>
</td></tr><tr><td>
</td><td>[[Ammonia]]
</td></tr><tr><td>
</td><td>[[water (molecule)|Water]]
</td></tr><tr><td>
</td><td>[[Ammonium hydrosulfide]](NH<sub>4</sub>SH)
</td></tr><tr><td>
</td><td>Methane (?)
</td></tr></table>
}}
'''Neptune''' is the eighth and outermost-known [[planet]] from the [[Sun]] in the [[Solar System]]. Named for the [[Neptune (mythology)|Roman god of the sea]], it is the fourth-largest planet by diameter and the third-largest by mass. Neptune is 17 times the mass of [[Earth]] and is slightly more massive than its near-twin [[Uranus]], which is 15 Earth masses and not as dense.<ref name=mass/> On average, Neptune orbits the Sun at a distance of 30.1 [[Astronomical Unit|AU]], approximately 30 times the Earth-Sun distance. Its [[astronomical symbol]] is [[Image:Neptune symbol.svg|20px|Astronomical symbol for Neptune.]], a stylized version of the [[Neptune (mythology)|god Neptune]]'s [[trident]].

Discovered on September 23, 1846,<ref name="Hamilton" /> Neptune was the first planet found by mathematical prediction rather than by [[empirical observation]]. Unexpected changes in the orbit of Uranus led astronomers to deduce that its orbit was subject to [[gravitation]]al [[perturbation (astronomy)|perturbation]] by an unknown planet. Neptune was subsequently found within a degree of its predicted position, and its largest moon, [[Triton (moon)|Triton]], was discovered shortly thereafter, though none of the planet's remaining 12 [[Moons of Neptune|moons]] was located telescopically until the 20th century. Neptune has been visited by only one spacecraft, ''[[Voyager 2]]'', which flew by the planet on August 25, 1989.

Neptune is similar in composition to [[Uranus]], and both have compositions which differ from those of the larger [[gas giant]]s [[Jupiter]] and [[Saturn]]. Neptune's atmosphere, while similar to Jupiter's and Saturn's in that it is composed primarily of [[hydrogen]] and [[helium]], along with traces of [[hydrocarbon]]s and possibly [[nitrogen]], contains a higher proportion of "ices" such as water, [[ammonia]] and [[methane]]. Astronomers sometimes categorize Uranus and Neptune as "[[ice giant]]s" in order to emphasize these distinctions.<ref name=atmo/> The interior of Neptune, like that of Uranus, is primarily composed of ices and rock.<ref name=Podolak1995>{{cite journal
 | last=Podolak | first=M.
 | coauthors=Weizman, A.; Marley, M.
 | title=Comparative models of Uranus and Neptune
 | journal=Planetary and Space Science | volume=43
 | issue=12 | pages=1517–1522 | year=1995
 | url=http://adsabs.harvard.edu/abs/1995P%26SS...43.1517P
 | doi=10.1016/0032-0633(95)00061-5}}</ref> Traces of methane in the outermost regions in part account for the planet's blue appearance.<ref name=bluecolour>{{cite web
 | first=Kirk | last=Munsell
 | coauthors=Smith, Harman; Harvey, Samantha
 | date=November 13, 2007
 | url=http://solarsystem.nasa.gov/planets/profile.cfm?Object=Neptune&Display=OverviewLong
 | title=Neptune overview | work=Solar System Exploration
 | publisher=NASA | accessdate=2008-02-20 }}</ref>

In contrast to the relatively featureless atmosphere of Uranus, Neptune's atmosphere is notable for its active and visible weather patterns. At the time of the 1989 ''Voyager 2'' [[planetary flyby|flyby]], for example, the planet's southern hemisphere possessed a [[Great Dark Spot]] comparable to the [[Great Red Spot]] on [[Jupiter]]. These weather patterns are driven by the strongest sustained winds of any planet in the Solar System, with recorded wind speeds as high as 2100&nbsp;km/h.<ref name="Suomi1991">{{cite journal
 | last=Suomi | first=V. E.
 | coauthors=Limaye, S. S.; Johnson, D. R. | year=1991
 | title=High Winds of Neptune: A possible mechanism
 | journal=[[Science (journal)|Science]] | volume=251
 | issue=4996 | pages=929–932 | publisher=AAAS (USA)
 | doi=10.1126/science.251.4996.929
 | pmid=17847386
 }}</ref> 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 {{convert|-218|°C|K|0|abbr=off|lk=on}}<!--note that the template is no good, abbr=off has no effect-->. Temperatures at the planet's centre, however, are approximately {{convert|5400|K|C|-3}}.<ref name=hubbard/><ref name="nettelmann"/> Neptune has a faint and fragmented [[planetary ring|ring system]], which may have been detected during the 1960s but was only indisputably confirmed in 1989 by ''Voyager 2''.<ref name=ring1 />

==History==
===Discovery===
{{mainarticle|Discovery of Neptune}}
[[Galileo Galilei|Galileo's]]  drawings show that he first observed Neptune on December 28, 1612, and again on January 27, 1613. On both occasions, Galileo mistook Neptune for a [[fixed star]] when it appeared very close—in [[Conjunction (astronomy and astrology)|conjunction]]—to Jupiter in the [[night sky]],<ref>{{cite book
 | first=Alan | last=Hirschfeld
 | title=Parallax: The Race to Measure the Cosmos | year=2001
 | publisher=Henry Holt | location=New York, New York
 | isbn=0-8050-7133-4 }}</ref> hence, he is not credited with Neptune's discovery. During the period of his first observation in December 1612, Neptune was stationary in the sky because it had just turned [[Retrograde and direct motion|retrograde]] that very day. This apparent backward motion is created when the orbit of the Earth takes it past an outer planet. Since Neptune was only beginning its yearly retrograde cycle, the motion of the planet was far too slight to be detected with Galileo's small [[telescope]].<ref>{{cite book
 | first=Mark | last=Littmann | coauthors=Standish, E. M.
 | title=Planets Beyond: Discovering the Outer Solar System
 | year=2004 | publisher=Courier Dover Publications
 | isbn=0-4864-3602-0 }}</ref> However, 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]].<ref>{{cite web
 | title=Galileo discovered Neptune, new theory claims
 | first=Robert Roy | last=Britt | year=2009
 | publisher=MSNBC News | accessdate=2009-07-10
 | url=http://www.msnbc.msn.com/id/31835303 }}</ref>

In 1821, [[Alexis Bouvard]] published astronomical tables of the [[orbit]] of Neptune's neighbor Uranus.<ref>{{cite book
 | first=A. | last=Bouvard | year=1821
 | title=Tables astronomiques publiées par le Bureau des Longitudes de France
 | publisher=Bachelier | location=Paris }}</ref> Subsequent observations revealed substantial deviations from the tables, leading Bouvard to hypothesize that an unknown body was [[Perturbation (astronomy)|perturbing]] the orbit through [[gravitation]]al interaction. In 1843, [[John Couch Adams]] calculated the orbit of a hypothesized eighth planet that would account for Uranus's motion. He sent his calculations to Sir [[George Biddell Airy|George Airy]], the [[Astronomer Royal]], who asked Adams for a clarification. Adams began to draft a reply but never sent it and did not aggressively pursue work on the Uranus problem.<ref>{{cite web
 | first=John J. | last=O'Connor
 | coauthors=Robertson, Edmund F. | month=March | year=2006
 | url=http://www-groups.dcs.st-and.ac.uk/~history/Extras/Adams_Neptune.html
 | title=John Couch Adams' account of the discovery of Neptune
 | publisher=University of St Andrews
 | accessdate=2008-02-18 }}
</ref><ref>{{cite journal
 | first=J. C. | last=Adams
 | url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1846MNRAS...7..149A&amp;db_key=AST&amp;data_type=HTML&amp;format=&amp;high=42c888df4622238
 | title=Explanation of the observed irregularities in the motion of Uranus, on the hypothesis of disturbance by a more distant planet
 | journal=Monthly Notices of the Royal Astronomical Society
 | volume=7 | pages=149 | date=November 13, 1846
 | accessdate=2008-02-18
 | publisher=Blackwell Publishing}}</ref>

[[Image:Urbain Le Verrier.jpg|thumb|upright|left||[[Urbain Le Verrier]]]]
In 1845–46, [[Urbain Le Verrier]], independently of Adams, developed his own calculations but also experienced difficulties in stimulating any enthusiasm in his compatriots. In June, however, upon seeing Le Verrier's first published estimate of the planet's longitude and its similarity to Adams's estimate, Airy persuaded Cambridge Observatory director [[James Challis]] to search for the planet. Challis vainly scoured the sky throughout August and September.<ref name=MNRAS7>{{cite journal
 | first=G. B. | last=Airy
 | url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1846MNRAS...7..121A&amp;db_key=AST&amp;data_type=HTML&amp;format=&amp;high=42c888df4622238
 | title=Account of some circumstances historically connected with the discovery of the planet exterior to Uranus
 | journal=Monthly Notices of the Royal Astronomical Society
 | volume=7 | pages=121–144 | date=November 13, 1846
 | accessdate=2008-02-18
 | publisher=Blackwell Publishing}}
</ref><ref>{{cite journal
 | first=Rev. J. | last=Challis
 | url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1846MNRAS...7..145C&amp;db_key=AST&amp;data_type=HTML&amp;format=&amp;high=42c888df4622238
 | title=Account of observations at the Cambridge observatory for detecting the planet exterior to Uranus
 | journal=Monthly Notices of the Royal Astronomical Society
 | volume=7 | pages=145–149 | date=November 13, 1846
 | accessdate=2008-02-18
 | publisher=Blackwell Publishing}}</ref>

Meantime, Le Verrier by letter urged [[Berlin Observatory]] astronomer [[Johann Gottfried Galle]] to search with the observatory's [[refractor]]. [[Heinrich Louis d'Arrest|Heinrich d'Arrest]], a student at the observatory, suggested to Galle that they could compare a recently drawn chart of the sky in the region of Le Verrier's predicted location with the current sky to seek the displacement characteristic of a [[planet]], as opposed to a fixed star. The very evening of the day of receipt of Le Verrier's letter on September 23, 1846, Neptune was discovered within 1° of where Le Verrier had predicted it to be, and about 12° from Adams' prediction. Challis later realized that he had observed the planet twice in August, failing to identify it owing to his casual approach to the work.<ref name=MNRAS7/><ref>{{cite journal
 | first=J. G. | last=Galle
 | url=http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1846MNRAS...7..153G&amp;db_key=AST&amp;data_type=HTML&amp;format=&amp;high=42c888df4622238
 |title=Account of the discovery of the planet of Le Verrier at Berlin
 | journal=Monthly Notices of the Royal Astronomical Society
 | volume=7 | pages=153 | date=November 13, 1846
 | accessdate=2008-02-18
 | publisher=Blackwell Publishing }}</ref>

In the wake of the discovery, there was much nationalistic rivalry between the French and the British over who had priority and deserved credit for the discovery. Eventually an international consensus emerged that both Le Verrier and Adams jointly deserved credit. However, the issue is being re-evaluated by historians with the rediscovery in 1998 of the "Neptune papers" (historical documents from the [[Royal Observatory, Greenwich]]), which had apparently been stolen by astronomer [[Olin J. Eggen]] and hoarded for nearly three decades, not to be rediscovered (in his possession) until immediately after his death.<ref name="Neptdisc"> {{cite web
 | url=http://www.ucl.ac.uk/sts/nk/neptune/index.htm
 | title=Neptune's Discovery. The British Case for Co-Prediction.
 | accessdate=2007-03-19 | first=Nick | last=Kollerstrom
 | year=2001 | publisher=University College London
 | archiveurl=http://web.archive.org/web/20051111190351/http://www.ucl.ac.uk/sts/nk/neptune/ | archivedate=2005-11-11 }}</ref> After reviewing the documents, some historians suggest that Adams does not deserve equal credit with Le Verrier. Since 1966 [[Dennis Rawlins]] has questioned the credibility of Adams's claim to co-discovery. In a 1992 article in his journal ''Dio'' he deemed the British claim "theft".<ref>{{cite web|title=The Neptune Conspiracy: British Astronomy's PostDiscovery Discovery|author=Rawlins, Dennis|year=1992|work=Dio|url=http://www.dioi.org/vols/w23.pdf|accessdate=2008-03-10|format=PDF}}</ref> "Adams had done some calculations but he was rather unsure about quite where he was saying Neptune was," said  [[Nicholas Kollerstrom]] of [[University College London]] in 2003.<ref>{{cite web|title=Lost letters' Neptune revelations|author=McGourty, Christine |work=BBC News|year=2003|url=http://news.bbc.co.uk/1/hi/sci/tech/2936663.stm|accessdate=2008-03-10}}</ref><ref>Summations following the Neptune documents' 1998 recovery appeared in [http://www.dioi.org/vols/w91.pdf ''DIO 9.1''] (1999) and William Sheehan, Nicholas Kollerstrom, Craig B. Waff (December 2004), [http://www.sciam.com/article.cfm?articleID=000CA850-8EA4-119B-8EA483414B7FFE9F The Case of the Pilfered Planet - Did the British steal Neptune?] ''Scientific American''.</ref>

===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 (mythology)|Janus]]''. In England, Challis put forward the name ''[[Oceanus]]''.<ref>Moore (2000):206</ref>

Claiming the right to name his discovery, Le Verrier quickly proposed the name ''Neptune'' for this new planet, while falsely stating that this had been officially approved by the French [[Bureau des Longitudes]].<ref>Littmann (2004):50</ref> 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]]. However, this suggestion met with stiff resistance outside [[France]].<ref>Baum & Sheehan (2003):109–110</ref> French almanacs quickly reintroduced the name ''Herschel'' for ''Uranus'', after that planet's discoverer Sir [[William Herschel]], and ''Leverrier'' for the new planet.<ref>{{cite journal
 | first=Owen | last=Gingerich
 | title=The Naming of Uranus and Neptune
 | journal=Astronomical Society of the Pacific Leaflets
 | year=1958 | volume=8 | pages=9–15
 | url=http://adsabs.harvard.edu/abs/1958ASPL....8....9G
 | accessdate=2008-02-19
}}</ref>

[[Friedrich Georg Wilhelm von Struve|Struve]]<!--- (1793–1864), or maybe his son Otto Wilhelm von Struve (1819–1905) ---> came out in favour of the name ''Neptune'' on December 29, 1846, to the [[Russian Academy of Sciences|Saint Petersburg Academy of Sciences]].<ref>{{cite journal
 | url=http://articles.adsabs.harvard.edu//full/seri/AN.../0025//0000164.000.html
 |title=Second report of proceedings in the Cambridge Observatory relating to the new Planet (Neptune)
 | year=1847 | journal=Astronomische Nachrichten
 | volume=25 | pages=309 | last=Hind | first=J. R.
 | accessdate=2008-02-18
 |doi=10.1002/asna.18470252102 }} Smithsonian/NASA Astrophysics Data System (ADS).</ref> Soon ''Neptune'' became the internationally accepted name. In [[Roman mythology]], [[Neptune (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 [[Graeco-Roman mythology|Graeco-Roman deities]].<ref name=USGS>{{cite web
 | first=Jennifer | last=Blue | date=December 17, 2008
 | url=http://planetarynames.wr.usgs.gov/append7.html
 | title=Planet and Satellite Names and Discoverers
 | publisher=USGS | accessdate=2008-02-18 }}</ref>

===Status===
From its discovery until 1930, Neptune was the farthest known planet. Upon the discovery of [[Pluto]] in 1930, Neptune became the penultimate planet, save for a 20-year period between 1979 and 1999 when Pluto fell within its orbit.<ref>{{cite web|title=Jan. 21, 1979: Neptune Moves Outside Pluto's Wacky Orbit|work=wired.com|url=http://www.wired.com/science/discoveries/news/2008/01/dayintech_0121
|author=Tony Long|year=2008|accessdate=2008-03-13}}</ref> However, the discovery of the [[Kuiper belt]] in 1992 led many astronomers to debate whether Pluto should be considered a planet in its own right or part of the belt's larger structure.<ref>{{cite web | author=Weissman, Paul R. | title=The Kuiper Belt| work=Annual Review of Astronomy and Astrophysics| url=http://adsabs.harvard.edu/abs/1995ARA&A..33..327W
| accessdate=2006-10-04}}</ref><ref>{{cite web | year=1999 | title=The Status of Pluto:A clarification | work=[[International Astronomical Union]], Press release | url=http://www.iau.org/STATUS_OF_PLUTO.238.0.html | accessdate=2006-05-25}}</ref> In 2006, the [[International Astronomical Union]] [[2006 definition of planet|defined the word "planet" for the first time]], reclassifying Pluto as a "[[dwarf planet]]" and making Neptune once again the last planet in the Solar System.<ref>{{cite news|url=http://www.iau.org/static/resolutions/Resolution_GA26-5-6.pdf|title=IAU 2006 General Assembly: Resolutions 5 and 6|date=August 24, 2006|publisher=IAU|format=PDF}}</ref>

==Composition and structure==
[[Image:Neptune, Earth size comparison.jpg|thumb|upright|A size comparison of Neptune and Earth]]
With a mass of 1.0243{{e|26}}&nbsp;[[kilogram|kg]],<ref name="fact"/> Neptune is an intermediate body between [[Earth]] and the larger [[gas giants]]: its mass is seventeen times that of the Earth but just 1/19th that of [[Jupiter]].<ref name=mass>The mass of the Earth is
5.9736{{e|24}}&nbsp;kg, giving a mass ratio of:
:<math>\begin{smallmatrix}\frac{M_{Neptune}}{M_{Earth}}
\ =\ \frac{1.02 \times 10^{26}}{5.97 \times 10^{24}}
\ =\ 17.09\end{smallmatrix}</math>
The mass of Uranus is 8.6810{{e|25}}&nbsp;kg, giving a mass ratio of:
:<math>\begin{smallmatrix}\frac{M_{Uranus}}{M_{Earth}}
\ =\ \frac{8.68 \times 10^{25}}{5.97 \times 10^{24}}
\ =\ 14.54\end{smallmatrix}</math>
The mass of Jupiter is 1.8986{{e|27}}&nbsp;kg, giving a mass ratio of:
:<math>\begin{smallmatrix}\frac{M_{Jupiter}}{M_{Neptune}}
\ =\ \frac{1.90 \times 10^{27}}{1.02 \times 10^{26}}
\ =\ 18.63\end{smallmatrix}</math>
See: {{cite web
 | last=Williams | first=David R.
 | date=November 29, 2007
 | url=http://nssdc.gsfc.nasa.gov/planetary/factsheet/
 | title=Planetary Fact Sheet - Metric | publisher=NASA
 | accessdate=2008-03-13 }}</ref> The planet's [[surface gravity]] is only surpassed by [[Jupiter]], making the two gas giants the only planets in the solar system with a surface gravity higher than the Earth.{{Fact|date=August 2009}} Neptune's [[equator]]ial radius of 24764&nbsp;km<ref name=Seidelmann2007/> is nearly four times that of the Earth. Neptune and [[Uranus]] are often considered a sub-class of gas giant termed "[[gas giant#Uranus_and_Neptune|ice giant]]s", due to their smaller size and higher concentrations of [[volatiles]] relative to Jupiter and [[Saturn]].<ref>See for example: {{cite journal
 | first=Alan P. | last=Boss
 | title=Formation of gas and ice giant planets
 | journal=Earth and Planetary Science Letters
 | year=2002 | volume=202 | issue=3–4
 | pages=513–523 | doi=10.1016/S0012-821X(02)00808-7 }}</ref> In the search for [[extrasolar planet]]s Neptune has been used as a [[metonymy|metonym]]: discovered bodies of similar mass are often referred to as "Neptunes",<ref>{{cite news
 | first=C. | last=Lovis | date=May 18, 2006
 | coauthors=Mayor, M.; Alibert Y.; Benz W.
 | url=http://www.eso.org/public/outreach/press-rel/pr-2006/pr-18-06.html
 | title=Trio of Neptunes and their Belt
 | publisher=[[European Southern Observatory|ESO]]
 | accessdate=2008-02-25}}</ref> just as astronomers refer to various extra-solar bodies as "Jupiters".

===Internal structure===
Neptune's internal structure resembles that of [[Uranus#Physical characteristics|Uranus]]. Its atmosphere forms about 5 to 10 percent of its mass and extends perhaps 10 to 20 percent of the way towards the core, where it reaches pressures of about 10&nbsp;[[Pascal (unit)|GPa]]. Increasing concentrations of [[methane]], [[ammonia]] and water are found in the lower regions of the atmosphere.<ref name=hubbard/>
[[Image:Neptune diagram.svg|325px|thumb|left| The internal structure of Neptune:<br /> 1. Upper atmosphere, top clouds<br /> 2. Atmosphere consisting of hydrogen, helium and methane gas<br /> 3. Mantle consisting of water, ammonia and methane ices<br />
4. Core consisting of rock and ice
]]
Gradually this darker and hotter region condenses into a superheated liquid [[mantle (geology)|mantle]], where temperatures reach 2,000&nbsp;K to 5,000&nbsp;K. The mantle is equivalent to 10 to 15 Earth masses and is rich in water, ammonia and methane.<ref name=Hamilton/> As is customary in planetary science, this mixture is referred to as [[volatiles|icy]] even though it is a hot, highly dense fluid. This fluid, which has a high electrical conductivity, is sometimes called a water-ammonia ocean.<ref name=Atreya2006>{{cite journal
 | last=Atreya | first=S. | coauthors=Egeler, P.; Baines, K.
 | title=Water-ammonia ionic ocean on Uranus and Neptune?
 | journal=Geophysical Research Abstracts
 | volume=8 | pages=05179 | year=2006 | format=pdf
 | url=http://www.cosis.net/abstracts/EGU06/05179/EGU06-J-05179-1.pdf}}</ref> At a depth of 7000&nbsp;km, the conditions may be such that methane decomposes into diamond crystals that then precipitate toward the core.<ref>{{cite journal
 | last=Kerr | first=Richard A.
 | title=Neptune May Crush Methane Into Diamonds
 | journal=Science | year=1999 | volume=286
 | issue=5437 | pages=25
 | url=http://www.sciencemag.org/cgi/content/full/286/5437/25a
 | accessdate=2007-02-26
 | doi=10.1126/science.286.5437.25a }}</ref>

The [[planetary core|core]] of Neptune is composed of [[iron]], [[nickel]] and [[silicate]]s, with an interior model giving a mass about 1.2 times that of the Earth.<ref name=pass43>{{cite journal
 | last=Podolak | first=M. | coauthors=Weizman, A.; Marley, M.
 | title=Comparative models of Uranus and Neptune
 | journal=Planetary and Space Science
 | year=1995 | volume=43 | issue=12 | pages=1517–1522
 | doi=10.1016/0032-0633(95)00061-5 }}</ref> The pressure at the centre is 7&nbsp;[[bar (unit)|Mbar]] (700 GPa), millions of times more than that on the surface of the Earth, and the temperature may be 5,400&nbsp;K.<ref name=hubbard/><ref name="nettelmann">{{cite web
 | last=Nettelmann | first=N.
 | coauthors=French, M.; Holst, B.; Redmer, R. | url=https://www-new.gsi.de/informationen/wti/library/plasma2006/PAPERS/TT-11.pdf
 | format=PDF
 | title=Interior Models of Jupiter, Saturn and Neptune
 | publisher=University of Rostock | accessdate=2008-02-25
}}</ref>

===Atmosphere===
At high altitudes, Neptune's atmosphere is 80% hydrogen and 19% helium.<ref name=hubbard>{{cite journal
 | last=Hubbard | first=W. B.
 | title=Neptune's Deep Chemistry
 | journal=Science | year=1997
 | volume=275 | issue=5304 | pages=1279–1280
 | url=http://www.sciencemag.org/cgi/content/full/275/5304/1279
 | accessdate=2008-02-19
 | doi=10.1126/science.275.5304.1279
 | pmid=9064785 }}</ref> A trace amount of methane is also present. Prominent absorption bands of methane occur at wavelengths above 600&nbsp;nm, in the red and infrared portion of the spectrum. As with Uranus, this absorption of red light by the [[atmospheric methane]] is part of what gives Neptune its blue hue,<ref>{{cite web
 | last=Crisp | first=D. | coauthors=Hammel, H. B.
 | date=June 14, 1995
 | url =http://hubblesite.org/newscenter/archive/releases/1995/09/image/a/
 | title =Hubble Space Telescope Observations of Neptune
 | publisher = Hubble News Center
 | accessdate = 2007-04-22
}}</ref>
although Neptune's vivid [[Azure (color)|azure]] differs from Uranus's milder [[aquamarine (color)|aquamarine]]. Since Neptune's atmospheric methane content is similar to that of Uranus, some unknown atmospheric constituent is thought to contribute to Neptune's colour.<ref name=bluecolour/> Neptune is, however, somewhat denser and heavier than Uranus, and models suggest that its hydrogen-helium atmosphere would be thinner, allowing more of the methane from the mantle to leak to the surface, leading to a richer colour.{{Fact|date=April 2009}}

Neptune's atmosphere is sub-divided into two main regions; the lower [[troposphere]], where temperature decreases with altitude, and the [[stratosphere]], where temperature increases with altitude. The boundary between the two, the [[tropopause]], occurs at a pressure of {{convert|0.1|bar|kPa}}.<ref name=atmo>{{cite web
 | title=The Atmospheres of Uranus and Neptune
 | last=Lunine | first=Jonathan I.
 | publisher=Lunar and Planetary Observatory, University of Arizona
 | year=1993 | format=PDF
 | url=http://articles.adsabs.harvard.edu/cgi-bin/nph-iarticle_query?1993ARA%26A..31..217L&amp;data_type=PDF_HIGH&amp;whole_paper=YES&amp;type=PRINTER&amp;filetype=.pdf
 | accessdate=2008-03-10}}</ref> The stratosphere then gives way to the [[thermosphere]] at a pressure lower than 10<sup>−5</sup> to 10<sup>−4</sup> microbars (1 to 10&nbsp;Pa).<ref name=atmo /> The thermosphere gradually transitions to the [[exosphere]].
[[Image:Neptune clouds.jpg|right|thumb|Bands of high-altitude clouds cast shadows on Neptune's lower cloud deck]]
Models suggest that Neptune's troposphere is banded by clouds of varying compositions depending on altitude. The upper-level clouds occur at pressures below one bar, where the temperature is suitable for methane to condense. For pressures between one and five bars (100 and 500 kPa), clouds of ammonia and [[hydrogen sulfide]] are believed to form. Above a pressure of five bars, the clouds may consist of ammonia, [[ammonium sulfide]], hydrogen sulfide and water. Deeper clouds of water ice should be found at pressures of about {{convert|50|bar|MPa}}, where the temperature reaches 0 °C. Underneath, clouds of ammonia and hydrogen sulfide may be found.<ref name=elkins-tanton/>

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&nbsp;km and lie about 50-110&nbsp;km above the cloud deck.<ref name=apj125/>

Neptune's [[spectra]] suggest that its lower stratosphere is hazy due to condensation of products of ultraviolet [[photolysis]] of methane, such as ethane and acetylene.<ref name= atmo /><ref name=hubbard/> The stratosphere is also home to trace amounts of [[carbon monoxide]] and [[hydrogen cyanide]].<ref name=atmo /><ref name=Encrenaz2003>{{cite journal|last=Encrenaz |first=Therese|title=ISO observations of the giant planets and Titan: what have we learnt?|journal=Planet. Space Sci.|volume=51|pages=89–103|year=2003|doi=10.1016/S0032-0633(02)00145-9| url=http://adsabs.harvard.edu/abs/2003P%26SS...51...89E}}</ref> The stratosphere of Neptune is warmer than that of Uranus due to the elevated concentration of hydrocarbons.<ref name=atmo/>

For reasons that remain obscure, the planet's thermosphere is at an anomalously high temperature of about 750&nbsp;K.<ref name=Broadfoot19989>{{cite journal|last=Broadfoot|first=A.L.|coauthors=Atreya, S.K.; Bertaux, J.L. et al.|title=Ultraviolet Spectrometer Observations of Neptune and Triton|journal=Science|volume=246|pages=1459–1456|year=1999| url=http://www-personal.umich.edu/~atreya/Articles/1989_Voyager_UV_Spectrometer.pdf|format=pdf|doi=10.1126/science.246.4936.1459|pmid=17756000}}</ref><ref name=Herbert1999>{{cite journal|last=Herbert|first=Floyd|coauthors=Sandel, Bill R. |title=Ultraviolet Observations of Uranus and Neptune|journal=Planet.Space Sci.|volume=47|pages=1119–1139|year=1999| url=http://adsabs.harvard.edu/abs/1999P%26SS...47.1119H |doi=10.1016/S0032-0633(98)00142-1}}</ref> The planet is too far from the Sun for this heat to be generated by [[ultraviolet]] radiation. One candidate for a heating mechanism is atmospheric interaction with ions in the planet's [[Magnetosphere|magnetic field]]. Other candidates are [[gravity wave]]s from the interior that dissipate in the atmosphere. The thermosphere contains traces of [[carbon dioxide]] and water, which may have been deposited from external sources such as [[meteorite]]s and dust.<ref name=elkins-tanton/><ref name=Encrenaz2003/>

===Magnetosphere===
Neptune also resembles Uranus in its [[magnetosphere]], with a [[magnetic field]] strongly tilted relative to its [[rotation]]al axis at 47° and offset at least 0.55&nbsp;radii, or about 13500&nbsp;km from the planet's physical centre. Before ''Voyager 2'''s arrival at Neptune, it was hypothesised that Uranus's tilted magnetosphere was the result of its sideways rotation. However, 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 [[convection|convective]] fluid motions in a thin spherical shell of
[[Electrical conductor|electrically conducting]] liquids (probably a combination of ammonia, methane and water)<ref name=elkins-tanton>Elkins-Tanton (2006):79–83.</ref> resulting in a [[dynamo]] action.<ref>{{cite journal
 | last=Stanley | first=Sabine | coauthors=Bloxham, Jeremy
 | title=Convective-region geometry as the cause of Uranus' and Neptune's unusual magnetic fields
 | journal=Nature | date=March 11, 2004
 | volume=428 | pages=151–153
 | doi=10.1038/nature02376 }}</ref>

The dipole component of the magnetic field at the magnetic equator of Neptune is about 14&nbsp;[[Tesla (unit)|microteslas]] (0.14&nbsp;[[Gauss (unit)|G]]).<ref name=Connerney1991>{{cite journal|last=Connerney|first=J.E.P.|coauthors=Acuna, Mario H.; Ness, Norman F.|title=The magnetic field of Neptune|year=1991|journal=Journal of Geophysics Research|volume=96|pages=19,023–42|url=http://adsabs.harvard.edu/abs/1991JGR....9619023C}}</ref> The dipole [[magnetic moment]] of Neptune is about 2.2{{Esp|17}}&nbsp;T·m<sup>3</sup> (14&nbsp;μT·''R''<sub>''N''</sub><sup>3</sup>, where ''R''<sub>''N''</sub> is the radius of Neptune). Neptune's magnetic field has a complex geometry that includes relatively large contributions from non-dipolar components, including a strong [[quadrupole]] moment that may exceed the [[Magnetic dipole moment|dipole moment]] in strength. By contrast, Earth, Jupiter and Saturn have only relatively small quadrupole moments, and their fields are less tilted from the polar axis. The large quadrupole moment of Neptune may be the result of offset from the planet's center and geometrical constraints of the field's dynamo generator.<ref name=science4936>{{cite journal
 | last=Ness | first=N. F.
 | coauthors=Acuña, M. H.; Burlaga, L. F.; Connerney, J. E. P.; Lepping, R. P.; Neubauer, F. M.
 | title=Magnetic Fields at Neptune
 | journal=Science | year=1989 | volume=246
 | issue=4936 | pages=1473–1478
 | url=http://www.sciencemag.org/cgi/content/abstract/246/4936/1473?ck=nck
 | accessdate=2008-02-25
 | doi=10.1126/science.246.4936.1473
 | pmid=17756002 }}
</ref><ref>{{cite web
 | last=Russell | first=C. T. | coauthors=Luhmann, J. G. | year=1997
 | url=http://www-ssc.igpp.ucla.edu/personnel/russell/papers/nep_mag.html
 | title=Neptune: Magnetic Field and Magnetosphere
 | publisher=University of California, Los Angeles
 | accessdate=2006-08-10
}}</ref>

Neptune's [[bow shock]], where the magnetosphere begins to slow the [[solar wind]], occurs at a distance of 34.9 times the radius of the planet. The [[magnetopause]], where the pressure of the magnetosphere counterbalances the solar wind, lies at a distance of 23–26.5 times the radius of Neptune. The tail of the magnetosphere extends out to at least 72 times the radius of Neptune, and very likely much farther.<ref name=science4936/>

===Planetary rings===
{{main|Rings of Neptune}}
[[Image:neptunerings.jpg|upright|thumb|right|Neptune's rings, taken by ''Voyager 2'']]
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.<ref>Cruikshank (1996):703–804</ref> The three main rings are the narrow Adams Ring, 63000&nbsp;km from the centre of Neptune, the Le Verrier Ring, at 53000&nbsp;km, and the broader, fainter Galle Ring, at 42000&nbsp;km. A faint outward extension to the Le Verrier Ring has been named Lassell; it is bounded at its outer edge by the Arago Ring at 57000&nbsp;km.<ref>{{cite web
 | last=Blue | first=Jennifer | date=December 8, 2004
 | url=http://planetarynames.wr.usgs.gov/append8.html
 | title=Nomenclature Ring and Ring Gap Nomenclature
 | work=Gazetteer of Planetary | publisher=USGS
 | accessdate=2008-02-28 }}</ref>

The first of these planetary rings was discovered in 1968 by a team led by [[Edward Guinan]],<ref name=ring1>{{cite news
 | last=Wilford | first=John N. | date=June 10, 1982
 | title=Data Shows 2 Rings Circling Neptune
 | publisher=The New York Times
 | url=http://query.nytimes.com/gst/fullpage.html?sec=technology&res=950DE3D71F38F933A25755C0A964948260&n=Top/News/Science/Topics/Space
 | accessdate=2008-02-29 }}</ref><ref>{{cite journal
 | last=Guinan | first=E. F.
 | coauthors=Harris, C. C.; Maloney, F. P.
 | title=Evidence for a Ring System of Neptune
 | journal=Bulletin of the American Astronomical Society
 | year=1982 | volume=14 | pages=658
 | url=http://adsabs.harvard.edu/abs/1982BAAS...14..658G
 | accessdate=2008-02-28
}}</ref> but it was later thought that this ring might be incomplete.<ref>{{cite journal
 | last=Goldreich | first=P.
 | coauthors=Tremaine, S.; Borderies, N. E. F.
 | title=Towards a theory for Neptune's arc rings
 | journal=Astronomical Journal
 | year=1986 | volume=92 | pages=490–494
 | url=http://articles.adsabs.harvard.edu/abs/1986AJ.....92..490G
 | accessdate=2008-02-28
 | doi=10.1086/114178
}}</ref> Evidence that the rings might have gaps first arose during a [[occultation|stellar occultation]] in 1984 when the rings obscured a star on immersion but not on emersion.<ref name="Nicholson90">{{cite journal
 | author=Nicholson, P. D. et al.
 | title=Five Stellar Occultations by Neptune: Further Observations of Ring Arcs
 | journal= Icarus | year= 1990 | volume= 87 | pages=1
 | url= http://adsabs.harvard.edu/cgi-bin/nph-bib_query?bibcode=1990Icar...87....1N&amp;db_key=AST&amp;data_type=HTML&amp;format=&amp;high=444b66a47d28395
 | accessdate= 2007-12-16
 | doi=10.1016/0019-1035(90)90020-A}}</ref> Images by ''Voyager 2'' in 1989 settled the issue by showing several faint rings. These rings have a clumpy structure,<ref name="Planetary Society">{{cite web
 | url=http://www.planetary.org/explore/topics/our_solar_system/neptune/missions.html
 | title=Missions to Neptune | year=2007
 | publisher=The Planetary Society | accessdate=2007-10-11}}</ref> the cause of which is not currently understood but which may be due to the gravitational interaction with small moons in orbit near them.<ref>{{cite news
 | last=Wilford | first=John Noble
 | date=December 15, 1989
 | title=Scientists Puzzled by Unusual Neptune Rings
 | publisher=Hubble News Desk
 | url=http://query.nytimes.com/gst/fullpage.html?res=950DE7DA1030F936A25751C1A96F948260
 | accessdate=2008-02-29 }}</ref>

The outermost ring, Adams, contains five prominent arcs now named ''Courage'', ''Liberté'', ''Egalité&nbsp;1'', ''Egalité&nbsp;2'' and ''Fraternité'' (Courage, Liberty, Equality and Fraternity).<ref>{{cite book
 | first=Arthur N. | last=Cox | year=2001
 | title=Allen's Astrophysical Quantities
 | publisher=Springer | isbn=0387987460 }}</ref> The existence of arcs was difficult to explain because the laws of motion would predict that arcs would spread out into a uniform ring over very short timescales. Astronomers now believe that the arcs are corralled into their current form by the gravitational effects of [[Galatea (moon)|Galatea]], a moon just inward from the ring.<ref>{{cite web
 | last=Munsell | first=Kirk | date=November 13, 2007
 | coauthors=Smith, Harman; Harvey, Samantha
 | url=http://solarsystem.nasa.gov/planets/profile.cfm?Object=Neptune&Display=Rings
 | title=Planets: Neptune: Rings
 | work=Solar System Exploration | publisher=NASA
 | accessdate=2008-02-29
}}</ref><ref>{{cite journal
 | last=Salo | first=Heikki | coauthors=Hänninen, Jyrki
 | title=Neptune's Partial Rings: Action of Galatea on Self-Gravitating Arc Particles
 | journal=Science | year=1998 | volume=282
 | issue=5391 | pages=1102–1104
 | url=http://www.scienceonline.org/cgi/content/full/282/5391/1102
 | accessdate=2008-02-29
 | doi=10.1126/science.282.5391.1102
 | pmid=9804544 }}</ref>

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.<ref>{{cite web
 | url=http://www.newscientist.com/channel/space/mg18524925.900
 | title=Neptune's rings are fading away
 | date=March 26, 2005 | author=Staff
 | publisher=New Scientist | accessdate=2007-08-06}}</ref>

==Climate==
One difference between Neptune and Uranus is the typical level of meteorological activity. When the ''Voyager 2'' spacecraft flew by Uranus in 1986, that planet was visually quite bland. In contrast Neptune exhibited notable weather phenomena during the 1989 ''Voyager 2'' fly-by.<ref name=spot />
[[Image:Neptune storms.jpg|thumb|upright|right|The [[Great Dark Spot]] (top), Scooter (middle white cloud),<ref name=scooter>{{cite web
 | first=Sue | last=Lavoie | date=January 8, 1998
 | title=PIA01142: Neptune Scooter
 | url=http://photojournal.jpl.nasa.gov/catalog/PIA01142
 | publisher=NASA | accessdate=2006-03-26}}</ref> and the [[Small Dark Spot]] (bottom)]]

Neptune's weather is characterized by extremely dynamic storm systems, with winds reaching speeds of almost 600&nbsp;m/s—nearly attaining [[supersonic]] flow.<ref>{{cite journal
 | last=Suomi | first=V. E. | coauthors=Limaye, S. S.; Johnson, D. R.
 | title=High Winds of Neptune: A Possible Mechanism
 | journal=Science | year=1991 | volume=251
 | issue=4996 | pages=929–932
 | url=http://www.sciencemag.org/cgi/content/abstract/251/4996/929
 | doi=10.1126/science.251.4996.929 | accessdate=2008-02-25
 | pmid=17847386
}}</ref> More typically, by tracking the motion of persistent clouds, wind speeds have been shown to vary from 20&nbsp;m/s in the easterly direction to 325&nbsp;m/s westward.<ref name="Hammel1989">{{cite journal
 | last=Hammel | first=H. B.
 | coauthors=Beebe, R. F.; De Jong, E. M.; Hansen, C. J.; Howell, C. D.; Ingersoll, A. P.; Johnson, T. V.; Limaye, S. S.; Magalhaes, J. A.; Pollack, J. B.; Sromovsky, L. A.; Suomi, V. E.; Swift, C. E.
 | title=Neptune's wind speeds obtained by tracking clouds in ''Voyager 2'' images
 | journal=Science | year=1989 | volume=245
 | pages=1367–1369
 | url=http://adsabs.harvard.edu/abs/1989Sci...245.1367H
 | accessdate=2008-02-27
 | doi=10.1126/science.245.4924.1367
 | pmid=17798743 }}</ref> At the cloud tops, the prevailing winds range in speed from 400&nbsp;m/s along the equator to 250&nbsp;m/s at the poles.<ref name=elkins-tanton/> Most of the winds on Neptune move in a direction opposite the planet's rotation.<ref name=burgess2>Burgess (1991):64–70.</ref> The general pattern of winds showed prograde rotation at high latitudes vs. retrograde rotation at lower latitudes. The difference in flow direction is believed to be a "skin effect" and not due to any deeper atmospheric processes.<ref name=atmo /> At 70° S latitude, a high-speed jet travels at a speed of 300&nbsp;m/s.<ref name=atmo />

The abundance of methane, ethane and acetylene 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.<ref name=atmo />

In 2007 it was discovered that the upper troposphere of Neptune's south pole was about 10°C warmer than the rest of Neptune, which averages approximately {{convert|-200|°C|K|-1|abbr=on}}.<ref>{{cite web|title=Evidence for methane escape and strong seasonal and dynamical perturbations of Neptune's atmospheric temperatures|author=Orton, G. S., Encrenaz T., Leyrat C., Puetter, R. and Friedson, A. J.|work=Astronomy and Astrophysics|url=http://www.aanda.org/index.php?option=article&access=doi&doi=10.1051/0004-6361:20078277
|year=2007|accessdate=2008-03-10}}</ref> The warmth differential is enough to let methane gas, which elsewhere lies frozen in Neptune's upper atmosphere, leak out through the south pole and into space. The relative "hot spot" is due to Neptune's [[axial tilt]], which has exposed the south pole to the [[Sun]] for the last quarter of Neptune's year, or roughly 40 Earth years. As Neptune slowly moves towards the opposite side of the Sun, the south pole will be darkened and the north pole illuminated, causing the methane release to shift to the north pole.<ref>{{cite news
 | first=Glenn | last=Orton | coauthors=Encrenaz, Thérèse
 | url=http://www.eso.org/public/outreach/press-rel/pr-2007/pr-41-07.html
 | title=A Warm South Pole? Yes, On Neptune!
 | publisher=ESO | date=September 18, 2007
 | accessdate=2007-09-20 }}</ref>

Because of seasonal changes, the cloud bands in the southern hemisphere of Neptune have been observed to increase in size and albedo. This trend was first seen in 1980 and is expected to last until about 2020. The long orbital period of Neptune results in seasons lasting forty years.<ref name=villard>{{cite news
 | last=Villard | first=Ray | coauthors=Devitt, Terry
 | date=May 15, 2003
 | title=Brighter Neptune Suggests A Planetary Change Of Seasons
 | publisher=Hubble News Center
 | url=http://hubblesite.org/newscenter/archive/releases/2003/17/text/
 | accessdate=2008-02-26 }}</ref>

===Storms===
[[Image:Neptune darkspot.jpg|thumb|left|''The Great Dark Spot'', as seen from ''Voyager 2'']]
In 1989, the [[Great Dark Spot]], an [[anticyclone|anti-cyclonic]] storm system spanning 13000×6600&nbsp;km,<ref name=spot>{{cite web
 | last=Lavoie | first=Sue | date=February 16, 2000
 | url=http://photojournal.jpl.nasa.gov/catalog/PIA02245
 | title=PIA02245: Neptune's blue-green atmosphere
 | publisher=NASA JPL | accessdate=2008-02-28 }}</ref> was discovered by [[NASA]]'s ''Voyager 2'' spacecraft. The storm resembled the [[Great Red Spot]] of Jupiter. Some five years later, however, on November 2, 1994, the [[Hubble Space Telescope]] did not see the Great Dark Spot on the planet. Instead, a new storm similar to the Great Dark Spot was found in the planet's northern hemisphere.<ref>{{cite journal
 | last=Hammel | first=H. B.
 | coauthors=Lockwood, G. W.; Mills, J. R.; Barnet, C. D.
 | title=Hubble Space Telescope Imaging of Neptune's Cloud Structure in 1994
 | journal=Science | year=1995 | volume=268
 | issue=5218 | pages=1740–1742
 | url=http://www.sciencemag.org/cgi/content/abstract/268/5218/1740
 | doi=10.1126/science.268.5218.1740
 | accessdate=2008-02-25
 | pmid=17834994 }}</ref>

The Scooter is another storm, a white cloud group farther south than the Great Dark Spot. Its nickname is due to the fact that when first detected in the months before the 1989 ''Voyager 2'' encounter it moved faster than the Great Dark Spot.<ref name=burgess2/> Subsequent images revealed even faster clouds. The [[Small Dark Spot]] is a southern cyclonic storm, the second-most-intense storm observed during the 1989 encounter. It initially was completely dark, but as ''Voyager 2'' approached the planet, a bright core developed and can be seen in most of the highest-resolution images.<ref>{{cite web
 | last=Lavoie | first=Sue | date=January 29, 1996
 | url=http://photojournal.jpl.nasa.gov/catalog/PIA00064
 | title=PIA00064: Neptune's Dark Spot (D2) at High Resolution
 | publisher=NASA JPL | accessdate=2008-02-28 }}</ref>

Neptune's dark spots are thought to occur in the [[troposphere]] at lower altitudes than the brighter cloud features,<ref>{{cite journal
 | last=S. G. | first=Gibbard
 | coauthors=de Pater, I.; Roe, H. G.; Martin, S.; Macintosh, B. A.; Max, C. E.
 | title=The altitude of Neptune cloud features from high-spatial-resolution near-infrared spectra
 | journal=Icarus | year=2003 | volume=166
 | issue=2 | pages=359–374
 | doi=10.1016/j.icarus.2003.07.006
 | url=http://cips.berkeley.edu/research/depater_altitude.pdf
 | format=PDF | accessdate=2008-02-26 }}</ref> 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.<ref name=apj125/> Often associated with dark spots are brighter, persistent methane clouds that form around the [[tropopause]] layer.<ref>{{cite journal
 | last=Stratman | first=P. W.
 | coauthors=Showman, A. P.; Dowling, T. E.; Sromovsky, L. A.
 | title=EPIC Simulations of Bright Companions to Neptune's Great Dark Spots
 | journal=Icarus | year=2001 | volume=151
 | issue=2 | pages=275–285 | doi=10.1006/icar.1998.5918
 | url=http://www.lpl.arizona.edu/~showman/publications/stratman-etal-2001.pdf
 | format=PDF | accessdate=2008-02-26 }}</ref> 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.<ref>{{cite journal
 | last=Sromovsky | first=L. A.
 | coauthors=Fry, P. M.; Dowling, T. E.; Baines, K. H.
 | title=The unusual dynamics of new dark spots on Neptune
 | journal=Bulletin of the American Astronomical Society
 | year=2000 | volume=32 | pages=1005
 | url=http://adsabs.harvard.edu/abs/2000DPS....32.0903S
 | accessdate=2008-02-29 }}</ref>

===Internal heat===
Neptune's more varied weather when compared to Uranus is believed to be due in part to its higher internal heat.<ref name=heat>{{cite web|title=Heat Sources within the Giant Planets|author=Williams, Sam|work=University of California, Berkeley |year=2004|url=http://64.233.179.104/scholar?hl=en&lr=&q=cache:jc66Q17QyAwJ:www.cs.berkeley.edu/~samw/projects/ay249/z_heat_sources/Paper.doc+neptune+uranus+weather+heat
|accessdate=2008-03-10}}</ref> Although Neptune lies half again as far from the Sun as Uranus, and receives only 40% its amount of sunlight,<ref name=atmo /> the two planets' surface temperatures are roughly equal.<ref name=heat /> The upper regions of Neptune's troposphere reach a low [[temperature]] of {{convert|-221.4|°C|K|abbr=on}}. At a depth where the atmospheric [[pressure]] equals {{convert|1|bar|kPa|lk=on}}, the temperature is {{convert|-201.15|°C|K|abbr=on}}.<ref>{{cite journal
 | last=Lindal | first=Gunnar F.
 | title=The atmosphere of Neptune - an analysis of radio occultation data acquired with Voyager 2
 | journal=Astronomical Journal
 | year=1992 | volume=103 | pages=967–982
 | url=http://adsabs.harvard.edu/abs/1992AJ....103..967L
 | accessdate=2008-02-25
 | doi=10.1086/116119 }}</ref> Deeper inside the layers of gas, however, the temperature rises steadily. As with Uranus, the source of this heating is unknown, but the discrepancy is larger: Uranus only radiates 1.1 times as much energy as it receives from the Sun;<ref>{{cite web
 | title=Class 12 - Giant Planets - Heat and Formation
 | work=3750 - Planets, Moons & Rings | year=2004
 | publisher=Colorado University, Boulder
 | url=http://lasp.colorado.edu/~bagenal/3750/ClassNotes/Class12/Class12.html
 | accessdate=2008-03-13}}</ref> while Neptune radiates about 2.61 times as much energy as it receives from the Sun.<ref>{{cite journal
 | last=Pearl | first=J. C. | coauthors=Conrath, B. J.
 | title=The albedo, effective temperature, and energy balance of Neptune, as determined from Voyager data
 | journal=Journal of Geophysical Research Supplement | year=1991
 | volume=96 | pages=18,921–18,930
 | url=http://adsabs.harvard.edu/abs/1991JGR....9618921P
 | accessdate=2008-02-20 }}</ref> Neptune is the farthest planet from the Sun, yet its internal energy is sufficient to drive the fastest planetary winds seen in the Solar System. Several possible explanations have been suggested, including [[radiogenic]] heating from the planet's core,<ref name=williams>{{cite paper
 | last=Williams | first=Sam
 | title=Heat Sources Within the Giant Planets
 | date=November 24, 2004 | publisher=UC Berkeley
 | url = http://64.233.179.104/scholar?hl=en&lr=&q=cache:jc66Q17QyAwJ:www.cs.berkeley.edu/~samw/projects/ay249/z_heat_sources/Paper.doc+%22Heat+Sources+Within+the+Giant+Planets%22
 | format=[[DOC (computing)|DOC]] | accessdate=2008-02-20 }}</ref> conversion of methane under high pressure into hydrogen, [[diamond]] and longer [[hydrocarbon]]s (the hydrogen and diamond would then rise and sink, respectively, releasing [[gravitational potential energy]]),<ref name=williams/><ref>{{cite journal
 | last=Scandolo | first=Sandro | coauthors=Jeanloz, Raymond
 | title=The Centers of Planets | journal=American Scientist
 | year=2003 | volume=91 | issue=6 | pages=516
 | doi=10.1511/2003.6.516 }}</ref> and [[convection]] in the lower atmosphere that causes [[gravity wave]]s to break above the tropopause.<ref>{{cite journal
 | last=McHugh | first=J. P. | month=September | year=1999
 | title=Computation of Gravity Waves near the Tropopause
 | journal=American Astronomical Society, DPS meeting #31, #53.07
 | url=http://adsabs.harvard.edu/abs/1999DPS....31.5307M
 | accessdate=2008-02-19 }}
</ref><ref>{{cite journal
 | last=McHugh | first=J. P. | coauthors=Friedson, A. J.
 | month=September | year=1996 | pages=1078
 | title=Neptune's Energy Crisis: Gravity Wave Heating of the Stratosphere of Neptune
 | journal=Bulletin of the American Astronomical Society
 | url=http://adsabs.harvard.edu/full/1996DPS....28.0507L
 | accessdate=2008-02-19 }}</ref>

==Orbit and rotation==
The average distance between Neptune and the Sun is 4.55&nbsp;billion&nbsp;km (about 30.1&nbsp;AU), and it completes an orbit every 164.79&nbsp;years. On July 12, 2011, Neptune will have completed the first full orbit since its discovery in 1846,<ref name="fact2">{{cite web
 | first=K.| last=Munsell | coauthors=Smith, H.; Harvey, S.
 | url = http://solarsystem.nasa.gov/planets/profile.cfm?Object=Neptune&Display=Facts
 | title = Neptune: Facts & Figures
 | date =November 13, 2007
 | publisher = NASA | accessdate = 2007-08-14}}</ref><ref name=Horizons2011>{{cite web
 | author=Anonymous | date=February 9, 2007
 | url=http://home.comcast.net/~kpheider/nept2011.txt
 | title=Horizons Output for Neptune 2010–2011
 | accessdate=2008-02-25 }}—Numbers generated using the Solar System Dynamics Group, Horizons On-Line Ephemeris System.</ref> although it will not appear at its exact discovery position in our sky because the Earth will be in a different location in its 365.25-day orbit.

The elliptical orbit of Neptune is inclined 1.77° compared to the Earth. Because of an [[orbital eccentricity|eccentricity]] of 0.011, the distance between Neptune and the Sun varies by 101&nbsp;million&nbsp;km between [[perihelion]] and [[aphelion]], the nearest and most distant points of the planet from the Sun along the orbital path, respectively.<ref name=horizons/>

The axial tilt of Neptune is 28.32°,<ref>{{cite web
 | last=Williams | first=David R. | date=January 6, 2005
 | url=http://nssdc.gsfc.nasa.gov/planetary/planetfact.html
 | title=Planetary Fact Sheets | publisher=NASA
 | accessdate=2008-02-28 }}</ref> which is similar to the tilts of Earth (23°) and Mars (25°). As a result, this planet experiences similar seasonal changes. However, the long orbital period of Neptune means that the seasons last for forty Earth years.<ref name=villard/> Its sidereal rotation period (day) is roughly 16.11&nbsp;hours.<ref name="fact2" /> Since its axial tilt is comparable to the Earth's, the variation in the length of its day over the course of its long year is not any more extreme.

Because Neptune is not a solid body, its atmosphere undergoes [[differential rotation]]. The wide equatorial zone rotates with a period of about 18&nbsp;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&nbsp;hours. This differential rotation is the most pronounced of any planet in the Solar System,<ref>{{cite journal
 | last=Hubbard | first=W. B.
 | coauthors=Nellis, W. J.; Mitchell, A. C.; Holmes, N. C.; McCandless, P. C.; Limaye, S. S.
 | title=Interior Structure of Neptune: Comparison with Uranus
 | journal=Science | year=1991 | volume=253
 | issue=5020 | pages=648–651
 | url=http://www.sciencemag.org/cgi/content/abstract/253/5020/648
 | accessdate=2008-02-28
 | doi=10.1126/science.253.5020.648
 | pmid=17772369 }}</ref> and it results in strong latitudinal wind shear.<ref name=apj125>{{cite journal
 | last=Max | first=C. E.
 | coauthors=Macintosh, B. A.; Gibbard, S. G.; Gavel, D. T.; Roe, H. G.; de Pater, I.; [[Andrea Ghez|Ghez, A. M.]]; Acton, D. S.; Lai, O.; Stomski, P.; Wizinowich, P. L.
 | title=Cloud Structures on Neptune Observed with Keck Telescope Adaptive Optics
 | journal=The Astronomical Journal,
 | year=2003 | volume=125 | issue=1 | pages=364–375
 | url=http://adsabs.harvard.edu/abs/2003AJ....125..364M
 | accessdate=2008-02-27
 | doi=10.1086/344943
}}</ref>

===Orbital resonances===
{{main|Kuiper belt}}
[[Image:TheKuiperBelt classes-en.svg|right|thumb|250px| A diagram showing the orbital resonances in the Kuiper belt caused by Neptune: the highlighted regions are the 2:3 resonance (Plutinos), the [[Classical Kuiper belt object|"classical belt"]], with orbits unaffected by Neptune, and the 1:2 resonance ([[twotino]]s).]]
Neptune's orbit has a profound impact on the region directly beyond it, known as the Kuiper belt. The Kuiper belt is a ring of small icy worlds, similar to the [[asteroid belt]] but far larger, extending from Neptune's orbit at 30&nbsp;AU out to about 55&nbsp;AU from the Sun.<ref>{{cite web
 | title=Collisional Erosion in the Primordial Edgeworth-Kuiper Belt and the Generation of the 30–50 AU Kuiper Gap
 | first=S. Alan | last=Stern
 | publisher=Geophysical, Astrophysical, and Planetary Sciences, Space Science Department, Southwest Research Institute
 | url=http://www.journals.uchicago.edu/doi/full/10.1086/304912
 | year=1997 | accessdate=2007-06-01}}</ref>  Much in the same way that Jupiter's gravity dominates the [[asteroid belt]], shaping its structure, so Neptune's gravity dominates the [[Kuiper belt]]. Over the age of the Solar System, certain regions of the Kuiper belt become destabilized by Neptune's gravity, creating gaps in the Kuiper belt's structure. The region between 40 and 42&nbsp;AU is an example.<ref>{{cite web
 | title=Large Scattered Planetesimals and the Excitation of the Small Body Belts
 | first=Jean-Marc | last=Petit | coauthors=Morbidelli, Alessandro; Valsecchi, Giovanni B.
 | url=http://www.oca.eu/morby/papers/6166a.pdf
 | year=1998 | accessdate=2007-06-23|format=PDF}}</ref>

There do, however, exist orbits within these empty regions where objects can survive for the age of the Solar System. These [[Orbital resonance|resonance]]s 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,<ref>{{cite web
 | title=List Of Transneptunian Objects
 | publisher=Minor Planet Center
 | url=http://www.cfa.harvard.edu/iau/lists/TNOs.html
 | accessdate=2007-06-23}}</ref> is the 2:3 resonance. Objects in this resonance complete 2 orbits for every 3 of Neptune, and are known as [[plutino]]s because the largest of the Kuiper belt objects, [[Pluto]], is among them.<ref>{{cite web
 | last=Jewitt | first=David | month=February | year=2004
 | url=http://www.ifa.hawaii.edu/faculty/jewitt/kb/plutino.html
 | title=The Plutinos | publisher=University of Hawaii
 | accessdate=2008-02-28 }}</ref> Although Pluto crosses Neptune's orbit regularly, the 2:3 resonance ensures they can never collide.<ref>{{cite journal
 | last=Varadi | first=F.
 | title=Periodic Orbits in the 3:2 Orbital Resonance and Their Stability
 | journal=The Astronomical Journal
 | year=1999 | volume=118 | pages=2526–2531
 | url=http://adsabs.harvard.edu/abs/1999AJ....118.2526V
 | accessdate=2008-02-28
 | doi=10.1086/301088 }}</ref> The 3:4, 3:5, 4:7 and 2:5 resonances are less populated.<ref>{{cite book|title=Beyond Pluto: Exploring the outer limits of the solar system |author=John Davies|publisher=Cambridge University Press|year=2001|pages=104}}</ref>

Neptune possesses a number of [[Neptune Trojan|trojan objects]], which occupy its {{L4}} and {{L5}} [[Lagrangian point]]s—gravitationally stable regions leading and trailing it in its orbit. Neptune trojans can be viewed as being in a 1:1 resonance with Neptune. Neptune trojans are remarkably stable in their orbits and are unlikely to have been captured by Neptune, but rather to have formed alongside it.<ref>{{cite web
 | title=Resonance Occupation in the Kuiper Belt: Case Examples of the 5 : 2 and Trojan Resonances
 | first=E. I. | last=Chiang | coauthors=Jordan, A. B.; Millis, R. L.; M. W. Buie; Wasserman, L. H.; Elliot, J. L.; Kern, S. D.; Trilling, D. E.; Meech, K. J.; Wagner, R. M.
 |year=2003
 | url=http://www.journals.uchicago.edu/doi/full/10.1086/375207
 | accessdate=2007-08-17}}</ref>

==Formation and migration==
{{main|Formation and evolution of the Solar System|Nice model}}

[[Image:Lhborbits.png|thumb|400px|A simulation showing Outer Planets and Kuiper Belt: a) Before Jupiter/Saturn 2:1 resonance b) Scattering of Kuiper Belt objects into the solar system after the orbital shift of Neptune c) After ejection of Kuiper Belt bodies by Jupiter]]
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 creation. One is that the ice giants were not created by core accretion but from instabilities within the original [[protoplanetary disc]], and later had their atmospheres blasted away by radiation from a nearby massive [[OB star]].<ref>{{cite web
 | title=Formation of gas and ice giant planets
 | first=Alan P. | last=Boss | date=2002-09-30 | url=http://www.sciencedirect.com/science?_ob=ArticleURL&_udi=B6V61-46SVX4B-1&_user=10&_rdoc=1&_fmt=&_orig=search&_sort=d&view=c&_acct=C000050221&_version=1&_urlVersion=0&_userid=10&md5=8356c9b57117437b185f44aca5ae71d0
 | accessdate=2008-03-05
 | work=Earth and Planetary Science Letters
 | publisher=ELSEVIER }}</ref>

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.<ref>{{cite web
 | title=The formation of Uranus and Neptune among Jupiter and Saturn
 | first=Edward W. |last=Thommes
 | coauthors=Duncan, Martin J.; Levison, Harold F.
 | year=2001 | url=http://arxiv.org/abs/astro-ph/0111290
 | accessdate=2008-03-05}}</ref> This hypothesis of migration after formation is currently favoured, due to its ability to better explain the occupancy of the populations of small objects observed in the trans-Neptunian region.<ref>{{cite web
 | title=Orbital shuffle for early solar system
 | first=Kathryn | last= Hansen | publisher=Geotimes
 | url=http://www.geotimes.org/june05/WebExtra060705.html
 | date=June 7, 2005 | accessdate=2007-08-26}}</ref> The current most widely accepted<ref name="Crida2009">
  {{cite journal
  |last=Crida
  |first=A.
  |date=2009
  |title=Solar System formation
  |journal=Invited review talk on Solar System formation, at the JENAM 2008 conference. Proceeding to appear in "Reviews in Modern Astronomy, 21"
  |url=http://adsabs.harvard.edu/abs/2009arXiv0903.3008C
}}</ref><ref name="Desch07">{{cite journal|last=Desch|first=S. J.|date=2007|title=Mass Distribution and Planet Formation in the Solar Nebula|journal=The Astrophysical Journal|volume=671|pages=878-893|url=http://www.iop.org/EJ/abstract/0004-637X/671/1/878|doi=10.1086/522825}}</ref><ref name="Smith2009">{{cite journal|last=Smith|first=R.|coauthors=L. J. Churcher; M. C. Wyatt; M. M. Moerchen; C. M. Telesco|date=2009|title=Resolved debris disc emission around $\eta $ Telescopii: a young solar system or ongoing planet formation?|journal=Astronomy and Astrophysics|volume=493|pages=299-308|doi=10.1051/0004-6361:200810706}}</ref> 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==
{{main|Moons of Neptune}}
:''For a timeline of discovery dates, see [[Timeline of discovery of Solar System planets and their moons]]''
[[Image:Voyager 2 Neptune and Triton.jpg|thumb|right|Neptune (top) and Triton (bottom)]]

Neptune has 13 known [[natural satellite|moons]].<ref name="fact"/> The largest by far, comprising more than 99.5 percent of the mass in orbit around Neptune<ref>Mass of Triton: 2.14{{e|22}}&nbsp;kg. Combined mass of 12 other known moons of Neptune: 7.53{{e|19}} kg, or 0.35 percent. The mass of the rings is negligible.</ref> and the only one massive enough to be [[spheroid]]al, is [[Triton (moon)|Triton]], discovered by [[William Lassell]] just 17&nbsp;days after the discovery of Neptune itself. Unlike all other large planetary moons in the Solar System, Triton has a [[retrograde orbit]], indicating that it was captured rather than forming in place; it probably was once a [[dwarf planet]] in the Kuiper belt.<ref>{{cite journal
 | first=Craig B. | last=Agnor | coauthors=Hamilton, Douglas P.
 | title=Neptune's capture of its moon Triton in a binary–planet gravitational encounter
 | journal=Nature | month=May | year=2006
 | volume=441 | issue=7090 | pages=192–194
 | doi=10.1038/nature04792
 | url= http://www.nature.com/nature/journal/v441/n7090/abs/nature04792.html
 | accessdate=2008-02-28
 | publisher=Nature Publishing Group }}</ref> It is close enough to Neptune to be locked into a [[synchronous rotation]], and it is slowly spiraling inward because of [[tidal acceleration]] and eventually will be torn apart, in about 3.6 billion years, when it reaches the [[Roche limit]].<ref>{{cite journal
 | first= Christopher F. | last=Chyba
 | coauthors=Jankowski, D. G.; Nicholson, P. D.
 | title = Tidal evolution in the Neptune-Triton system
 | journal = Astronomy and Astrophysics
 | month = July | year = 1989 | volume = 219
 | issue = 1–2 | pages=L23–L26
 | url = http://adsabs.harvard.edu/abs/1989A&A...219L..23C
 | accessdate = 2006-05-10
 | publisher=EDP Sciences }}</ref> In 1989, Triton was the coldest object that had yet been measured in the solar system,<ref>{{cite news
 | last=Wilford | first=John N. | publisher=The New York Times
 | date=August 29, 1989
 | title=Triton May Be Coldest Spot in Solar System
 | url=http://query.nytimes.com/gst/fullpage.html?res=950DE4DC1138F93AA1575BC0A96F948260
 | accessdate=2008-02-29 }}</ref> with estimated temperatures of {{convert|−235|°C|K|0}}.<ref>{{cite journal
 | last=R. M. | first=Nelson
 | coauthors=Smythe, W. D.; Wallis, B. D.; Horn, L. J.; Lane, A. L.; Mayo, M. J.
 | title=Temperature and Thermal Emissivity of the Surface of Neptune's Satellite Triton
 | journal=Science | year=1990 | volume=250
 | issue=4979 | pages=429–431
 | url=http://www.sciencemag.org/cgi/content/abstract/250/4979/429
 | accessdate=2008-02-29
 | publisher=AAAS (USA)
 | doi=10.1126/science.250.4979.429
 | pmid=17793020}}</ref>

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.<ref><math>\begin{smallmatrix}\frac{r_{a}}{r_{p}} = \frac{2}{1-e}-1 = 2/0.2488-1=7.039.\end{smallmatrix}</math></ref>

[[Image:Proteus (Voyager 2).jpg|thumb|upright|left|Neptune's moon [[Proteus (moon)|Proteus]]]]
From July to September 1989, ''Voyager 2'' discovered six new Neptunian moons.<ref name="science4936">{{cite journal
 | last=Stone | first=E. C. | coauthors=Miner, E. D.
 | title=The Voyager 2 Encounter with the Neptunian System
 | journal=Science | year=1989 | volume=246
 | issue=4936 | pages=1417–1421
 | url=http://www.sciencemag.org/cgi/content/abstract/246/4936/1417
 | accessdate=2008-02-29
 | publisher=AAAS (USA)
 | doi=10.1126/science.246.4936.1417
 | pmid=17755996}}</ref> 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.<ref name=Brown>{{cite web
 | url=http://web.gps.caltech.edu/~mbrown/dwarfplanets/
 | title=The Dwarf Planets
 | first=Michael E. | last=Brown | authorlink=Michael E. Brown
 | publisher=California Institute of Technology, Department of Geological Sciences
 | accessdate=2008-02-09 }}</ref> Although the second-most-massive Neptunian moon, it is only one-quarter of one percent the mass of Triton. Neptune's innermost four moons—[[Naiad (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, it was found to have been caused by the moon. Five new irregular moons discovered between 2002 and 2003 were announced in 2004.<ref>{{cite journal
 | author=Holman, Matthew J. et al.
 | date=August 19, 2004  | url=http://www.nature.com/nature/journal/v430/n7002/abs/nature02832.html
 | title=Discovery of five irregular moons of Neptune
 | journal=Nature | volume=430 | pages=865–867
 | accessdate=2008-02-09
 | publisher=Nature Publishing Group
 | doi=10.1038/nature02832 }}
</ref><ref>{{cite news
 | author=Staff
 | url=http://news.bbc.co.uk/2/hi/science/nature/3578210.stm
 | title=Five new moons for planet Neptune
 | date=August 18, 2004
 | publisher=BBC News |accessdate=2007-08-06 }}</ref> As Neptune was the Roman god of the sea, the planet's moons have been named after lesser sea gods.<ref name=USGS/>

==Observation==
Neptune is never visible to the [[naked eye]], having a brightness between [[Apparent magnitude|magnitudes]] +7.7 and +8.0,<ref name="fact"/><ref name=ephemeris>{{cite web
 | last=Espenak | first=Fred | date=July 20, 2005
 | url=http://sunearth.gsfc.nasa.gov/eclipse/TYPE/TYPE.html
 | title=Twelve Year Planetary Ephemeris: 1995–2006
 | publisher=NASA | accessdate=2008-03-01 }}</ref> which can be outshone by Jupiter's [[Galilean moons]], the [[dwarf planet]] [[Ceres (dwarf planet)|Ceres]] and the [[asteroid]]s [[4 Vesta]], [[2 Pallas]], [[7 Iris]], [[3 Juno]] and [[6 Hebe]].<ref>See the respective articles for magnitude data.</ref> A telescope or strong binoculars will resolve Neptune as a small blue disk, similar in appearance to Uranus.<ref>Moore (2000):207.</ref>

Because of the distance of Neptune from the Earth, the [[angular diameter]] of the planet only ranges from 2.2–2.4&nbsp;[[arcsecond]]s;<ref name="fact"/><ref name=ephemeris/> the smallest of the Solar System planets. Its small apparent size has made it challenging to study visually; most telescopic data was fairly limited until the advent of [[Hubble Space Telescope]] and large ground-based telescopes with [[adaptive optics]].<ref>In 1977, for example, even the rotation period of Neptune remained uncertain.
See: {{cite journal
 | last=Cruikshank | first=D. P.
 | title=On the rotation period of Neptune
 | journal=Astrophysical Journal, Part 2 - Letters to the Editor
 | date=March 1, 1978 | volume=220
 | pages=L57–L59
 | url=http://adsabs.harvard.edu/abs/1978ApJ...220L..57C
 | accessdate=2008-03-01
 | publisher=University of Chicago Press
 | doi=10.1086/182636 }}</ref><ref>{{cite journal
 | last=Max | first=C.
 | title=Adaptive Optics Imaging of Neptune and Titan with the W.M. Keck Telescope
 | journal=Bulletin of the American Astronomical Society
 | month=December | year=1999 | volume=31 | pages=1512
 | url=http://adsabs.harvard.edu/abs/1999BAAS...31.1512M
 | accessdate=2008-03-01
 | publisher=American Astronomical Society}}</ref>

From the Earth, Neptune goes through apparent [[Retrograde and direct motion|retrograde motion]] every 367&nbsp;days, resulting in a looping motion against the background stars during each [[Opposition (astronomy and astrology)|opposition]]. These loops will carry it close to the 1846 discovery coordinates in April and July 2010 and in October and November 2011.<ref name=Horizons2011/>

Observation of Neptune in the [[radio]] frequency band shows that the planet is a source of both continuous emission and irregular bursts. Both sources are believed to originate from the planet's rotating magnetic field.<ref name=elkins-tanton/> 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.<ref>{{cite journal
 | last= Gibbard | first=S. G.
 | coauthors=Roe, H.; de Pater, I.; Macintosh, B.; Gavel, D.; Max, C. E.; Baines, K. H.; Ghez, A.
 | title=High-Resolution Infrared Imaging of Neptune from the Keck Telescope
 | journal=Icarus | year=1999 | volume=156 | pages=1–15
 | url=http://linkinghub.elsevier.com/retrieve/pii/S0019103501967661
 | accessdate=2008-03-01
 | publisher=Elsevier
 | doi=10.1006/icar.2001.6766}}</ref>

==Exploration==
{{main|Exploration of Neptune}}
''[[Voyager 2]]''<nowiki>'</nowiki>s closest approach to Neptune occurred on August 25, 1989. Since this was the last major planet the spacecraft could visit, it was decided to make a close flyby of the moon Triton, regardless of the consequences to the trajectory, similarly to what was done for ''[[Voyager 1]]''<nowiki>'</nowiki>s encounter with [[Saturn]] and its moon [[Titan (moon)|Titan]]. The images relayed back to Earth from ''Voyager 2'' became the basis of a 1989 [[Public Broadcasting Service|PBS]] all-night program, ''Neptune All Night''.<ref>{{cite web
 | last=Phillips | first=Cynthia | date=August 5, 2003
 | url=http://www.seti.org/about-us/voices/phillips-080503.php
 | title=Fascination with Distant Worlds
 | accessdate=2007-10-03 | publisher=[[SETI Institute]] }}</ref>

[[Image:Triton moon mosaic Voyager 2 (large).jpg|right|thumb|A ''Voyager 2'' image of Triton]]
During the encounter, signals from the spacecraft required 246&nbsp;minutes to reach the Earth. Hence, for the most part, the ''Voyager 2'' mission relied on pre-loaded commands for the Neptune encounter. The spacecraft performed a near-encounter with the moon [[Nereid (moon)|Nereid]] before it came within 4400&nbsp;km of Neptune's atmosphere on August 25, then passed close to the planet's largest moon [[Triton (moon)|Triton]] later the same day.<ref name=burgess>Burgess (1991):46–55.</ref>

The spacecraft verified the existence of a magnetic field surrounding the planet and discovered that the field was offset from the centre and tilted in a manner similar to the field around Uranus. The question of the planet's rotation period was settled using measurements of radio emissions. ''Voyager 2'' also showed that Neptune had a surprisingly active weather system. Six new moons were discovered, and the planet was shown to have more than one ring.<ref name="science4936"/><ref name=burgess/>
[[Image:SunsetNeptune.png|thumb|left|Voyager 2 passes Neptune in 1989.]]

In 2003, there was a proposal to [[NASA]]'s "Vision Missions Studies" to implement a "[[Neptune Orbiter|Neptune Orbiter with Probes]]" mission that does ''[[Cassini–Huygens|Cassini]]''-level science without fission-based electric power or propulsion. The work is being done in conjunction with [[Jet Propulsion Laboratory|JPL]] and the [[California Institute of Technology]].<ref>{{cite journal
 | last=Spilker | first=T. R. | coauthors=Ingersoll, A. P.
 | title=Outstanding Science in the Neptune System From an Aerocaptured Vision Mission
 | journal=Bulletin of the American Astronomical Society
 | year=2004 | volume=36 | pages=1094
 | url=http://adsabs.harvard.edu/abs/2004DPS....36.1412S
 | accessdate=2008-02-26
 | publisher=American Astronomical Society }}</ref>

==See also==
{{portal|Solar System|Solar system.jpg}}
* [[Planets in astrology#Neptune|Neptune in astrology]]
* [[Neptune in fiction]]
* [[Colonization of the outer Solar System#Neptune|Colonization of Neptune]]
* ''[[Neptune Orbiter]]'' – proposed space probe to Neptune (not before 2035- may have been indefinitely postponed, as [http://solarsystem.nasa.gov/missions/future1.cfm NASA's Strategic Exploration Plan] no longer includes it)
* [[Neptune Trojan]] – asteroids orbiting in Neptune's Lagrangian points
* ''[[The Planets]]'' – Neptune is one of the seven movements in Gustav Holst's orchestral suite, ''The Planets''

==References==
{{reflist|2}}

==Further reading==
*{{cite book
 | last=Baum | first=Richard | coauthors=Sheehan, William
 | year=2003
 | title=In Search of Planet Vulcan: The Ghost in Newton's Clockwork Universe
 | publisher=Basic Books | isbn=0738208892 }}
*{{cite book
 | last=Burgess | first=Eric | year=1991
 | title=Far Encounter: The Neptune System
 | publisher=Columbia University Press | isbn=0-231-07412-3 }}
*{{cite book
 | last=Cruikshank | first=Dale P. | title=Neptune and Triton
 | publisher=University of Arizona Press | year=1996
 | isbn=0-8165-1525-5 }}
*{{cite book
 | last=Elkins-Tanton | first=Linda T. | year=2006
 | title=Uranus, Neptune, Pluto, and the Outer Solar System
 | publisher=Chelsea House | location=New York
 | isbn=0-8160-5197-6 }}
*{{cite book
 | last=Littmann | first=Mark | year=2004
 | title=Planets Beyond, Exploring the Outer Solar System
 | publisher=Courier Dover Publications
 | isbn=0486436020 }}
*{{cite book
 | last=Miner | first=Ellis D. | coauthors=Wessen, Randii R.
 | title=Neptune: The Planet, Rings, and Satellites
 | year=2002 | publisher=Springer-Verlag
 | isbn=1-85233-216-6 }}
*{{cite book
 | last=Moore | first=Patrick | authorlink=Patrick Moore
 | title=The Data Book of Astronomy | year=2000
 | publisher = CRC Press | isbn=0-7503-0620-3 }}
*{{cite book
 | last=Standage | first=Tom
 | title=The Neptune File | year=2001
 | publisher = Penguin }}

==External links==
{{sisterlinks|Neptune}}
* [http://nssdc.gsfc.nasa.gov/planetary/factsheet/neptunefact.html NASA's Neptune fact sheet]
* [http://www.nasa.gov/worldbook/neptune_worldbook.html "Neptune."] Smith, Bradford A. ''World Book Online Reference Center''. 2004. World Book, Inc. (NASA.gov)
* [http://www.nineplanets.org/neptune.html Neptune] from Bill Arnett's nineplanets.org
* [http://www.astronomycast.com/astronomy/episode-63-neptune/ Neptune] [[Astronomy Cast]] episode #63, includes full transcript.
* [http://solarsystem.nasa.gov/planets/profile.cfm?Object=Neptune Neptune Profile] by [http://solarsystem.nasa.gov NASA's Solar System Exploration]
* [http://www.projectshum.org/Planets/neptune.html Planets – Neptune] A kid's guide to Neptune.

{{Neptune}}
{{Solar System}}
{{Atmospheres}}
{{featured article}}

[[Category:Neptune| ]]
[[Category:Gas giant planets]]

{{Link FA|it}}
{{Link FA|sl}}

[[af:Neptunus (planeet)]]
[[als:Neptun (Planet)]]
[[ang:Neptune (planēta)]]
[[ar:نبتون]]
[[frp:Neptuno (planèta)]]
[[ast:Neptunu (planeta)]]
[[az:Neptun (planet)]]
[[bn:নেপচুন গ্রহ]]
[[zh-min-nan:Hái-ông-chheⁿ]]
[[be:Планета Нептун]]
[[be-x-old:Нэптун (плянэта)]]
[[bs:Neptun]]
[[br:Neizhan (planedenn)]]
[[bg:Нептун (планета)]]
[[ca:Neptú (planeta)]]
[[cs:Neptun (planeta)]]
[[cy:Neifion (planed)]]
[[da:Neptun (planet)]]
[[de:Neptun (Planet)]]
[[et:Neptuun]]
[[el:Ποσειδώνας (πλανήτης)]]
[[es:Neptuno (planeta)]]
[[eo:Neptuno (planedo)]]
[[eu:Neptuno (planeta)]]
[[fa:نپتون]]
[[fo:Neptun]]
[[fr:Neptune (planète)]]
[[ga:Neiptiún (pláinéad)]]
[[gv:Neptune]]
[[gd:Neiptiùn]]
[[gl:Neptuno]]
[[gan:海王星]]
[[gu:નૅપ્ચ્યુન (ગ્રહ)]]
[[ko:해왕성]]
[[hy:Նեպտուն]]
[[hi:वरुण (ग्रह)]]
[[hr:Neptun (planet)]]
[[io:Neptuno]]
[[ilo:Neptuno (planeta)]]
[[id:Neptunus]]
[[is:Neptúnus (reikistjarna)]]
[[it:Nettuno (astronomia)]]
[[he:נפטון]]
[[jv:Neptunus]]
[[kn:ನೆಪ್ಚೂನ್]]
[[pam:Neptune]]
[[ka:ნეპტუნი (პლანეტა)]]
[[csb:Neptun]]
[[kk:Нептун (ғаламшар)]]
[[kw:Nevyon (planet)]]
[[sw:Neptun]]
[[ht:Neptin (planèt)]]
[[ku:Neptûn (gerstêrk)]]
[[la:Neptunus (planeta)]]
[[lv:Neptūns (planēta)]]
[[lb:Neptun (Planéit)]]
[[lt:Neptūnas]]
[[lij:Nettun (astrònomia)]]
[[li:Neptunus (planeet)]]
[[jbo:neptune]]
[[hu:Neptunusz]]
[[mk:Нептун]]
[[ml:നെപ്റ്റ്യൂണ്‍]]
[[mt:Nettunu (pjaneta)]]
[[mr:नेपच्यून ग्रह]]
[[arz:نيبتون]]
[[ms:Neptun]]
[[mdf:Нептун]]
[[mn:Далайн ван]]
[[my:နက်ပကျွန်းဂြိုဟ်]]
[[nah:Tlāloccītlalli]]
[[nl:Neptunus (planeet)]]
[[nds-nl:Neptunus (planeet)]]
[[ne:नेप्च्युन]]
[[new:वरुण]]
[[ja:海王星]]
[[no:Neptun (planet)]]
[[nn:Planeten Neptun]]
[[nov:Neptune (planete)]]
[[oc:Neptun (planeta)]]
[[uz:Neptun]]
[[pa:ਵਰੁਣ]]
[[nds:Neptun (Planet)]]
[[pl:Neptun]]
[[pt:Neptuno (planeta)]]
[[ksh:Nepptuun (Planneet)]]
[[ro:Neptun (planetă)]]
[[rm:Neptun (planet)]]
[[qu:Niptun (puriq quyllur)]]
[[ru:Нептун (планета)]]
[[sah:Нептун]]
[[stq:Neptun]]
[[sq:Neptuni]]
[[scn:Nettunu (pianeta)]]
[[simple:Neptune]]
[[sk:Neptún]]
[[sl:Neptun]]
[[szl:Neptůn]]
[[sr:Нептун (планета)]]
[[sh:Neptun (planeta)]]
[[fi:Neptunus]]
[[sv:Neptunus]]
[[tl:Neptuno]]
[[ta:நெப்டியூன்]]
[[te:నెప్ట్యూన్]]
[[th:ดาวเนปจูน]]
[[tg:Нептун]]
[[tr:Neptün (gezegen)]]
[[uk:Нептун (планета)]]
[[ur:نیپچون]]
[[ug:نېپتۇن]]
[[vi:Sao Hải Vương]]
[[fiu-vro:Neptun]]
[[war:Neptuno]]
[[yi:נעפטון]]
[[zh-yue:海王星]]
[[bat-smg:Neptūns]]
[[zh:海王星]]