View source for Neptune

{{Short description|Eighth planet from the Sun}}
{{About|the planet|the Roman god|Neptune (mythology)|other uses}}
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{{Use British English|date=August 2024}}
{{Use dmy dates|date=August 2024}}
{{Infobox planet
| name = Neptune
| symbol = [[File:Neptune symbol (bold).svg|24px|♆|class=skin-invert]], [[File:Neptune monogram (fixed width).svg|24px|⯉|class=skin-invert]]
| image = Neptune Voyager2 color calibrated.png
| caption = Neptune in true colour;<ref>{{Cite journal |last1=Irwin |first1=Patrick G. J. |last2=Dobinson |first2=Jack |last3=James |first3=Arjuna |last4=Teanby |first4=Nicholas A. |last5=Simon |first5=Amy A. |last6=Fletcher |first6=Leigh N. |last7=Roman |first7=Michael T. |last8=Orton |first8=Glenn S. |last9=Wong |first9=Michael H. |last10=Toledo |first10=Daniel |last11=Pérez-Hoyos |first11=Santiago |last12=Beck |first12=Julie |date=23 December 2023 |title=Modelling the seasonal cycle of Uranus's colour and magnitude, and comparison with Neptune |url=https://academic.oup.com/mnras/article/527/4/11521/7511973 |journal=Monthly Notices of the Royal Astronomical Society |volume=527 |issue=4 |pages=11521–11538 |doi=10.1093/mnras/stad3761 |doi-access=free |hdl=20.500.11850/657542 |hdl-access=free }}</ref> at center is the [[Great Dark Spot]]{{efn|Neptune's dark spots are not permanent features; the large dark spot observed by ''Voyager 2'' was designated GDS-89 for "Great Dark Spot 1989".}}
| background = LightBlue
| discovery_ref = <ref name="Hamilton">{{cite web |first=Calvin J. |last=Hamilton |date=4 August 2001 |url=http://www.solarviews.com/eng/neptune.htm |title=Neptune |publisher=Views of the Solar System |access-date=13 August 2007 |archive-url=https://web.archive.org/web/20070715223035/http://www.solarviews.com/eng/neptune.htm |archive-date=15 July 2007 |url-status=dead}}</ref>
| discoverer = {{plainlist|
* [[Johann Gottfried Galle|Johann Galle]]
* [[Urbain Le Verrier]]
* [[John Couch Adams]]
 }}
| discovered = 23 September 1846
| pronounced = {{IPAc-en|audio=en-us-Neptune.ogg|us|ˈ|n|ɛ|p|t|uː|n}}, 
{{IPAc-en|uk|-|tj|uː|n}}<ref>
{{cite dictionary |title=Neptune |first=Elizabeth |last=Walter |date=21 April 2003 |dictionary=Cambridge Advanced Learner's Dictionary |publisher=Cambridge University Press |edition=2nd |isbn=978-0-521-53106-1}}</ref>
| adjectives = Neptunian ({{IPAc-en|n|ɛ|p|ˈ|tj|uː|n|i|ən}}),<ref>{{OED|Neptunian}}</ref> Poseidean<ref name=Poseidean>{{Cite web |url=http://solarsystem.nasa.gov/multimedia/downloads/Small_RPS_Report.pdf |title=Enabling Exploration with Small Radioisotope Power Systems |date=September 2004 |access-date=26 January 2016|publisher=NASA |archive-url=https://web.archive.org/web/20161222125722/http://solarsystem.nasa.gov/multimedia/downloads/Small_RPS_Report.pdf |archive-date=22 December 2016 |url-status=dead}}</ref>
| named_after = Latin {{lang|la|Neptunus}}, via French {{lang|fr|Neptune}}
| orbit_ref = <ref name=Horizons>{{cite web |first=Donald K. |last=Yeomans |url=https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%278%27&TABLE_TYPE=%27ELEMENTS%27&START_TIME=%272000-01-01%27&STOP_TIME=%272000-01-02%27&STEP_SIZE=%27200%20years%27&CENTER=%27@0%27&OUT_UNITS=%27AU-D%27 |title=HORIZONS Web-Interface for Neptune Barycenter (Major Body=8) |publisher=[[JPL Horizons On-Line Ephemeris System]] |access-date=18 July 2014 |archive-date=7 September 2021 |archive-url=https://web.archive.org/web/20210907055935/https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%278%27&TABLE_TYPE=%27ELEMENTS%27&START_TIME=%272000-01-01%27&STOP_TIME=%272000-01-02%27&STEP_SIZE=%27200%20years%27&CENTER=%27%400%27&OUT_UNITS=%27AU-D%27 |url-status=live }}—Select "Ephemeris Type: Orbital Elements", "Time Span: 2000-01-01 12:00 to 2000-01-02". ("Target Body: Neptune Barycenter" and "Center: Solar System Barycenter (@0)".)</ref><ref group=lower-alpha name=barycentre>Orbital elements refer to the Neptune barycentre and Solar System barycentre. These 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 the motion of the moons.</ref>
| epoch = [[J2000]]
| apsis = helion
| aphelion = {{convert|30.33|AU|e9km|2|abbr=unit|lk=in}}
| perihelion = {{convert|29.81|AU|e9km|2|abbr=unit}}
| time_periastron = 2042-Sep-04<ref name=horizons-perihelion>{{Cite web|url=https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%27899%27&START_TIME=%272042-08-20%27&STOP_TIME=%272042-09-15%27&STEP_SIZE=%273%20hours%27&QUANTITIES=%2719%27|title=HORIZONS Planet-center Batch call for September 2042 Perihelion|website=ssd.jpl.nasa.gov|type=Perihelion for Neptune's planet-center (899) occurs on 2042-Sep-04 at 29.80647406au during a rdot flip from negative to positive|publisher=NASA/JPL|access-date=7 September 2021|archive-date=7 September 2021|archive-url=https://web.archive.org/web/20210907220759/https://ssd.jpl.nasa.gov/horizons_batch.cgi?batch=1&COMMAND=%27899%27&START_TIME=%272042-08-20%27&STOP_TIME=%272042-09-15%27&STEP_SIZE=%273%20hours%27&QUANTITIES=%2719%27|url-status=live}}</ref>
| semimajor = {{convert|30.07|AU|e9km|2|abbr=unit}}
| eccentricity = {{val|0.008678}}
| period = {{plainlist|
* 164.8 [[julian year (astronomy)|yr]]
* 60,195 days
* 89,666 Neptunian [[solar day]]s<ref name="planet_years">{{cite web |last=Seligman |first=Courtney |url=http://cseligman.com/text/sky/rotationvsday.htm |title=Rotation Period and Day Length |access-date=13 August 2009 |archive-url=https://web.archive.org/web/20110728200555/http://cseligman.com/text/sky/rotationvsday.htm |archive-date=28 July 2011 |url-status=dead }}</ref>
 }}
| synodic_period = 367.49&nbsp;days<ref name="fact">{{cite web |last=Williams |first=David R. |date=1 September 2004 |url=http://nssdc.gsfc.nasa.gov/planetary/factsheet/neptunefact.html |title=Neptune Fact Sheet |publisher=NASA |access-date=14 August 2007 |archive-url=https://web.archive.org/web/20100701192119/http://nssdc.gsfc.nasa.gov/planetary/factsheet/neptunefact.html |archive-date=1 July 2010 |url-status=live }}</ref>
| avg_speed = {{val|5.43|u=km/s}}<ref name="fact"/>
| mean_anomaly = {{val|259.883|u=°}}
| inclination = {{val|1.770|u=°}} to [[ecliptic]]<br/>6.43° to [[Sun]]'s equator<br/>0.74° to [[invariable plane]]<ref name=Souami_Souchay_2012>{{cite journal
 | title=The solar system's invariable plane
 | last1=Souami | first1=D. | last2=Souchay | first2=J.
 | journal=Astronomy & Astrophysics
 | volume=543 | id=A133 | pages=11 | date=July 2012
 | doi=10.1051/0004-6361/201219011 | bibcode=2012A&A...543A.133S | doi-access=free }}</ref>
| asc_node = {{val|131.783|u=°}}
| arg_peri = {{val|273.187|u=°}}
| satellites = [[Moons of Neptune|16]]
| mean_radius = {{nowrap|{{val|fmt=commas|24622|19|u=km}}}}<ref name="Seidelmann Archinal A'hearn et al. 2007"/><ref group=lower-alpha name=1bar/>
| equatorial_radius = {{nowrap|{{val|fmt=commas|24764|15|u=km}}}}<ref name="Seidelmann Archinal A'hearn et al. 2007"/><ref group=lower-alpha name=1bar>Refers to the level of {{convert|1|bar|kPa}} atmospheric pressure</ref><br/>3.883 Earths
| polar_radius = {{nowrap|{{val|fmt=commas|24341|30|u=km}}}}<ref name="Seidelmann Archinal A'hearn et al. 2007"/><ref group=lower-alpha name=1bar/><br/>3.829 Earths
| flattening = {{val|0.0171|0.0013}} <!-- calculated from data in ref name="Seidelmann Archinal A'hearn et al. 2007" -->
| surface_area = {{val|7.6183|e=9|u=km2}}<ref name="fact2" /><ref group=lower-alpha name=1bar/><br/>14.98 Earths
| volume = {{val|6.253|e=13|u=km3}}<ref name="fact"/><ref group=lower-alpha name=1bar/><br/>57.74 Earths
| mass = {{val|1.02409|e=26|u=kg}}<ref name="fact"/><br/>17.147 Earths<br/>5.15{{e|-5}}&nbsp;Suns
| density = {{val|1.638|u=g/cm3}}<ref name="fact"/><ref group=lower-alpha name=1bar_b>Based on the volume within the level of 1 bar atmospheric pressure</ref>
| surface_grav = {{cvt|11.15|m/s2|g0|lk=out}}<ref name="fact"/><ref group=lower-alpha name=1bar/>
| moment_of_inertia_factor = {{val|0.23}}<ref name="PS15">{{cite book |last2=Lissauer |first2=Jack J. |last1=de Pater |first1=Imke |title=Planetary Sciences |date=2015 |url=https://books.google.com/books?id=stFpBgAAQBAJ&pg=PA250 |page=250 |publisher=Cambridge University Press |location=New York |isbn=978-0-521-85371-2 |edition=2nd updated |access-date=17 August 2016 |archive-url=https://web.archive.org/web/20161126120848/https://books.google.com/books?id=stFpBgAAQBAJ&pg=PA250 |archive-date=26 November 2016 |url-status=live }}</ref> (estimate)
| escape_velocity = 23.5&nbsp;km/s<ref name="fact"/><ref group=lower-alpha name=1bar/>
| rotation = {{val|0.67125|u=days}}<br/>
16 h 6 m 36 s<ref name="planet_years"/>
| sidereal_day = 0.6713&nbsp;day<ref name="fact"/><br/>16&nbsp;h 6&nbsp;min 36&nbsp;s
| rot_velocity = {{val|2.68|u=km/s}}<ref name="g142">{{cite book | last=Kennett | first=Carolyn | title=Uranus and Neptune | publisher=Reaktion Books | year=2022 | isbn=978-1-78914-642-4 | page=185}}</ref>
| axial_tilt = 28.32° (to orbit)<ref name="fact"/>
| right_asc_north_pole = {{RA|19|57|20}}<ref name="Seidelmann Archinal A'hearn et al. 2007"/><br/>299.36°<ref name="iau2015">{{Cite journal |last1=Archinal |first1=B. A. |last2=Acton |first2=C. H. |last3=A'Hearn |first3=M. F. |last4=Conrad |first4=A. |last5=Consolmagno |first5=G. J. |last6=Duxbury |first6=T. |last7=Hestroffer |first7=D. |last8=Hilton |first8=J. L. |last9=Kirk |first9=R. L. |last10=Klioner |first10=S. A. |last11=McCarthy |first11=D. |last12=Meech |first12=K. |last13=Oberst |first13=J. |last14=Ping |first14=J. |last15=Seidelmann |first15=P. K. |date=2018 |title=Report of the IAU Working Group on Cartographic Coordinates and Rotational Elements: 2015 |url=http://link.springer.com/10.1007/s10569-017-9805-5 |journal=Celestial Mechanics and Dynamical Astronomy |language=en |volume=130 |issue=3 |page=22 |doi=10.1007/s10569-017-9805-5 |bibcode=2018CeMDA.130...22A }}</ref>
| declination = 43.46°<ref name="iau2015" />
| albedo = 0.290 ([[Bond albedo|bond]])<ref name="Pearl_et_al_Neptune"/><br/>0.442 ([[Geometric albedo|geom.]])<ref name="Mallama_et_al"/>
| temp_name1 = 1 bar level
| mean_temp_1 = {{cvt|72|K|C|0|lk=in}}<ref name="fact"/>
| temp_name2 = {{cvt|0.1|bar|kPa}}
| mean_temp_2 = {{cvt|55|K|C|0|lk=in}}<ref name="fact"/>
| magnitude = 7.67<ref name="Mallama_and_Hilton"/> to 8.00<ref name="Mallama_and_Hilton"/>
| abs_magnitude = −6.9<ref name="IMCCE">{{cite web | title=Encyclopedia - the brightest bodies | website=IMCCE | url=https://promenade.imcce.fr/en/pages5/572.html | access-date=29 May 2023 | archive-date=24 July 2023 | archive-url=https://web.archive.org/web/20230724225002/https://promenade.imcce.fr/en/pages5/572.html | url-status=live }}</ref>
| angular_size = 2.2–2.4″<ref name="fact"/><ref name=ephemeris>{{cite web |last=Espenak |first=Fred |date=20 July 2005 |url=http://eclipse.gsfc.nasa.gov/TYPE/TYPE.html |title=Twelve Year Planetary Ephemeris: 1995–2006 |publisher=NASA |access-date=1 March 2008 |archive-url=https://archive.today/20121205061717/http://eclipse.gsfc.nasa.gov/TYPE/TYPE.html |archive-date=5 December 2012 |url-status=live }}</ref>
| atmosphere_ref = <ref name="fact"/>
| scale_height = {{val|19.7|0.6|u=km}}
| atmosphere_composition = {{plainlist|
* {{val|80|3.2|u=%}} [[hydrogen]]
* {{val|19|3.2|u=%}} [[helium]]
* {{val|1.5|0.5|u=%}} [[methane]]
* ~0.019% [[hydrogen deuteride]]
* ~0.00015% [[ethane]]
* Icy [[Volatile (astrogeology)|volatiles]]: {{cslist|[[ammonia]]|[[ice|water ice]]|[[ammonium hydrosulfide]]}}}}
}}
'''Neptune''' is the eighth and farthest known [[planet]] from the [[Sun]]. It is the [[List of Solar System objects by size|fourth-largest planet]] in the [[Solar System]] by diameter, the third-most-massive planet, and the densest [[giant planet]]. It is 17&nbsp;times the [[mass of Earth]]. Compared to its fellow [[ice giant]] [[Uranus]], Neptune is slightly more massive, but denser and smaller. Being composed primarily of gases and liquids,<ref name="c102">{{cite web | title=Neptune| website=NASA Science | date=10 November 2017 | url=https://science.nasa.gov/neptune/neptune-facts/ | access-date=19 July 2024}}</ref> it has no well-defined solid surface, and orbits the Sun once every 164.8&nbsp;[[julian year (astronomy)|years]] at an [[orbital distance]] of {{convert|30.1|AU|e9km e9mi|1|lk=in|abbr=off}}. It is named after [[Neptune (mythology)|the Roman god of the sea]] and has the [[astronomical symbol]] {{nowrap|[[File:Neptune symbol (fixed width).svg|16px|♆]],}} representing [[Trident of Poseidon|Neptune's trident]].{{efn|A second symbol, an ‘LV’ monogram [[file:Neptune monogram (fixed width).svg|16px|⯉]] for 'Le&nbsp;Verrier', analogous to the ‘H’ monogram [[file:Uranus monogram (fixed width).svg|16px|♅]] for Uranus. It was never much used outside of France and is now archaic.}}

Neptune is not visible to the unaided eye and is the only planet in the Solar System that was not initially observed by direct [[empirical observation]]. Rather, unexpected changes in the orbit of Uranus led [[Alexis Bouvard]] to hypothesise that its orbit was subject to gravitational [[Perturbation (astronomy)|perturbation]] by an unknown planet. After Bouvard's death, the position of Neptune was mathematically predicted from his observations, independently, by [[John Couch Adams]] and [[Urbain Le Verrier]]. Neptune was subsequently directly observed with a telescope on 23&nbsp;September 1846<ref name="Hamilton"/> by [[Johann Gottfried Galle]] within a [[Degree (angle)|degree]] of the position predicted by Le Verrier. Its largest moon, [[Triton (moon)|Triton]], was discovered shortly thereafter, though none of the planet's [[Moons of Neptune|remaining moons]] were located telescopically until the 20th&nbsp;century.

The planet's distance from Earth gives it a small [[apparent size]], and its distance from the Sun renders it very dim, making it challenging to study with Earth-based telescopes. Only the advent of the [[Hubble Space Telescope]] and of large ground-based telescopes with [[adaptive optics]] allowed for detailed observations. Neptune was visited by ''[[Voyager 2|Voyager&nbsp;2]]'', which [[Planetary flyby|flew by]] the planet on 25&nbsp;August 1989; ''Voyager&nbsp;2'' remains the only spacecraft to have visited it.<ref name="NYT-20141018-KC">{{cite news |last=Chang |first=Kenneth |title=Dark Spots in Our Knowledge of Neptune |url=https://www.nytimes.com/2014/08/19/science/dark-spots-in-our-knowledge-of-neptune.html |date=18 October 2014 |newspaper=[[The New York Times]] |access-date=21 October 2014 |url-status=live |archive-url=https://web.archive.org/web/20141028062618/http://www.nytimes.com/2014/08/19/science/dark-spots-in-our-knowledge-of-neptune.html |archive-date=28 October 2014}}</ref><ref>{{cite web|url=https://solarsystem.nasa.gov/planets/neptune/exploration?page=0&per_page=10&order=launch_date+desc,title+asc&search=&tags=Neptune&category=33|title=Exploration {{!}} Neptune|website=NASA Solar System Exploration|access-date=3 February 2020|quote="In 1989, NASA's Voyager 2 became the first-and only-spacecraft to study Neptune up close."|archive-date=17 July 2020|archive-url=https://web.archive.org/web/20200717140051/https://solarsystem.nasa.gov/planets/neptune/exploration/?page=0&per_page=10&order=launch_date+desc,title+asc&search=&tags=Neptune&category=33|url-status=live}}</ref> Like the [[gas giant]]s ([[Jupiter]] and [[Saturn]]), Neptune's atmosphere is composed primarily of [[hydrogen]] and [[helium]], along with traces of [[hydrocarbon]]s and possibly [[nitrogen]], but contains a higher proportion of ''[[Volatile (astrogeology)|ices]]'' such as water, [[ammonia]] and [[methane]]. Similar to Uranus, its interior is primarily composed of ices and rock;<ref name="Podolak Weizman et al. 1995"/> both planets are normally considered "ice giants" to distinguish them.<ref name="Lunine 1993"/> Along with [[Rayleigh scattering]], traces of methane in the outermost regions make Neptune appear faintly blue.<ref name="bluecolour">{{Cite web |last1=Munsell |first1=Kirk |last2=Smith |first2=Harman |last3=Harvey |first3=Samantha |date=13 November 2007 |title=Neptune overview |url=http://solarsystem.nasa.gov/planets/profile.cfm?Object=Neptune&Display=OverviewLong |url-status=dead |archive-url=https://web.archive.org/web/20080303045911/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Neptune&Display=OverviewLong |archive-date=3 March 2008 |access-date=20 February 2008 |website=Solar System Exploration |publisher=NASA}}</ref><ref>{{Cite web |title=Gemini North Telescope Helps Explain Why Uranus and Neptune Are Different Colors - Observations from Gemini Observatory, a Program of NSF's NOIRLab, and other telescopes reveal that excess haze on Uranus makes it paler than Neptune |url=https://www.noirlab.edu/public/news/noirlab2211/ |date=31 May 2022 |website=noirlab.edu |language=en |access-date=30 July 2022 |archive-date=30 July 2022 |archive-url=https://web.archive.org/web/20220730153653/https://www.noirlab.edu/public/news/noirlab2211/ |url-status=live }}</ref>

In contrast to the strongly seasonal atmosphere of Uranus, which can be featureless for long periods of time, Neptune's atmosphere has active and consistently visible weather patterns. At the time of the ''Voyager&nbsp;2'' flyby in 1989, the planet's southern hemisphere had a [[Great Dark Spot]] comparable to the [[Great Red Spot]] on Jupiter. In 2018, a newer main dark spot and smaller dark spot were identified and studied.<ref name="NYT-20201222" /> These weather patterns are driven by the strongest sustained winds of any planet in the Solar System, as high as {{cvt|2100|km/h|m/s mph}}.<ref name="Suomi1991">{{Cite journal |last1=Suomi |first1=V. E. |last2=Limaye |first2=S. S. |last3=Johnson |first3=D. R. |date=22 February 1991 |title=High Winds of Neptune: A Possible Mechanism |journal=[[Science (journal)|Science]] |volume=251 |issue=4996 |pages=929–932 |bibcode=1991Sci...251..929S |doi=10.1126/science.251.4996.929 |issn=0036-8075 |pmid=17847386 |s2cid=46419483}}</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 {{cvt|55|K|C F|0|lk=on}}. Temperatures at the planet's centre are approximately {{cvt|5400|K|C F|-2}}.<ref name=hubbard/><ref name="nettelmann" /> Neptune has [[Rings of Neptune|a faint and fragmented ring system]] (labelled "arcs"), discovered in 1984 and confirmed by ''Voyager&nbsp;2''.<ref name=ring1/>

== History ==

=== Discovery ===
{{Main|Discovery of Neptune}}
{{Multiple image
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| image1            = Galileo.arp.300pix.jpg
| alt1              = 
| caption1          = [[Galileo Galilei]] may have been the first person to sight Neptune
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| image2            = Le_Verrier_colored.jpg
| alt2              = 
| caption2          = [[Urbain Le Verrier]] mostly successfully predicted Neptune's position
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| image3            = JohannGalle.jpg
| alt3              = 
| caption3          = [[Johann Gottfried Galle]] was requested by Le Verrier to look for Neptune
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| image4            = John_Couch_Adams_Cornwall.jpg
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| caption4          = [[John Couch Adams]] independently calculated Neptune's position
}}

Some of the earliest known [[telescope|telescopic]] observations ever, Galileo's drawings on 28 Dec. 1612 and 27 Jan. 1613 ([[Old Style and New Style dates|New Style]]) contain plotted points that match what is now known to have been the positions of Neptune on those dates. Both times, Galileo seems to have mistaken Neptune for a [[fixed star]] when it appeared close—in [[Conjunction (astronomy)|conjunction]]—to Jupiter in the [[night sky]].<ref>
{{cite book |first=Alan |last=Hirschfeld |title=Parallax: The Race to Measure the Cosmos |date=2001 |publisher=Henry Holt |location=New York, New York |isbn=978-0-8050-7133-7}}</ref> Hence, he is not credited with Neptune's discovery. At his first observation in Dec. 1612, Neptune was almost stationary in the sky because it had just turned [[Apparent retrograde motion|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.<ref>{{Cite book |last=Littmann |first=Mark |title=Planets Beyond: Discovering the Outer Solar System |date=2004 |publisher=[[Dover Publications]] |isbn=978-0-486-43602-9 |location=Mineola, N.Y}}</ref> In 2009, a study suggested that Galileo was at least aware that the "star" he had observed had moved relative to fixed stars.<ref>{{cite web |title=Galileo discovered Neptune, new theory claims |first=Robert Roy |last=Britt |date=2009 |publisher=NBC News News |access-date=10 July 2009 |url=https://www.nbcnews.com/id/wbna31835303 |archive-date=4 November 2013 |archive-url=https://web.archive.org/web/20131104140532/http://www.nbcnews.com/id/31835303 |url-status=live }}</ref>

In 1821, [[Alexis Bouvard]] published astronomical tables of the [[orbit]] of [[Uranus]].<ref>
{{cite book |first=A. |last=Bouvard |date=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.<ref name=MNRAS7/> In 1843, [[John Couch Adams]] began work on the orbit of Uranus using the data he had. He requested extra data from Sir [[George Biddell Airy|George Airy]], the [[Astronomer Royal]], who supplied it in February 1844. Adams continued to work in 1845–1846 and produced several different estimates for the position of an undiscovered planet beyond Uranus.<ref>{{Cite web |last1=O'Connor |first1=John J. |last2=Robertson |first2=Edmund F. |date=2006 |title=John Couch Adams' account of the discovery of Neptune |url=http://www-groups.dcs.st-and.ac.uk/~history/Extras/Adams_Neptune.html |url-status=live |archive-url=https://web.archive.org/web/20080126124928/http://www-groups.dcs.st-and.ac.uk/~history/Extras/Adams_Neptune.html |archive-date=26 January 2008 |access-date=18 February 2008 |publisher=[[University of St Andrews]]}}</ref><ref>{{cite journal |first=J.C. |last=Adams |bibcode=1846MNRAS...7..149A |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 |issue=9 |pages=149–52 |date=13 November 1846 |doi=10.1093/mnras/7.9.149 |url=https://zenodo.org/record/1431905 |access-date=25 August 2019 |archive-url=https://web.archive.org/web/20190502014753/https://zenodo.org/record/1431905/files/article.pdf |archive-date=2 May 2019 |url-status=live |doi-access=free }}</ref>

Independently from Adams, [[Urbain Le Verrier]] developed his own calculations in 1845–1846 that pointed to an undiscovered planet, but aroused no enthusiasm among his compatriots. In June 1846, upon seeing Le&nbsp;Verrier's first published estimate of a suspected undiscovered planet's longitude and its similarity to Adams's estimate, Airy persuaded [[James Challis]] to search for the it. Challis vainly scoured the sky throughout August and September.<ref name=MNRAS7>{{cite journal |first=G.B. |last=Airy |bibcode=1846MNRAS...7..121A |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 |issue=10 |pages=121–44 |date=13 November 1846 |doi= 10.1093/mnras/7.9.121|doi-access=free }}</ref><ref>{{cite journal |first=Rev. J. |last=Challis |bibcode=1846MNRAS...7..145C |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 |issue=9 |pages=145–149 |date=13 November 1846 |doi=10.1093/mnras/7.9.145 |url=https://zenodo.org/record/1431903 |access-date=25 August 2019 |archive-url=https://web.archive.org/web/20190504065619/https://zenodo.org/record/1431903/files/article.pdf |archive-date=4 May 2019 |url-status=live |doi-access=free }}</ref> Challis had, in fact, observed Neptune a year before the planet's subsequent discoverer, [[Johann Gottfried Galle]], and on two occasions, 4 and 12 August 1845. However, his out-of-date star maps and poor observing techniques meant that he failed to recognize the observations as such until he carried out later analysis. Challis was full of remorse but blamed his neglect on his maps and the fact that he was distracted by his concurrent work on comet observations.<ref>{{cite web |url=http://scihi.org/james-challis-neptune/ |title=James Challis and his failure to discover the planet Neptune |first=Harald |last=Sack |website=scihi.org |date=12 December 2017 |access-date=15 November 2021 |archive-date=15 November 2021 |archive-url=https://web.archive.org/web/20211115120435/http://scihi.org/james-challis-neptune/ |url-status=live }}</ref><ref name=MNRAS7/><ref>{{cite journal |first=J.G. |last=Galle |bibcode=1846MNRAS...7..153G |title=Account of the discovery of the planet of Le&nbsp;Verrier at Berlin |journal=Monthly Notices of the Royal Astronomical Society |volume=7 |issue=9 |page=153 |date=13 November 1846 |doi=10.1093/mnras/7.9.153|doi-access=free }}</ref>

Meanwhile, Le Verrier sent a letter and urged [[Berlin Observatory]] astronomer 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&nbsp;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 just northeast of [[Iota Aquarii]], 1° from the "''five degrees east of [[Delta Capricorni|Delta Capricorn]]''" position Le&nbsp;Verrier had predicted it to be,<ref name="auto">{{cite web | url=https://www.space.com/12249-neptune-completes-orbit-discovery-1846.html | title=Neptune Completes First Orbit Since Its Discovery in 1846 | date=12 July 2011 | first=Geoff | last=Gaherty | publisher=space.com | access-date=3 September 2019 | archive-url=https://web.archive.org/web/20190825071538/https://www.space.com/12249-neptune-completes-orbit-discovery-1846.html | archive-date=25 August 2019 | url-status=live }}</ref><ref>{{cite book |last=Levenson |first=Thomas |title=The Hunt for Vulcan ... and how Albert Einstein Destroyed a Planet, Discovered Relativity, and Deciphered the Universe |publisher=Random House |year=2015 |page=38}}</ref> about 12° from Adams's prediction, and on the border of [[Aquarius (constellation)|Aquarius]] and [[Capricornus]] according to the modern [[IAU designated constellations|IAU constellation]] boundaries.
[[File:Deutsches Museum München - 15803973590.jpg|thumb|upright|The 9" refractor used by Galle to discover Neptune]]

In the wake of the discovery, there was a nationalistic rivalry between the French and the British over who deserved credit for the discovery. Eventually, an international consensus emerged that Le&nbsp;Verrier and Adams deserved joint credit.<ref>{{Cite web |last=Williams |first=Matt |date=14 September 2015 |title=The gas (and ice) giant Neptune |url=https://www.universetoday.com/tag/urbain-le-verrier/ |archive-url=https://web.archive.org/web/20230927053751/https://www.universetoday.com/tag/urbain-le-verrier/ |archive-date=27 September 2023 |access-date=26 April 2024 |website=Universe Today |language=en}}</ref> 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]].<ref name="Neptdisc">
{{cite web |last=Kollerstrom |first=Nick |date=2001 |title=Neptune's Discovery. The British Case for Co-Prediction |url=http://www.ucl.ac.uk/sts/nk/neptune/index.htm |archive-url=https://web.archive.org/web/20051111190351/http://www.ucl.ac.uk/sts/nk/neptune/ |archive-date=11 November 2005 |access-date=19 March 2007 |publisher=University College London}}</ref><ref>{{Cite magazine |last1=Sheehan |first1=William |last2=Kollerstrom |first2=Nicholas |last3=Waff |first3=Craig B. |date=December 2004 |title=The case of the pilfered planet – did the British steal Neptune? |url=http://www.scientificamerican.com/article.cfm?id=the-case-of-the-pilfered |url-status=live |archive-url=https://web.archive.org/web/20110319155153/http://www.scientificamerican.com/article.cfm?id=the-case-of-the-pilfered |archive-date=19 March 2011 |access-date=20 January 2011 |magazine=[[Scientific American]] |jstor=26060804}}</ref>

=== Naming ===
Shortly after its discovery, Neptune was referred to simply as "the planet exterior to Uranus" or as "Le&nbsp;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|Moore]] (2000):206</ref>

Claiming the right to name his discovery, Le&nbsp;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]].<ref>{{Cite book |last=Littmann |first=Mark |title=Planets Beyond, Exploring the Outer Solar System |date=2004 |publisher=[[Dover Publications]] |isbn=978-0-486-43602-9 |location=Mineola, N.Y |page=50}}</ref> In October, he sought to name the planet ''Le&nbsp;Verrier'', after himself, and he had loyal support in this from the observatory director, [[François Arago]]. This suggestion met with stiff resistance outside France.<ref>{{Cite book |last1=Baum |first1=Richard |title=In Search of Planet Vulcan: The ghost in Newton's clockwork universe |last2=Sheehan |first2=William |date=2003 |publisher=Basic Books |isbn=978-0-7382-0889-3 |pages=109–10 |ref=Baum}}</ref> French almanacs quickly reintroduced the name ''Herschel'' for Uranus, after that planet's discoverer Sir&nbsp;[[William Herschel]], and ''Leverrier'' for the new planet.<ref>{{cite journal |title=The naming of Uranus and Neptune |journal=Astronomical Society of the Pacific Leaflets |first=Owen |last=Gingerich |volume=8 |issue=352 |pages=9–15 |date=October 1958 |bibcode=1958ASPL....8....9G}}</ref>

[[Friedrich Georg Wilhelm von Struve|Struve]]<!-- (1793–1864), or maybe his son Otto Wilhelm von Struve (1819–1905). (Having looked at the original text it seems like it's most likely the elder of the Struve, but the text is ambiguous). --> came out in favour of the name ''Neptune'' on 29&nbsp;December 1846, to the [[Russian Academy of Sciences|Saint Petersburg Academy of Sciences]],<ref>{{cite journal |title=Second report of proceedings in the Cambridge Observatory relating to the new Planet (Neptune) |date=1847 |journal=Astronomische Nachrichten |volume=25 |issue=21 |pages=309–14 |last=Hind |first=J.R. |doi=10.1002/asna.18470252102 |bibcode=1847AN.....25..309. |url=https://zenodo.org/record/1424623 |access-date=12 June 2019 |archive-date=29 September 2021 |archive-url=https://web.archive.org/web/20210929080353/https://zenodo.org/record/1424623 |url-status=live }}</ref> after the colour of the planet as viewed through a telescope.<ref>{{cite book | chapter=Neptune: More Surprises | isbn=978-1-4020-5544-7 | first1=Gunter | last1=Faure | first2=Teresa M. | last2=Mensing | title=Introduction to Planetary Science | date=2007 | pages=385–399 | doi=10.1007/978-1-4020-5544-7_19 | publisher=Springer | publication-place=Dordrecht }}</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 were named for deities in [[Greek mythology|Greek]] and Roman mythology.<ref group="lower-alpha">One could argue that it is true except for the 'Earth', which in the English language is the name of a Germanic deity, [[Jörð|Erda]]. The [[IAU]] policy is that one may call the Earth and the Moon by any name commonly used in the language. According to IAU, 'Terra' and 'Luna' are ''not'' the official names of planet Earth and its moon; {{Cite web |title=Naming of Astronomical Objects |url=https://www.iau.org/public/themes/naming/ |url-status=live |archive-url=https://web.archive.org/web/20240321174041/https://www.iau.org/public/themes/naming/ |archive-date=21 March 2024 |access-date=27 April 2024 |website=International Astronomical Union}}</ref><ref name="USGS">{{cite web |url=http://planetarynames.wr.usgs.gov/Page/Planets |title=Planet and Satellite Names and Discoverers |publisher=U.S. Geological Survey |series=Gazetteer of Planetary Nomenclature |date=17 December 2008 |archive-url=https://web.archive.org/web/20180809110639/https://planetarynames.wr.usgs.gov/Page/Planets |archive-date=9 August 2018 |url-status=live |access-date=26 March 2012 }}</ref>

Most languages today use some variant of the name "Neptune" for the planet. In Chinese, Vietnamese, Japanese, and Korean, the planet's name was translated as "sea king star" ({{lang|zh|海王星}}).<ref>{{cite web |title=Planetary linguistics |publisher=nineplanets.org |url=http://nineplanets.org/days.html |access-date=8 April 2010 |archive-url=https://web.archive.org/web/20100407154905/http://www.nineplanets.org/days.html |archive-date=7 April 2010 |url-status=live }}</ref><ref>{{cite web |url=http://kenh14.vn/kham-pha/sao-hai-vuong-cuc-bang-khong-lo-xa-tit-tap-2010103010172747.chn |title=Sao Hải Vương – "Cục băng" khổng lồ xa tít tắp |publisher=Kenh14 |language=vi |date=31 October 2010 |access-date=30 July 2018 |archive-url=https://web.archive.org/web/20180730170641/http://kenh14.vn/kham-pha/sao-hai-vuong-cuc-bang-khong-lo-xa-tit-tap-2010103010172747.chn |archive-date=30 July 2018 |url-status=live }}</ref> In [[Mongolian language|Mongolian]], Neptune is called {{transliteration|mn|Dalain van}} ({{lang|mn|Далайн ван}}), reflecting its namesake god's role as the ruler of the sea. In modern [[Greek language|Greek]], the planet is called ''Poseidon'' ({{lang|el|Ποσειδώνας}}, {{transliteration|el|Poseidonas}}), the Greek counterpart of Neptune.<ref>{{cite web |url=http://www.greek-names.info/greek-names-of-the-planets/ |title=Greek Names of the Planets |access-date=14 July 2012 |quote=Neptune or ''Poseidon'' as is its Greek name, was the God of the Seas. It is the eight planet from the sun&nbsp;... |date=25 April 2010 |archive-url=https://web.archive.org/web/20100509164917/http://www.greek-names.info/greek-names-of-the-planets/ |archive-date=9 May 2010 |url-status=live }}</ref> [[hebrew astronomy|In Hebrew]], {{transliteration|he|[[Rahab (term)|Rahab]]}} ({{lang|he|רהב}}), from [[Rahab (Egypt)#Jewish folklore|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 {{transliteration|he|Neptun}} ({{lang|he|נפטון}}) is commonly used.<ref name="Haaretz">{{cite news |first=Yair |last=Ettinger |date=31 December 2009 |title=Uranus and Neptune get Hebrew names at last |newspaper=[[Haaretz]] |url=https://www.haaretz.com/1.4758704 |access-date=16 August 2018 |archive-url=https://web.archive.org/web/20180625050641/https://www.haaretz.com/1.4758704 |archive-date=25 June 2018 |url-status=live }}</ref><ref name="Hayadan.org.il">{{cite news |url=https://www.hayadan.org.il/hebrew-names-to-uranus-and-neptune-3112090 |title=אוראנוס הוא מהיום אורון ונפטון מעתה רהב |trans-title=Uranus is now Oron and Neptune is now Rahav |newspaper=[[Hayadan]] |language=he |first=Avi |last=Belizovsky |date=31 December 2009 |access-date=16 August 2018 |archive-url=https://web.archive.org/web/20180624204205/https://www.hayadan.org.il/hebrew-names-to-uranus-and-neptune-3112090 |archive-date=24 June 2018 |url-status=live }}</ref> In [[Māori language|Māori]], the planet is called {{lang|mi|Tangaroa}}, named after the [[Tangaroa|Māori god of the sea]].<ref name="Appendix 5: Planetary Linguistics">{{Cite web |last=Ian |date=25 September 2019 |title=Planetary Linguistics {{!}} Latin, Greek, Sanskrit & Different Languages |url=https://nineplanets.org/planetary-linguistics/ |access-date=1 February 2024 |website=The Nine Planets |language=en-US |archive-date=2 February 2019 |archive-url=https://web.archive.org/web/20190202091251/https://nineplanets.org/days.html |url-status=live }}</ref> In [[Nahuatl language|Nahuatl]], the planet is called {{lang|nah|Tlāloccītlalli}}, named after the rain god [[Tlāloc]].<ref name="Appendix 5: Planetary Linguistics"/> In [[Thai language|Thai]], Neptune is referred to by the Westernised name {{transliteration|th|Dao Nepchun/Nepjun}} ({{lang|th|ดาวเนปจูน}}) but is also called {{transliteration|th|Dao Ket}} ({{lang|th|ดาวเกตุ}}, {{lit|star of Ketu}}), after [[Ketu (mythology)|Ketu]] ({{lang|th|केतु}}), the descending [[lunar node]], who plays a role in [[Hindu astrology]]. In [[Malay language|Malay]], the name {{lang|ms|Waruna}}, after the [[Varuna|Hindu god of seas]], is attested as far back as the 1970s,<ref>{{cite encyclopedia|dictionary=Kamus Kebangsaan Ejaan Baru, Inggeris-bahasa Malaysia, Bahasa Malaysia-Inggeris|url=https://books.google.com/books?id=gCtNAQAAIAAJ&q=%22waruna%22+neptune|title=Waruna|last=Mohamed|first=Kadir|year=1975|publisher=Titiwangsa|pages=299, 857|access-date=29 May 2021|archive-date=29 September 2021|archive-url=https://web.archive.org/web/20210929080319/https://books.google.com/books?id=gCtNAQAAIAAJ&q=%22waruna%22+neptune|url-status=live}}</ref> but was eventually superseded by the Latinate equivalents {{lang|ms|Neptun}} (in [[Malaysian language|Malaysian]]<ref>{{cite encyclopedia|title=Neptun|dictionary=Kamus Dewan|edition=4th|url=https://prpm.dbp.gov.my/Cari1?keyword=Neptun|year=2017|publisher=Dewan Bahasa dan Pustaka Malaysia|access-date=5 May 2021|archive-date=7 May 2021|archive-url=https://web.archive.org/web/20210507122704/https://prpm.dbp.gov.my/Cari1?keyword=Neptun|url-status=live}}</ref>) or {{lang|id|Neptunus}} (in [[Indonesian language|Indonesian]]<ref>{{cite encyclopedia|title=Neptunus|dictionary=Kamus Besar Bahasa Indonesia|edition=3rd|url=https://kbbi.kemdikbud.go.id/entri/neptunus|year=2016|publisher=Badan Pengembangan dan Pembinaan Bahasa Indonesia|access-date=5 May 2021|archive-date=29 September 2021|archive-url=https://web.archive.org/web/20210929080334/https://kbbi.kemdikbud.go.id/entri/neptunus|url-status=live}}</ref>).

The usual adjectival form is ''Neptunian''. The [[nonce word|nonce]] form ''Poseidean'' ({{IPAc-en|p|ə|ˈ|s|aɪ|d|i|ən}}), from [[Poseidon]], has also been used,<ref name=Poseidean/> though the usual adjectival form of ''Poseidon'' is ''Poseidonian'' ({{IPAc-en|ˌ|p|ɒ|s|aɪ|ˈ|d|oʊ|n|i|ən}}).<ref>''The Century Dictionary'' (1914)</ref>

=== Status ===
From its discovery in 1846 until the [[discovery of Pluto]] in 1930, Neptune was the farthest known planet. When [[Pluto]] was discovered, it was considered a planet, and Neptune thus became the second-farthest known planet, except for a 20-year period between 1979 and 1999 when Pluto's elliptical orbit brought it closer than Neptune to the Sun, making Neptune the ninth planet from the Sun during this period.<ref>{{cite magazine |title=Jan. 21, 1979: Neptune Moves Outside Pluto's Wacky Orbit |magazine=Wired |url=https://www.wired.com/science/discoveries/news/2008/01/dayintech_0121 |last=Long |first=Tony |access-date=13 March 2008 |date=21 January 2008 |archive-url=https://web.archive.org/web/20080327014628/http://www.wired.com/science/discoveries/news/2008/01/dayintech_0121 |archive-date=27 March 2008 |url-status=live }}</ref><ref>{{Cite web|url=https://www.space.com/3137-neptune-data-sheet.html|title=Neptune Data Sheet|date=17 November 2006|website=Space.com|access-date=11 February 2023|archive-date=11 February 2023|archive-url=https://web.archive.org/web/20230211220514/https://www.space.com/3137-neptune-data-sheet.html|url-status=live}}</ref> The increasingly accurate estimations of Pluto's mass from ten times [[Earth mass|that of Earth's]] to far less than [[Lunar mass|that of the Moon]]<ref>{{cite book|last1 = Stern|first1 = Alan|last2 = Tholen|first2 = David James|title = Pluto and Charon|date = 1997|publisher=University of Arizona Press|isbn = 978-0-8165-1840-1|pages=206–208}}</ref> and 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.<ref>{{Cite journal |last=Weissman |first=Paul R. |date=September 1995 |title=The Kuiper Belt |journal=[[Annual Review of Astronomy and Astrophysics]] |volume=33 |issue=1 |pages=327–357 |bibcode=1995ARA&A..33..327W |doi=10.1146/annurev.aa.33.090195.001551 |issn=0066-4146}}</ref><ref>{{cite web |date=1999 |title=The Status of Pluto:A clarification |website=[[International Astronomical Union]], Press release |url=http://www.iau.org/STATUS_OF_PLUTO.238.0.html |access-date=25 May 2006 |archive-url=https://web.archive.org/web/20060615200253/http://www.iau.org/STATUS_OF_PLUTO.238.0.html |archive-date=15 June 2006 |url-status=dead}}</ref> In 2006, the [[International Astronomical Union]] [[IAU definition of planet|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.<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=24 August 2006 |publisher=IAU |access-date=22 July 2008 |archive-url=https://web.archive.org/web/20080625043626/http://www.iau.org/static/resolutions/Resolution_GA26-5-6.pdf |archive-date=25 June 2008 |url-status=live }}</ref>

== Physical characteristics ==
[[File:Neptune, Earth size comparison true color.jpg|thumb|A size comparison of Neptune and Earth]]
Neptune's mass of 1.0243{{e|26}}&nbsp;kg<ref name="fact"/> is intermediate between Earth and the larger [[gas giant]]s: it is 17 times that of Earth but just 1/19th [[Jupiter mass|that of Jupiter]].<ref group="lower-alpha" name="mass">The mass of Earth is 5.9736{{e|24}}&nbsp;kg, giving a mass ratio

:<math>\tfrac{M_\text{Neptune}}{M_\text{Earth}} = \tfrac{1.02 \times 10^{26}}{5.97 \times 10^{24}} = 17.09.</math>
The mass of Uranus is 8.6810{{e|25}}&nbsp;kg, giving a mass ratio

:<math>\tfrac{M_\text{Uranus}}{M_\text{Earth}} = \tfrac{8.68 \times 10^{25}}{5.97 \times 10^{24}} = 14.54.</math>
The mass of Jupiter is 1.8986{{e|27}}&nbsp;kg, giving a mass ratio

:<math>\tfrac{M_\text{Jupiter}}{M_\text{Neptune}} = \tfrac{1.90 \times 10^{27}}{1.02 \times 10^{26}} = 18.63.</math>
Mass values from {{cite web |last=Williams |first=David R. |date=29 November 2007 |url=http://nssdc.gsfc.nasa.gov/planetary/factsheet/ |title=Planetary Fact Sheet&nbsp;– Metric |publisher=NASA |access-date=13 March 2008 |archive-url=https://web.archive.org/web/20140905165712/http://nssdc.gsfc.nasa.gov/planetary/factsheet/ |archive-date=5 September 2014 |url-status=live }}</ref> Its [[gravity]] at 1 bar is 11.15&nbsp;m/s<sup>2</sup>, 1.14 times the [[surface gravity]] of Earth,<ref>{{cite web |url=http://nssdc.gsfc.nasa.gov/planetary/factsheet/neptunefact.html |title=Neptune Fact Sheet |publisher=NASA |access-date=22 September 2005 |archive-url=https://web.archive.org/web/20100701192119/http://nssdc.gsfc.nasa.gov/planetary/factsheet/neptunefact.html |archive-date=1 July 2010 |url-status=live }}</ref> and surpassed only by Jupiter.<ref name="Unsöld & Baschek 2001"/> Neptune's [[equator]]ial radius of 24,764&nbsp;km<ref name="Seidelmann Archinal A'hearn et al. 2007"/> is nearly four times [[Earth radius|that of Earth]]. Neptune, like [[Uranus]], is an [[ice giant]], a subclass of [[giant planet]], because they are smaller and have higher concentrations of [[Volatile (astrogeology)|volatiles]] than Jupiter and Saturn.<ref name=Boss>{{cite journal |first=Alan P. |last=Boss |title=Formation of gas and ice giant planets |journal=Earth and Planetary Science Letters |date=2002 |volume=202 |issue=3–4 |pages=513–23 |doi=10.1016/S0012-821X(02)00808-7 |bibcode=2002E&PSL.202..513B}}</ref> In the search for [[exoplanet]]s, Neptune has been used as a [[metonymy|metonym]]: discovered bodies of similar mass are often referred to as "Neptunes",<ref>{{Cite news |last1=Lovis |first1=C. |last2=Mayor |first2=M. |last3=Alibert |first3=Y. |last4=Benz |first4=W. |date=18 May 2006 |title=Trio of Neptunes and their Belt |url=http://www.eso.org/public/news/eso0618/ |url-status=live |archive-url=https://web.archive.org/web/20100113120656/http://www.eso.org/public/news/eso0618/ |archive-date=13 January 2010 |access-date=25 February 2008 |publisher=[[European Southern Observatory|ESO]]}}</ref> just as scientists refer to various extrasolar bodies as "Jupiters".

=== Internal structure ===
{{See also|Extraterrestrial diamonds}}
Neptune's internal structure resembles [[Uranus#Internal structure|that of Uranus]]. Its atmosphere forms about 5 to 10% of its mass and extends perhaps 10 to 20% of the way towards the core. Pressure in the atmosphere reaches about 10&nbsp;[[Pascal (unit)|GPa]], or about 10{{sup|5}} atmospheres. Increasing concentrations of [[methane]], [[ammonia]] and water are found in the lower regions of the atmosphere.<ref name=hubbard/>
[[File:Another Neptune diagram.svg|thumb|upright=1.5|Illustration of the physical component of Neptune's interior and its surroundings in [[False color|false colours]]]]
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 called [[Volatile (astrogeology)|icy]] even though it is a hot, dense [[supercritical fluid]]. This fluid, which has a high electrical conductivity, is sometimes called a water–ammonia ocean.<ref name=Atreya2006>{{cite journal |url=http://www.cosis.net/abstracts/EGU06/05179/EGU06-J-05179-1.pdf |title=Water-ammonia ionic ocean on Uranus and Neptune? |journal=Geophysical Research Abstracts |first1=S. |last1=Atreya |first2=P. |last2=Egeler |first3=K. |last3=Baines |volume=8 |at=05179 |date=2006 |access-date=7 November 2007 |archive-url=https://web.archive.org/web/20120205194205/http://www.cosis.net/abstracts/EGU06/05179/EGU06-J-05179-1.pdf |archive-date=5 February 2012 |url-status=live }}</ref> The mantle may consist of a layer of ionic water in which the water molecules break down into a soup of hydrogen and [[oxygen]] [[ion]]s, and deeper down [[superionic water]] in which the oxygen crystallizes but the [[hydrogen ion]]s float around freely within the oxygen lattice.<ref>{{cite news |url=https://www.newscientist.com/article/mg20727764.500-weird-water-lurking-inside-giant-planets/ |title=Weird water lurking inside giant planets |magazine=[[New Scientist]] |first=David |last=Shiga |issue=2776 |date=1 September 2010 |access-date=11 February 2018 |archive-url=https://web.archive.org/web/20180212143537/https://www.newscientist.com/article/mg20727764.500-weird-water-lurking-inside-giant-planets/ |archive-date=12 February 2018 |url-status=live }}</ref> At a depth of 7,000&nbsp;km, the conditions may be such that methane decomposes into diamond crystals that rain downwards like hailstones.<ref>{{cite journal |title=Neptune May Crush Methane into Diamonds |journal=Science |last=Kerr |first=Richard A. |volume=286 |issue=5437 |pages=25a–25 |date=October 1999 |doi=10.1126/science.286.5437.25a |pmid=10532884|s2cid=42814647 }}</ref><ref name="WP-20170825">{{cite news |url=https://www.washingtonpost.com/news/speaking-of-science/wp/2017/08/25/it-rains-solid-diamonds-on-uranus-and-neptune/ |title=It rains solid diamonds on Uranus and Neptune |newspaper=[[The Washington Post]] |last=Kaplan |first=Sarah |date=25 August 2017 |access-date=27 August 2017 |archive-url=https://web.archive.org/web/20170827011901/https://www.washingtonpost.com/news/speaking-of-science/wp/2017/08/25/it-rains-solid-diamonds-on-uranus-and-neptune/ |archive-date=27 August 2017 |url-status=live }}</ref><ref>{{Cite journal |title=Formation of diamonds in laser-compressed hydrocarbons at planetary interior conditions |journal=Nature Astronomy |last1=Kraus |first1=D. |last2=Vorberger |first2=J. |last3=Pak |first3=A. |last4=Hartley |first4=N.J. |last5=Fletcher |first5=L.B. |last6=Frydrych |first6=S. |last7=Galtier |first7=E. |last8=Gamboa |first8=E.J. |last9=Gericke |first9=D.O. |last10=Glenzer |first10=S.H. |last11=Granados |first11=E. |last12=MacDonald |first12=M.J. |last13=MacKinnon |first13=A.J. |last14=McBride |first14=E.E. |last15=Nam |first15=I. |last16=Neumayer |first16=P. |last17=Roth |first17=M. |last18=Saunders |first18=A.M. |last19=Schuster |first19=A.K. |last20=Sun |first20=P. |last21=van Driel |first21=T. |last22=Döppner |first22=T. |last23=Falcone |first23=R.W. |display-authors=1 |volume=1 |issue=9 |pages=606–11 |date=September 2017 |doi=10.1038/s41550-017-0219-9 |bibcode=2017NatAs...1..606K |s2cid=46945778 |url=https://cloudfront.escholarship.org/dist/prd/content/qt1k79p6pk/qt1k79p6pk.pdf?t=p5nsjk |access-date=25 August 2018 |archive-url=https://web.archive.org/web/20180725002534/https://cloudfront.escholarship.org/dist/prd/content/qt1k79p6pk/qt1k79p6pk.pdf?t=p5nsjk |archive-date=25 July 2018 |url-status=live }}</ref> Scientists believe that this kind of diamond rain occurs on Jupiter, Saturn, and Uranus.<ref>{{Cite web |last=Kane |first=Sean |date=29 April 2016 |title=Lightning storms make it rain diamonds on Saturn and Jupiter |url=https://www.businessinsider.com/diamond-rain-saturn-jupiter-2016-4 |url-status=live |archive-url=https://web.archive.org/web/20190626211704/https://www.businessinsider.com/diamond-rain-saturn-jupiter-2016-4 |archive-date=26 June 2019 |access-date=22 May 2019 |publisher=[[Business Insider]]}}</ref><ref name="WP-20170825"/> Very-high-pressure experiments at [[Lawrence Livermore National Laboratory]] suggest that the top of the mantle may be an ocean of liquid carbon with floating solid 'diamonds'.<ref>{{cite news |url=http://www.astronomynow.com/news/n1001/21diamond/ |title=Oceans of diamond possible on Uranus and Neptune |website=Astronomy Now |first=Emily |last=Baldwin |date=21 January 2010 |archive-url=https://web.archive.org/web/20131203065217/http://www.astronomynow.com/news/n1001/21diamond/ |archive-date=3 December 2013}}</ref><ref name="Eggert 2004">{{cite journal |title=Shock Compressing Diamond to a Conducting Fluid |doi=10.1103/physrevlett.93.195506 |pmid=15600850 |journal=Physical Review Letters |date=30 July 2004 |last1=Bradley |first1=D.K. |last2=Eggert |first2=J.H. |last3=Hicks |first3=D.G. |last4=Celliers |first4=P.M. |url=https://e-reports-ext.llnl.gov/pdf/310197.pdf |access-date=16 March 2016 |volume=93 |issue=19 |pages=195506 |bibcode=2004PhRvL..93s5506B |hdl=1959.3/380076 |s2cid=6203103 |archive-url=https://web.archive.org/web/20161221011243/https://e-reports-ext.llnl.gov/pdf/310197.pdf |archive-date=21 December 2016 |url-status=dead }}</ref><ref name="Eggert 2010">{{cite journal |title=Melting temperature of diamond at ultrahigh pressure |journal=Nature Physics |date=8 November 2009 |last1=Eggert |first1=J.H. |last2=Hicks |first2=D.G. |last3=Celliers |first3=P.M. |last4=Bradley |first4=D.K. |volume=6 |issue=40 |pages=40–43 |doi=10.1038/nphys1438 |display-authors=etal |bibcode=2010NatPh...6...40E|doi-access=free }}</ref>

The [[planetary core|core]] of Neptune is likely composed of iron, nickel and [[silicate]]s, with an interior [[Scientific model|model]] giving a mass about 1.2x that of Earth.<ref name="Podolak Weizman et al. 1995" /> The pressure at the centre is 7&nbsp;[[bar (unit)|Mbar]] (700&nbsp;GPa), about twice as high as that at the centre of Earth, and the temperature may be {{cvt|5400|K|C F|-2}}.<ref name=hubbard/><ref name="nettelmann">{{Cite web |last1=Nettelmann |first1=N. |last2=French |first2=M. |last3=Holst |first3=B. |last4=Redmer |first4=R. |title=Interior Models of Jupiter, Saturn and Neptune |url=https://www.gsi.de/informationen/wti/library/plasma2006/PAPERS/TT-11.pdf |url-status=dead |archive-url=https://web.archive.org/web/20110718120920/https://www.gsi.de/informationen/wti/library/plasma2006/PAPERS/TT-11.pdf |archive-date=18 July 2011 |access-date=25 February 2008 |publisher=University of Rostock}}</ref>

=== Atmosphere ===
{{Multiple image
| direction   = vertical
| total_width = 230
| image1      = Neptune-Methane.jpg
| caption1    = Combined colour and near-[[infrared]] image of Neptune, showing bands of [[methane]] in its [[atmosphere]], and four of its moons, [[Proteus (moon)|Proteus]], [[Larissa (moon)|Larissa]], [[Galatea (moon)|Galatea]], and [[Despina (moon)|Despina]]
| image2      = Neptune's Dynamic Environment.webm
| caption2    = A timelapse video of Neptune and its moons
| image3      = Neptune Cloud Cover Over Three Decades (2023-019).png
| caption3    = Neptune's cloud cover over three decades (1994–2020).<ref name="NYT-20230818" /> False colour image based on data from [[WFPC2]] and [[WFC3]] instruments of the [[Hubble Space Telescope]].
| image4      = Neptune clouds.jpg
| caption4    = Bands of high-altitude clouds cast shadows on Neptune's lower cloud deck. The colour is exaggerated to show the clouds more clearly.
}}

At high altitudes, Neptune's atmosphere is 80% [[hydrogen]] and 19% [[helium]].<ref name=hubbard>{{cite journal |last=Hubbard |first=W.B. |title=Neptune's Deep Chemistry |journal=Science |date=1997 |volume=275 |issue=5304 |pages=1279–80 |doi=10.1126/science.275.5304.1279 |pmid=9064785|s2cid=36248590 }}</ref> A trace amount of [[methane]] is present. Prominent absorption bands of methane exist at wavelengths above 600&nbsp;nm, in the red and infrared portion of the spectrum. As with Uranus, this absorption of red light by atmospheric methane is part of what gives Neptune its faint blue hue,<ref>{{Cite web |last1=Crisp |first1=D. |last2=Hammel |first2=H.B. |date=14 June 1995 |title=Hubble Space Telescope Observations of Neptune |url=http://hubblesite.org/newscenter/archive/releases/1995/09/image/a/ |url-status=live |archive-url=https://web.archive.org/web/20070802132430/http://hubblesite.org/newscenter/archive/releases/1995/09/image/a |archive-date=2 August 2007 |access-date=22 April 2007 |publisher=Hubble News Center}}</ref> which is more pronounced for Neptune's due to concentrated haze in Uranus's atmosphere.<ref name="NYT-20240104">{{cite news |last=Ferreira |first=Becky |title=Uranus and Neptune Reveal Their True Colors - Neptune is not as blue as you've been led to believe, and Uranus's shifting colors are better explained, in new research. |url=https://www.nytimes.com/2024/01/04/science/uranus-neptune-colors-blue.html |date=4 January 2024 |work=[[The New York Times]] |url-status=live |archiveurl=https://archive.today/20240105004738/https://www.nytimes.com/2024/01/04/science/uranus-neptune-colors-blue.html |archivedate=5 January 2024 |accessdate=5 January 2024 }}</ref><ref name=":0">{{cite web
 | author=NASA Science Editorial Team
 | title=Why Uranus and Neptune Are Different Colors
 | date=31 May 2022
 | publisher=NASA
 | url=https://science.nasa.gov/solar-system/planets/neptune/why-uranus-and-neptune-are-different-colors/
 | access-date=30 October 2023
 | archive-date=31 October 2023
 | archive-url=https://web.archive.org/web/20231031014930/https://science.nasa.gov/solar-system/planets/neptune/why-uranus-and-neptune-are-different-colors/
 | url-status=live
 }}</ref>

Neptune's atmosphere is subdivided 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]], lies at a pressure of {{convert|0.1|bar|kPa}}.<ref name="Lunine 1993"/> The stratosphere then gives way to the [[thermosphere]] at a pressure lower than 10<sup>−5</sup> to 10<sup>−4</sup>&nbsp;bars (1 to 10&nbsp;Pa).<ref name="Lunine 1993"/> The thermosphere gradually transitions to the [[exosphere]].<ref name="l086">{{cite web | last=Williams | first=Matt | title=The gas (and ice) giant Neptune | website=Phys.org | date=14 September 2015 | url=https://phys.org/news/2015-09-gas-ice-giant-neptune.html | access-date=27 August 2024}}</ref>

Models suggest that Neptune's troposphere is banded by clouds of varying compositions depending on altitude.<ref name="NYT-20230818">{{cite news |last=Andrews |first=Robin George |title=Neptune's Clouds Have Vanished, and Scientists Think They Know Why - A recent study suggests a relationship between solar cycles and the atmosphere of the solar system's eighth planet. |url=https://www.nytimes.com/2023/08/18/science/neptune-clouds-sun.html |date=18 August 2023 |work=[[The New York Times]] |url-status=live |archiveurl=https://archive.today/20230818192313/https://www.nytimes.com/2023/08/18/science/neptune-clouds-sun.html |archivedate=18 August 2023 |accessdate=21 August 2023 }}</ref> The upper-level clouds lie at pressures below one bar, where the temperature is suitable for methane to condense. For pressures between one and five bars (100 and 500&nbsp;kPa), clouds of [[ammonia]] and [[hydrogen sulfide]] are thought to form. Above a pressure of five bars, the clouds may consist of ammonia, [[ammonium sulfide]], hydrogen sulfide and water. Deeper clouds of water ice should be found at pressures of about {{convert|50|bar|MPa}}, where the temperature reaches {{Convert|273|K|C F}}. 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 high-altitude cloud bands that wrap around the planet at constant latitudes. These circumferential bands have widths of 50–150&nbsp;km and lie about 50–110&nbsp;km above the cloud deck.<ref name=apj125/> These altitudes are in the layer where weather occurs, the troposphere. Weather does not occur in the higher stratosphere or thermosphere. In August 2023, the high-altitude clouds of Neptune vanished, prompting a study spanning thirty years of observations by the Hubble Space Telescope and ground-based telescopes. The study found that Neptune's high-altitude cloud activity is bound to [[Solar cycle]]s, and not to the planet's seasons.<ref name="NYT-20230818" /><ref>{{cite web |last1=Gianopoulos |first1=Andrea |title=Neptune's Disappearing Clouds Linked to the Solar Cycle |url=https://www.nasa.gov/feature/goddard/2023/hubble-neptunes-disappearing-clouds-linked-to-the-solar-cycle |website=NASA |access-date=24 August 2023 |date=16 August 2023 |archive-date=24 August 2023 |archive-url=https://web.archive.org/web/20230824000143/https://www.nasa.gov/feature/goddard/2023/hubble-neptunes-disappearing-clouds-linked-to-the-solar-cycle/ |url-status=live }}</ref><ref>{{cite journal |last1=Chavez |first1=Erandi |last2=de Pater |first2=Imke |last3=Redwing |first3=Erin |last4=Molter |first4=Edward M. |last5=Roman |first5=Michael T. |last6=Zorzi |first6=Andrea |last7=Alvarez |first7=Carlos |last8=Campbell |first8=Randy |last9=de Kleer |first9=Katherine |last10=Hueso |first10=Ricardo |last11=Wong |first11=Michael H. |last12=Gates |first12=Elinor |last13=Lynam |first13=Paul David |last14=Davies |first14=Ashley G. |last15=Aycock |first15=Joel |last16=Mcilroy |first16=Jason |last17=Pelletier |first17=John |last18=Ridenour |first18=Anthony |last19=Stickel |first19=Terry |title=Evolution of Neptune at near-infrared wavelengths from 1994 through 2022 |journal=Icarus |date=1 November 2023 |volume=404 |pages=115667 |doi=10.1016/j.icarus.2023.115667 |arxiv=2307.08157 |bibcode=2023Icar..40415667C |s2cid=259515455 |url=https://www.sciencedirect.com/science/article/abs/pii/S0019103523002440 |access-date=24 August 2023 |quote=The clear positive correlation we find between cloud activity and Solar Lyman-Alpha (121.56 nm) irradiance lends support to the theory that the periodicity in Neptune’s cloud activity results from photochemical cloud/haze production triggered by Solar ultraviolet emissions. |archive-date=24 August 2023 |archive-url=https://web.archive.org/web/20230824073844/https://www.sciencedirect.com/science/article/abs/pii/S0019103523002440 |url-status=live }}</ref>

Neptune's [[Visible spectrum|spectra]] suggest that its lower stratosphere is hazy due to condensation of products of ultraviolet [[photolysis]] of methane, such as [[ethane]] and [[acetylene|ethyne]].<ref name="Lunine 1993"/><ref name=hubbard/> The stratosphere is home to trace amounts of [[carbon monoxide]] and [[hydrogen cyanide]].<ref name="Lunine 1993"/><ref name="Encrenaz 2003"/> The stratosphere of Neptune is warmer than that of Uranus due to the elevated concentration of hydrocarbons.<ref name="Lunine 1993"/>

For reasons that remain obscure, the planet's thermosphere is at an anomalously high temperature of about {{Convert|750|K|C F|sigfig=3}}.<ref name="Broadfoot19989">{{Cite journal |last1=Broadfoot |first1=A.L. |last2=Atreya |first2=S.K. |last3=Bertaux |first3=J.L. |display-authors=etal |date=1999 |title=Ultraviolet Spectrometer Observations of Neptune and Triton |url=http://www-personal.umich.edu/~atreya/Articles/1989_Voyager_UV_Spectrometer.pdf |url-status=live |journal=Science |volume=246 |issue=4936 |pages=1459–66 |bibcode=1989Sci...246.1459B |doi=10.1126/science.246.4936.1459 |pmid=17756000 |s2cid=21809358 |archive-url=https://web.archive.org/web/20080528020007/http://www-personal.umich.edu/~atreya/Articles/1989_Voyager_UV_Spectrometer.pdf |archive-date=28 May 2008 |access-date=12 March 2008}}</ref><ref name="Herbert & Sandel 1999"/> 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="Encrenaz 2003"/>

=== Colour ===
Neptune's atmosphere is faintly blue in the [[Visible spectrum|optical spectrum]], only slightly more saturated than the blue of Uranus's atmosphere. Early renderings of the two planets greatly exaggerated Neptune's colour contrast "to better reveal the clouds, bands and winds", making it seem deep blue compared to Uranus's off-white. The two planets had been imaged with different systems, making it hard to directly compare the resulting [[Compositing|composite images]]. This was revisited with the colour normalised over time, most comprehensively in late 2023.<ref name=":0" /><ref name=":1">{{Cite journal |last1=Irwin |first1=Patrick G. J. |last2=Dobinson |first2=Jack |last3=James |first3=Arjuna |last4=Teanby |first4=Nicholas A. |last5=Simon |first5=Amy A. |last6=Fletcher |first6=Leigh N. |last7=Roman |first7=Michael T. |last8=Orton |first8=Glenn S. |last9=Wong |first9=Michael H. |last10=Toledo |first10=Daniel |last11=Pérez-Hoyos |first11=Santiago |last12=Beck |first12=Julie |date=February 2024 |title=Modelling the seasonal cycle of Uranus's colour and magnitude, and comparison with Neptune |journal=Monthly Notices of the Royal Astronomical Society |language=en |volume=527 |issue=4 |pages=11521–11538 |doi=10.1093/mnras/stad3761 |doi-access=free |hdl=20.500.11850/657542 |hdl-access=free }}</ref>

<gallery widths="200" heights="200" class="center">
File:Neptune Full.jpg|Original 2-colour (orange-green) NASA/JPL image from [[Voyager 2|''Voyager&nbsp;2'']], with exaggerated colour<ref>{{Cite web |title=Catalog Page for PIA01492 |url=https://photojournal.jpl.nasa.gov/catalog/PIA01492 |access-date=2024-02-05 |website=photojournal.jpl.nasa.gov |archive-date=22 July 2023 |archive-url=https://web.archive.org/web/20230722014419/https://photojournal.jpl.nasa.gov/catalog/PIA01492 |url-status=live }}</ref>
File:Neptune - Voyager 2 (29347980845) flatten crop.jpg|Colour recalibrated in 2016 (Justin Cowart), preserving some enhancement for contrast<ref>{{Cite web |title=The subtle color difference between Uranus and Neptune |url=https://www.planetary.org/articles/uranus-neptune-color-difference |access-date=2024-02-05 |website=The Planetary Society |language=en |archive-date=5 February 2024 |archive-url=https://web.archive.org/web/20240205080048/https://www.planetary.org/articles/uranus-neptune-color-difference |url-status=live }}</ref>
File:Neptune Voyager2 color calibrated.png|Colour recalibrated in 2023 (Patrick Irwin), approximating the true colour<ref>{{Cite web |last=Oxford |first=University of |title=New images reveal what Neptune and Uranus really look like |url=https://phys.org/news/2024-01-images-reveal-neptune-uranus.html |access-date=2024-02-05 |website=phys.org |language=en |archive-date=5 February 2024 |archive-url=https://web.archive.org/web/20240205000820/https://phys.org/news/2024-01-images-reveal-neptune-uranus.html |url-status=live }}</ref>
</gallery>

=== Magnetosphere ===
Neptune's [[magnetosphere]] consists of a [[magnetic field]] that is strongly tilted relative to its [[Rotation|rotational axis]] at 47° and offset of at least 0.55&nbsp;radius (~13,500&nbsp;km) from the planet's physical centre—resembling Uranus's magnetosphere. Before the arrival of ''Voyager 2'' to Neptune, it was hypothesised that Uranus's sideways rotation caused its tilted magnetosphere. In comparing the magnetic fields of the two planets, scientists now think the extreme orientation may be characteristic of flows in the planets' interiors. This field may be generated 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>{{cite book |last=Elkins-Tanton |first=Linda T. |date=2006 |title=Uranus, Neptune, Pluto, and the Outer Solar System |publisher=Chelsea House |location=New York |isbn=978-0-8160-5197-7 |pages=[https://archive.org/details/uranusneptuneplu00elki/page/79 79–83] |url=https://archive.org/details/uranusneptuneplu00elki/page/79 }}</ref> resulting in a [[dynamo]] action.<ref>{{Cite journal |last1=Stanley |first1=Sabine |author-link1=Sabine Stanley |last2=Bloxham |first2=Jeremy |date=March 2004 |title=Convective-region geometry as the cause of Uranus' and Neptune's unusual magnetic fields |journal=[[Nature (journal)|Nature]] |volume=428 |issue=6979 |pages=151–153 |bibcode=2004Natur.428..151S |doi=10.1038/nature02376 |issn=0028-0836 |pmid=15014493 |s2cid=33352017}}</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 |last1=Connerney |first1=J.E.P. |last2=Acuña |first2=Mario H. |last3=Ness |first3=Norman F. |title=The magnetic field of Neptune |date=1991 |journal=Journal of Geophysical Research |volume=96 |pages=19,023–42 |bibcode=1991JGR....9619023C |doi=10.1029/91JA01165}}</ref> The dipole [[magnetic moment]] of Neptune is about 2.2{{E-sp|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|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 an offset from the planet's centre and geometrical constraints of the field's dynamo generator.<ref name="science4936">{{Cite journal |last1=Ness |first1=Norman F. |last2=Acuña |first2=Mario H. |last3=Burlaga |first3=Leonard F. |last4=Connerney |first4=John E. P. |last5=Lepping |first5=Ronald P. |last6=Neubauer |first6=Fritz M. |date=15 December 1989 |title=Magnetic Fields at Neptune |url=https://zenodo.org/record/1230994 |url-status=live |journal=Science |volume=246 |issue=4936 |pages=1473–1478 |bibcode=1989Sci...246.1473N |doi=10.1126/science.246.4936.1473 |issn=0036-8075 |pmid=17756002 |s2cid=20274953 |archive-url=https://web.archive.org/web/20190710235516/https://zenodo.org/record/1230994/files/article.pdf |archive-date=10 July 2019 |access-date=25 August 2019}}</ref><ref>{{Cite web |last1=Russell |first1=C.T. |last2=Luhmann |first2=J.G. |date=1997 |title=Neptune: Magnetic Field and Magnetosphere |url=http://www-ssc.igpp.ucla.edu/personnel/russell/papers/nep_mag.html |url-status=live |archive-url=https://web.archive.org/web/20190629123808/http://www-ssc.igpp.ucla.edu/personnel/russell/papers/nep_mag.html |archive-date=29 June 2019 |access-date=10 August 2006 |publisher=University of California, Los Angeles}}</ref>

Measurements by ''Voyager 2'' in extreme-ultraviolet and radio frequencies revealed that Neptune has faint and weak but complex and unique [[aurora]]e; however, these observations were limited in time and did not contain infrared. Subsequent astronomers using the Hubble Space Telescope have not glimpsed the aurorae, in contrast to the more well-defined aurorae of Uranus.<ref name="Lamy 2020 p. 20190481">{{cite journal | last=Lamy | first=L. | title=Auroral emissions from Uranus and Neptune | journal=Philosophical Transactions of the Royal Society A: Mathematical, Physical and Engineering Sciences | publisher=The Royal Society | volume=378 | issue=2187 | date=9 November 2020 | doi=10.1098/rsta.2019.0481 | page=20190481| doi-access=free | pmid=33161867 | pmc=7658782 | bibcode=2020RSPTA.37890481L }}</ref><ref name="esa">{{cite web|url=http://www.esa.int/esaCP/SEMLQ71DU8E_index_0.html|title=ESA Portal – Mars Express discovers auroras on Mars|publisher=European Space Agency|date=11 August 2004|access-date=5 August 2010|archive-date=19 October 2012|archive-url=https://web.archive.org/web/20121019183824/http://www.esa.int/esaCP/SEMLQ71DU8E_index_0.html|url-status=live}}</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 likely much farther.<ref name=science4936/>

== Climate ==
[[File:Neptune storms.jpg|thumb|The [[Great Dark Spot]] (top), Scooter (middle white cloud),<ref name=scooter>{{cite web |first=Sue |last=Lavoie |date=8 January 1998 |title=PIA01142: Neptune Scooter |url=http://photojournal.jpl.nasa.gov/catalog/PIA01142 |publisher=NASA |access-date=26 March 2006 |archive-url=https://web.archive.org/web/20131029200323/http://photojournal.jpl.nasa.gov/catalog/PIA01142 |archive-date=29 October 2013 |url-status=live }}</ref> and the [[Small Dark Spot]] (bottom), with contrast exaggerated]]

Neptune's weather is characterized by extremely dynamic storm systems, with winds reaching speeds of almost {{cvt|600|m/s|km/h mph}}—exceeding [[supersonic speed|supersonic]] flow.<ref name="Suomi1991" /> 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 |last1=Hammel |first1=H. B. |last2=Beebe |first2=R. F. |last3=De Jong |first3=E. M. |last4=Hansen |first4=C. J. |last5=Howell |first5=C. D. |last6=Ingersoll |first6=A. P. |last7=Johnson |first7=T. V. |last8=Limaye |first8=S. S. |last9=Magalhães |first9=J. A. |last10=Pollack |first10=J. B. |last11=Sromovsky |first11=L. A. |last12=Suomi |first12=V. E. |last13=Swift |first13=C. E. |display-authors=4 |date=22 September 1989 |title=Neptune's Wind Speeds Obtained by Tracking Clouds in Voyager Images |journal=Science |volume=245 |issue=4924 |pages=1367–1369 |bibcode=1989Sci...245.1367H |doi=10.1126/science.245.4924.1367 |issn=0036-8075 |pmid=17798743 |s2cid=206573894}}</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|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 thought to be a "[[skin effect]]" and not due to any deeper atmospheric processes.<ref name="Lunine 1993"/> At 70°S latitude, a high-speed jet travels at a speed of 300&nbsp;m/s.<ref name="Lunine 1993"/> Due to 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. The long orbital period of Neptune results in seasons lasting 40 Earth years.<ref name="villard">{{Cite news |last1=Villard |first1=Ray |last2=Devitt |first2=Terry |date=15 May 2003 |title=Brighter Neptune Suggests A Planetary Change of Seasons |url=http://hubblesite.org/newscenter/archive/releases/2003/17/text/ |url-status=live |archive-url=https://web.archive.org/web/20080228081218/http://hubblesite.org/newscenter/archive/releases/2003/17/text/ |archive-date=28 February 2008 |access-date=26 February 2008 |publisher=Hubble News Center}}</ref>

Neptune differs from Uranus in its typical level of [[Meteorology|meteorological]] activity. ''Voyager 2'' observed weather phenomena on Neptune during its 1989 flyby,<ref name=spot/> but no comparable phenomena on Uranus during its 1986 flyby.

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, as photochemistry cannot account for the distribution without meridional circulation.<ref name="Lunine 1993"/>

In 2007, it was discovered that the upper troposphere of Neptune's south pole was about 10 K warmer than the rest of its atmosphere, which averages about {{Convert|73|K|C|round=5}}. The temperature differential is enough to let methane, which elsewhere is frozen in the troposphere, escape into the stratosphere near the pole.<ref>{{Cite journal |last1=Orton |first1=G. S. |last2=Encrenaz |first2=T. |author-link2=Thérèse Encrenaz |last3=Leyrat |first3=C. |last4=Puetter |first4=R. |last5=Friedson |first5=A. J. |display-authors=4 |name-list-style=amp |date=October 2007 |title=Evidence for methane escape and strong seasonal and dynamical perturbations of Neptune's atmospheric temperatures |url=https://hal.science/hal-03730672/file/aa8277-07.pdf |url-status=live |journal=[[Astronomy & Astrophysics]] |volume=473 |issue=1 |pages=L5–L8 |bibcode=2007A&A...473L...5O |doi=10.1051/0004-6361:20078277 |issn=0004-6361 |s2cid=54996279 |archive-url=https://web.archive.org/web/20240201222014/https://hal.science/hal-03730672/file/aa8277-07.pdf |archive-date=1 February 2024 |access-date=1 February 2024 |doi-access=free}}</ref> 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 |last1=Orton |first1=Glenn |last2=Encrenaz |first2=Thérèse |author-link2=Thérèse Encrenaz |date=18 September 2007 |title=A Warm South Pole? Yes, On Neptune! |url=http://www.eso.org/public/news/eso0741/ |url-status=live |archive-url=https://web.archive.org/web/20100323211637/http://www.eso.org/public/news/eso0741/ |archive-date=23 March 2010 |access-date=20 September 2007 |publisher=[[ESO]]}}</ref>

=== Storms ===
[[File:Neptune's Great Dark Spot.jpg|thumb|256x256px|The Great Dark Spot imaged by ''Voyager 2'' in an enhanced colour image]]
In 1989, the [[Great Dark Spot]], an [[anticyclone|anticyclonic]] storm system spanning {{convert|13000 x 6,600|km|mi|abbr=on}},<ref name=spot>{{cite web |last=Lavoie |first=Sue |date=16 February 2000 |url=http://photojournal.jpl.nasa.gov/catalog/PIA02245 |title=PIA02245: Neptune's blue-green atmosphere |publisher=NASA JPL |access-date=28 February 2008 |archive-url=https://web.archive.org/web/20130805215059/http://photojournal.jpl.nasa.gov/catalog/PIA02245 |archive-date=5 August 2013 |url-status=live }}</ref> was discovered by [[NASA]]'s ''Voyager 2'' spacecraft. The storm resembled the [[Great Red Spot]] of Jupiter. Some five years later, on 2 November 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 Neptune's northern hemisphere.<ref>{{cite journal |last1=Hammel |first1=H. B. |last2=Lockwood | first2=G. W. | last3=Mills | first3=J. R. | last4=Barnet | first4=C. D. |title=Hubble Space Telescope Imaging of Neptune's Cloud Structure in 1994 |journal=Science |date=1995 |volume=268 |issue=5218 |pages=1740–42 |doi=10.1126/science.268.5218.1740 |pmid=17834994 |bibcode=1995Sci...268.1740H|s2cid=11688794 }}</ref>

The {{vanchor|Scooter}} is another storm, a white cloud group farther south than the Great Dark Spot. This nickname first arose during the months leading up to the ''Voyager 2'' encounter in 1989, when they were observed moving at speeds faster than the Great Dark Spot (and images acquired later would subsequently reveal the presence of clouds moving even faster than those that had initially been detected by ''Voyager 2'').<ref name="burgess2" /> The [[Small Dark Spot]] is a southern cyclonic storm, the second-most-intense storm observed during the 1989 encounter. It was initially completely dark, but as ''Voyager 2'' approached the planet, a bright core developed, which can be seen in most of the highest-resolution images.<ref>{{cite web |last=Lavoie |first=Sue |date=29 January 1996 |url=http://photojournal.jpl.nasa.gov/catalog/PIA00064 |title=PIA00064: Neptune's Dark Spot (D2) at High Resolution |publisher=NASA JPL |access-date=28 February 2008 |archive-url=https://web.archive.org/web/20130927090039/http://photojournal.jpl.nasa.gov/catalog/PIA00064 |archive-date=27 September 2013 |url-status=live }}</ref> In 2018, a newer main dark spot and smaller dark spot were identified and studied.<ref name="NYT-20201222">{{Cite news |last=Stirone |first=Shannon |author-link=Shannon Stirone |date=22 December 2020 |title=Neptune's Weird Dark Spot Just Got Weirder – While observing the planet's large inky storm, astronomers spotted a smaller vortex they named Dark Spot Jr. |url=https://www.nytimes.com/2020/12/22/science/neptune-dark-spot.html |url-status=live |archive-url=https://web.archive.org/web/20201222101200/https://www.nytimes.com/2020/12/22/science/neptune-dark-spot.html |archive-date=22 December 2020 |access-date=22 December 2020 |work=[[The New York Times]]}}</ref> In 2023, the first ground-based observation of a dark spot on Neptune was announced.<ref>{{Cite web |title=Mysterious Neptune dark spot detected from Earth for the first time |url=https://www.eso.org/public/news/eso2314/ |access-date=26 August 2023 |publisher=[[ESO]] |archive-date=26 August 2023 |archive-url=https://web.archive.org/web/20230826075848/https://www.eso.org/public/news/eso2314/ |url-status=live }}</ref>

{{Multiple image
| direction         = horizontal
| total_width       = 370
| image1            = A storm is coming Neptune.tif
| caption1          = The Northern Great Dark Spot imaged by Hubble in 2018
| alt1              = Neptune with its Northern Great Dark Spot visible.
| image2            = Neptune Dark Spot Jr. Hubble.png
| caption2          = The Northern Great Dark Spot and a smaller companion storm imaged by Hubble in 2020
| image3            = Neptune’s shrinking vortex.jpg
| caption3          = Neptune's SDS-2015<ref>''SDS-2015'' meant that it was a Southern Dark Spot that was discovered in 2015.<br>{{Cite journal |last1=H. Wong |first1=Michael |last2=Tollefson |first2=Joshua |last3=I. Hsu |first3=Andrew |last4=de Pater |first4=Imke |last5=A. Simon |first5=Amy |last6=Hueso |first6=Ricardo |last7=Sánchez-Lavega |first7=Agustín |last8=Sromovsky |first8=Lawrence |last9=Fry |first9=Patrick |last10=Luszcz-Cook |first10=Statia |date=15 February 2018 |title=A New Dark Vortex on Neptune |url=https://iopscience.iop.org/article/10.3847/1538-3881/aaa6d6/meta |journal=The American Astronomical Society |volume=155 |issue=3 |at=Abstract section |doi=10.3847/1538-3881|doi-broken-date=1 November 2024 |doi-access=free }}</ref> vortex in around two years, which has appeared to be shrinking and disappearing
| align             = left| perrow            = 2 / 1
}}

Neptune's dark spots are thought to occur in the [[troposphere]] at lower altitudes than the brighter cloud features,<ref>{{cite journal |last1=S.G. |first1=Gibbard | last2=de Pater | first2=I. | last3=Roe | first3=H. G. | last4=Martin | first4=S. | last5=Macintosh | first5=B.A. | last6=Max | first6=C.E. | display-authors=4 |date=2003 |title=The altitude of Neptune cloud features from high-spatial-resolution near-infrared spectra |url=http://cips.berkeley.edu/research/depater_altitude.pdf |url-status=dead |journal=Icarus |volume=166 |issue=2 |pages=359–74 |bibcode=2003Icar..166..359G |doi=10.1016/j.icarus.2003.07.006 |archive-url=https://web.archive.org/web/20120220052413/http://cips.berkeley.edu/research/depater_altitude.pdf |archive-date=20 February 2012 |access-date=26 February 2008}}</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 |last1=Stratman |first1=P.W. | last2=Showman | first2=A.P. | last3=Dowling | first3=T.E. | last4=Sromovsky | first4=L.A. |date=2001 |title=EPIC Simulations of Bright Companions to Neptune's Great Dark Spots |url=http://www.lpl.arizona.edu/~showman/publications/stratman-etal-2001.pdf |url-status=live |journal=Icarus |volume=151 |issue=2 |pages=275–85 |bibcode=1998Icar..132..239L |doi=10.1006/icar.1998.5918 |archive-url=https://web.archive.org/web/20080227115523/http://www.lpl.arizona.edu/~showman/publications/stratman-etal-2001.pdf |archive-date=27 February 2008 |access-date=26 February 2008}}</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 |last1=Sromovsky |first1=L.A. |last2=Fry |first2=P.M. |last3=Dowling |first3=T.E. |last4=Baines |first4=K.H. |title=The unusual dynamics of new dark spots on Neptune |journal=Bulletin of the American Astronomical Society |date=2000 |volume=32 |page=1005 |bibcode=2000DPS....32.0903S}}</ref>

[[File:Different Faces Neptune.jpg|thumb|Four images taken a few hours apart with the NASA/ESA ''Hubble Space Telescope''{{'s}} [[Wide Field Camera 3]]. Near-infrared radiation data has been used as a red channel.<ref>{{cite web |url=http://www.spacetelescope.org/images/ann1115a/ |title=Happy birthday Neptune |publisher=ESA/Hubble |access-date=13 July 2011 |archive-url=https://web.archive.org/web/20110715204613/http://www.spacetelescope.org/images/ann1115a/ |archive-date=15 July 2011 |url-status=live }}</ref>|231x231px]]

In 1989, ''Voyager 2''<nowiki/>'s [[Voyager 2#Scientific instruments|Planetary Radio Astronomy]] (PRA) experiment observed around 60 lightning flashes, or Neptunian electrostatic discharges emitting energies over {{val|7|e=8|ul=J}}.<ref>{{cite journal |last1=Borucki |first1=W.J. |title=Predictions of lightning activity at Neptune |journal=Geophysical Research Letters |date=1989 |volume=16 |issue=8 |pages=937–939 |doi=10.1029/gl016i008p00937|bibcode=1989GeoRL..16..937B }}</ref> A plasma wave system (PWS) detected 16 electromagnetic wave events with a frequency range of {{val|50|–|12|u=kHz}} at magnetic latitudes 7–33˚.<ref name="Atmospheric Electricity at the Ice">{{cite journal |last1=Aplin |first1=K.L. |last2=Fischer |first2=G. |last3=Nordheim |first3=T.A. |last4=Konovalenko | first4=A. |last5=Zakharenko |first5=V. |last6=Zarka |first6= P.|title=Atmospheric Electricity at the Ice Giants |journal=Space Science Reviews |date=2020 |volume=216 |issue=2 |page=26 |doi=10.1007/s11214-020-00647-0 |arxiv=1907.07151 |bibcode=2020SSRv..216...26A }}</ref><ref name="Whistlers in Neptune's magnetospher">{{cite journal |title=Whistlers in Neptune's magnetosphere: Evidence of atmospheric lightning |journal=Journal of Geophysical Research: Space Physics |date=1990 |volume=95 |pages=20967–20976 |doi=10.1029/ja095ia12p20967|bibcode=1990JGR....9520967G |hdl=2060/19910002329 |hdl-access=free |last1=Gurnett |first1=D. A. |last2=Kurth |first2=W. S. |last3=Cairns |first3=I. H. |last4=Granroth |first4=L. J. |issue=A12 }}</ref> These plasma wave detections were possibly triggered by lightning over 20 minutes in the ammonia clouds of the magnetosphere.<ref name="Whistlers in Neptune's magnetospher"/>

During ''Voyager 2''’s closest approach to Neptune, the PWS instrument provided Neptune's first plasma wave detections at a sample rate of 28,800 samples per second.<ref name="Whistlers in Neptune's magnetospher"/> The measured plasma densities range from {{nowrap|10<sup>−3</sup> – 10<sup>−1</sup> cm<sup>3</sup>}}.<ref name="Whistlers in Neptune's magnetospher"/><ref>{{cite journal |last1=Belcher |first1=J.W. |last2=Bridge |first2=H.S. |last3=Bagenal |first3=F. |last4=Coppi |first4=B. |last5=Divers |first5=O. |last6=Eviatar |first6=A. |last7=Gordon |first7=G.S. |last8=Lazarus |first8=A.J. |last9=McNutt |first9=R.L. |last10=Ogilvie |first10= K.W. |last11=Richardson |first11= J.D. |last12= Siscoe |first12=G.L. |last13=Sittler |first13=E.C. |last14=Steinberg |first14=J.T. |last15=Sullivan |first15=J.D. |last16=Szabo |first16=A. |last17=Villanueva |first17=L. |last18=Vasyliunas |first18=V.M. |last19=Zhang |first19=M. |title= Plasma observations near Neptune: Initial results from Voyager 2 |journal=Science |date=1989 |volume=246 |issue=4936 |pages=1478–1483 |doi=10.1126/science.246.4936.1478 |pmid=17756003 |bibcode=1989Sci...246.1478B }}</ref> Neptunian lightning may occur in three cloud layers,<ref>{{cite journal |last1=Gibbard |first1=S.G. |last2=Levy |first2=E.H. |last3=Lunine |first3=J.I. |last4=de Pater |first4=I. |title=Lightning on Neptune |journal=Icarus |date=1999 |volume=139 |issue=2 |pages=227–234 |doi=10.1006/icar.1999.6101 |bibcode=1999Icar..139..227G }}</ref> with microphysical modelling suggesting that most of these occurrences happen in the water clouds of the troposphere or the shallow ammonia clouds of the magnetosphere.<ref name="Atmospheric Electricity at the Ice"/><ref>{{cite journal |last1=Aglyamov |first1=Y.S. |last2=Lunine |first2=J. |last3=Atreya |first3=S. |last4=Guillot | first4=T. |last5=Becker |first5=H.N. |last6=Levin |first6=S.|last7=Bolton |first7=S.J. |title=Atmospheric Electricity at the Ice Giants |journal=Space Science Reviews |date=2020 |volume=216 |issue=2 |doi=10.1007/s11214-020-00647-0 |arxiv=1907.07151 |bibcode=2020SSRv..216...26A }}</ref> Neptune is predicted to have 1/19 the lightning flash rate of Jupiter and to display most of its lightning activity at high latitudes. However, lightning on Neptune seems to resemble lightning on Earth rather than Jovian lightning.<ref>{{cite journal |last=Borucki |first=W.J. |title=Predictions of lightning activity at Neptune |journal=Geophysical Research Letters |date=1989 |volume=16 |issue=8 |pages=937–939 |doi=10.1029/gl016i008p00937|bibcode=1989GeoRL..16..937B }}</ref>

{{Clear left}}

=== Internal heating ===
Neptune's more varied weather when compared to Uranus is due in part to its higher [[internal heating]]. The upper regions of Neptune's troposphere reach a low temperature of {{cvt|51.8|K|C}}. At a depth where the [[atmospheric pressure]] equals {{Cvt|1|bar|kPa|lk=on}}, the temperature is {{cvt|72.00|K|C}}.<ref>{{cite journal |last=Lindal |first=Gunnar F. |title=The atmosphere of Neptune&nbsp;– an analysis of radio occultation data acquired with Voyager 2 |journal=Astronomical Journal |date=1992 |volume=103 |pages=967–82 |bibcode=1992AJ....103..967L |doi=10.1086/116119|doi-access=free }}</ref> Deeper inside the layers of gas, the temperature rises steadily. As with Uranus, the source of this heating is unknown, but the discrepancy is larger: Uranus only radiates 1.1 times as much energy as it receives from the Sun;<ref>{{cite web |title=Class 12&nbsp;– Giant Planets&nbsp;– Heat and Formation |website=3750&nbsp;– Planets, Moons & Rings |date=2004 |publisher=Colorado University, Boulder |url=http://lasp.colorado.edu/~bagenal/3750/ClassNotes/Class12/Class12.html |access-date=13 March 2008 |archive-url=https://web.archive.org/web/20080621120100/http://lasp.colorado.edu/~bagenal/3750/ClassNotes/Class12/Class12.html |archive-date=21 June 2008 |url-status=live }}</ref> whereas Neptune radiates about 2.61 times as much energy as it receives from the Sun.<ref>{{cite journal |last1=Pearl |first1=J.C. |last2=Conrath |first2=B.J. |title=The albedo, effective temperature, and energy balance of Neptune, as determined from Voyager data |journal=Journal of Geophysical Research: Space Physics |date=1991 |volume=96 |pages=18,921–30 |bibcode=1991JGR....9618921P |doi=10.1029/91ja01087}}</ref>

Neptune is over 50% farther from the Sun than Uranus and receives only ~40% of Uranus's amount of sunlight;<ref name="Lunine 1993" /> however, its internal energy is still enough for the fastest planetary winds in the Solar System. Depending on the thermal properties of its interior, the heat left over from Neptune's formation may be sufficient to explain its current heat flow, though it is harder to explain Uranus's lack of internal heat while preserving the apparent similarity between the two planets.<ref>{{Cite book |last1=Pater |first1=Imke de |url=https://books.google.com/books?id=RaJdy3_VINQC&dq=planet+neptune+heat+flow&pg=PA224 |title=Planetary Sciences |last2=Lissauer |first2=Jack J. |date=6 December 2001 |publisher=Cambridge University Press |isbn=978-0-521-48219-6 |pages=224 |language=en |access-date=15 March 2023 |archive-date=29 September 2021 |archive-url=https://web.archive.org/web/20210929080319/https://books.google.com/books?id=RaJdy3_VINQC&pg=PA224&lpg=PA224&dq=planet+neptune+heat+flow |url-status=live }}</ref>

== Orbit and rotation ==
[[File:Neptune Orbit.gif|thumb|Neptune (red arc) completes one orbit around the Sun (centre) for every 164.79 orbits of Earth. The light blue dot represents Uranus.|210x210px]]

The average distance between Neptune and the Sun is {{nobr|4.5 billion km}} (about 30.1&nbsp;[[astronomical unit]]s (AU), the mean distance from the Earth to the Sun), and it completes an orbit on average every 164.79&nbsp;years, subject to a variability of around ±0.1&nbsp;years. The perihelion distance is 29.81&nbsp;AU, and the aphelion distance is 30.33&nbsp;AU.{{efn|The last three aphelia were 30.33&nbsp;AU, the next is 30.34&nbsp;AU. The perihelia are even more stable at 29.81&nbsp;AU.<ref name=AA>{{cite book |first=Jean |last=Meeus |title=Astronomical Algorithms |place=Richmond, VA |publisher=Willmann-Bell |year=1998 |page=273}} Supplemented by further use of VSOP87.</ref>}} Neptune's [[orbital eccentricity]] is only 0.008678, making it the planet in the Solar System with the second most circular orbit after [[Venus]].<ref>{{cite web |title=Planetary Fact Sheet |url=https://nssdc.gsfc.nasa.gov/planetary/factsheet/index.html |url-status=live |archive-url=https://web.archive.org/web/20240202030708/https://nssdc.gsfc.nasa.gov/planetary/factsheet/index.html |archive-date=2 February 2024 |access-date=2 January 2024}}</ref> The orbit of Neptune is [[Orbital inclination|inclined]] 1.77° compared to that of Earth.

On 11&nbsp;July 2011, Neptune completed its first full [[Barycenter|barycentric]] orbit since its discovery in 1846;<ref name="obsjul711">{{cite news |first=Robin |last=McKie |url=https://www.theguardian.com/science/2011/jul/10/neptune-orbit-anniversary-astronomy |title=Neptune's first orbit: a turning point in astronomy |date=9 July 2011 |newspaper=[[The Guardian]] |access-date=15 December 2016 |archive-url=https://web.archive.org/web/20160823175306/https://www.theguardian.com/science/2011/jul/10/neptune-orbit-anniversary-astronomy |archive-date=23 August 2016 |url-status=live }}</ref> it did not appear at its exact discovery position in the sky because Earth was in a different location in its 365.26-day orbit. Because of the motion of the Sun in relation to the barycentre of the Solar System, on 11&nbsp;July, Neptune was not at its exact discovery position in relation to the Sun—if the more common [[Solar coordinate systems|heliocentric coordinate system]] is used, the discovery longitude was reached on 12&nbsp;July 2011.<ref name="fact2">{{Cite web |last1=Munsell |first1=K. |last2=Smith |first2=H. |last3=Harvey |first3=S. |date=13 November 2007 |title=Neptune: Facts & Figures |url=http://solarsystem.nasa.gov/planets/profile.cfm?Object=Neptune&Display=Facts |url-status=dead |archive-url=https://web.archive.org/web/20140409164451/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Neptune&Display=Facts |archive-date=9 April 2014 |access-date=14 August 2007 |publisher=NASA}}</ref><ref name="UT2010-08">{{Cite news |last=Atkinson |first=Nancy |date=26 August 2010 |title=Clearing the Confusion on Neptune's Orbit |url=https://www.universetoday.com/72088/clearing-the-confusion-on-neptunes-orbit/ |access-date=1 February 2024 |website=Universe Today |language=en-US |archive-date=29 September 2023 |archive-url=https://web.archive.org/web/20230929002150/https://www.universetoday.com/72088/clearing-the-confusion-on-neptunes-orbit/ |url-status=live }}</ref><ref>{{Cite tweet |number=21525820626 |user=elakdawalla |title=Doh! RT @lukedones: From Bill Folkner at JPL: Neptune will reach the same ecliptic longitude it had on Sep. 23, 1846, on July 12, 2011. |first=Emily |last=Lakdawalla |date=19 August 2010}}</ref><ref name=Horizons2011>{{cite web |date=16 November 2007 |url=http://home.surewest.net/kheider/astro/nept2011.txt |title=Horizons Output for Neptune 2010–2011 |access-date=25 February 2008 |url-status=dead |archive-url=https://web.archive.org/web/20130502171825/http://home.surewest.net/kheider/astro/nept2011.txt |archive-date=2 May 2013 }}—Numbers generated using the Solar System Dynamics Group, Horizons On-Line Ephemeris System.</ref>

The [[axial tilt]] of Neptune is 28.32°,<ref>{{cite web |last=Williams |first=David R. |date=6 January 2005 |url=http://nssdc.gsfc.nasa.gov/planetary/planetfact.html |title=Planetary Fact Sheets |publisher=NASA |access-date=28 February 2008 |archive-url=https://web.archive.org/web/20080925071832/http://nssdc.gsfc.nasa.gov/planetary/planetfact.html |archive-date=25 September 2008 |url-status=live }}</ref> which is similar to the tilts of Earth (23°) and Mars (25°). As a result, Neptune experiences seasonal changes similar to those on Earth. The long orbital period of Neptune means that the seasons last for forty Earth years.<ref name=villard/> Its sidereal rotation period (day) is roughly 16.11&nbsp;hours.<ref name="fact2" /> Because its axial tilt is comparable to Earth's, the variation in the length of its day over the course of its long year is not any more extreme.

Because Neptune is not a solid body, its atmosphere undergoes [[differential rotation]]. The wide equatorial zone rotates with a period of about 18&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 |last1=Hubbard |first1=W. B. |last2=Nellis |first2=W. J. |last3=Mitchell |first3=A. C. |last4=Holmes |first4=N. C. |last5=Limaye |first5=S. S. |last6=McCandless |first6=P. C. |display-authors=4 |date=9 August 1991 |title=Interior Structure of Neptune: Comparison with Uranus |url=https://zenodo.org/record/1231004 |url-status=live |journal=Science |language=en |volume=253 |issue=5020 |pages=648–651 |bibcode=1991Sci...253..648H |doi=10.1126/science.253.5020.648 |issn=0036-8075 |pmid=17772369 |s2cid=20752830 |archive-url=https://web.archive.org/web/20181023120218/https://zenodo.org/record/1231004 |archive-date=23 October 2018 |access-date=12 June 2019}}</ref> and it results in strong latitudinal wind shear.<ref name="apj125">{{Cite journal |last1=Max |first1=C. E. |last2=Macintosh |first2=B. A. |last3=Gibbard |first3=S. G. |last4=Gavel |first4=D. T. |last5=Roe |first5=H. G. |last6=de Pater |first6=I. |last7=Ghez |first7=A. M. |author-link7=Andrea Ghez |last8=Acton |first8=D. S. |last9=Lai |first9=O. |last10=Stomski |first10=P. |last11=Wizinowich |first11=P. L. |display-authors=4 |date=January 2003 |title=Cloud Structures on Neptune Observed with Keck Telescope Adaptive Optics |journal=[[The Astronomical Journal]] |volume=125 |issue=1 |pages=364–375 |bibcode=2003AJ....125..364M |doi=10.1086/344943 |doi-access=free}}</ref>

== Formation and resonances ==

=== Formation ===
{{Main|Formation and evolution of the Solar System|Nice model}}
[[File:Lhborbits.png|thumb|upright=2|A simulation showing the outer planets and Kuiper belt: a) before Jupiter and Saturn reached a 2:1 resonance; b) after inward scattering of Kuiper belt objects following the orbital shift of Neptune; c) after ejection of scattered Kuiper belt bodies by Jupiter]]

The formation of the ice giants, Neptune and Uranus, has been difficult to model precisely. Current models suggest that the matter density in the outer regions of the Solar System was too low to account for the formation of such large bodies from the traditionally accepted method of core [[Accretion (astrophysics)|accretion]], and various hypotheses have been advanced to explain their formation. One is that the ice giants were not formed by core accretion but from instabilities within the original [[protoplanetary disk|protoplanetary disc]] and later had their atmospheres blasted away by radiation from a nearby massive [[OB star]].<ref name="Boss" />

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 journal |last1=Thommes |first1=E. W. |last2=Duncan |first2=M. J. |last3=Levison |first3=H. F. |date=May 2002 |title=The Formation of Uranus and Neptune among Jupiter and Saturn |journal=[[The Astronomical Journal]] |volume=123 |issue=5 |pages=2862–2883 |arxiv=astro-ph/0111290 |bibcode=2002AJ....123.2862T |doi=10.1086/339975 |s2cid=17510705}}</ref> This hypothesis of migration after formation is favoured due to its ability to better explain the occupancy of the populations of small objects observed in the trans-Neptunian region.<ref>{{cite web |last=Hansen |first=Kathryn |date=7 June 2005 |title=Orbital shuffle for early solar system |url=http://www.geotimes.org/june05/WebExtra060705.html |url-status=live |archive-url=https://web.archive.org/web/20070927212314/http://www.geotimes.org/june05/WebExtra060705.html |archive-date=27 September 2007 |access-date=26 August 2007 |publisher=Geotimes}}</ref> The current most widely accepted<ref name="Crida2009">
 {{Cite book |last=Crida |first=A. |title=Reviews in Modern Astronomy |date=2009 |isbn=978-3-527-62919-0 |volume=21 |page=3008 |chapter=Solar System Formation |bibcode=2009RvMA...21..215C |doi=10.1002/9783527629190.ch12 |arxiv=0903.3008 |s2cid=118414100}}</ref><ref name="Desch07">{{cite journal |last=Desch |first=S.J. |date=2007 |title=Mass Distribution and Planet Formation in the Solar Nebula |url=https://pdfs.semanticscholar.org/054e/675d459212463f2ec9770f858eaaf3c4505b.pdf |url-status=dead |journal=The Astrophysical Journal |volume=671 |issue=1 |pages=878–93 |bibcode=2007ApJ...671..878D |doi=10.1086/522825 |s2cid=120903003 |archive-url=https://web.archive.org/web/20200207103938/https://pdfs.semanticscholar.org/054e/675d459212463f2ec9770f858eaaf3c4505b.pdf |archive-date=7 February 2020}}</ref><ref name="Smith2009">{{Cite journal |last1=Smith |first1=R. |last2=Churcher |first2=L. J. |last3=Wyatt |first3=M. C. |last4=Moerchen |first4=M. M. |last5=Telesco |first5=C. M. |display-authors=4 |date=January 2009 |title=Resolved debris disc emission around η Telescopii: a young solar system or ongoing planet formation? |journal=[[Astronomy & Astrophysics]] |volume=493 |issue=1 |pages=299–308 |arxiv=0810.5087 |bibcode=2009A&A...493..299S |doi=10.1051/0004-6361:200810706 |issn=0004-6361 |s2cid=6588381}}</ref> explanation of the details of this hypothesis is known as the [[Nice model]], which is a [[Formation and evolution of the Solar System|dynamical evolution]] scenario that explores the potential effect of a migrating Neptune and the other giant planets on the structure of the Kuiper belt.

=== Orbital resonances ===
{{Main|Kuiper belt|resonant trans-Neptunian object|Neptune trojan}}
[[File:TheKuiperBelt classes-en.svg|thumb|A diagram showing the major orbital resonances in the [[Kuiper belt]] caused by Neptune: the highlighted regions are the 2:3 resonance ([[plutino]]s), the nonresonant [[Classical Kuiper belt object|"classical belt"]] (cubewanos), 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 journal |title=Collisional Erosion in the Primordial Edgeworth-Kuiper Belt and the Generation of the 30–50 AU Kuiper Gap |first1=S. Alan |last1=Stern |doi=10.1086/304912 |date=1997 |last2=Colwell |first2=Joshua E. |journal=The Astrophysical Journal |volume=490 |issue=2 |pages=879–82 |bibcode=1997ApJ...490..879S|doi-access=free }}</ref> Much in the same way that [[Jupiter#Impacts|Jupiter's gravity dominates the asteroid belt]], Neptune's gravity dominates the Kuiper belt. Over the age of the Solar System, certain regions of the Kuiper belt became destabilised by Neptune's gravity, creating gaps in its structure. The region between 40 and 42&nbsp;AU is an example.<ref>{{Cite journal |last1=Petit |first1=Jean-Marc |last2=Morbidelli |first2=Alessandro |last3=Valsecchi |first3=Giovanni B. |date=1999 |title=Large Scattered Planetesimals and the Excitation of the Small Body Belts |url=https://www.oca.eu/morby/papers/6166a.pdf |url-status=dead |journal=Icarus |volume=141 |issue=2 |pages=367–87 |bibcode=1999Icar..141..367P |doi=10.1006/icar.1999.6166 |archive-url=https://web.archive.org/web/20071201013047/http://www.oca.eu/morby/papers/6166a.pdf |archive-date=1 December 2007 |access-date=23 June 2007}}</ref>

There do exist orbits within these empty regions where objects can survive for the age of the Solar System. These [[Orbital resonance|resonances]] occur when Neptune's orbital period is a precise fraction of that of the object, such as 1:2, or 3:4. If, say, an object orbits the Sun once for every two Neptune orbits, it will only complete half an orbit by the time Neptune returns to its original position. The most heavily populated resonance in the Kuiper belt, with over 200 known objects,<ref>{{cite web |title=List of Transneptunian Objects |publisher=Minor Planet Center |url=http://www.minorplanetcenter.org/iau/lists/TNOs.html |access-date=25 October 2010 |archive-url=https://archive.today/20101027133511/http://www.minorplanetcenter.org/iau/lists/TNOs.html |archive-date=27 October 2010 |url-status=live }}</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 known Kuiper belt objects, [[Pluto]], is among them.<ref>{{cite web |last=Jewitt |first=David |date=2004 |url=http://www2.ess.ucla.edu/~jewitt/kb/plutino.html |title=The Plutinos |publisher=UCLA |access-date=28 February 2008 |archive-url=https://web.archive.org/web/20070419234021/http://www.ifa.hawaii.edu/~jewitt/kb/plutino.html |archive-date=19 April 2007 |url-status=live }}</ref> Although Pluto crosses Neptune's orbit regularly, the 2:3 resonance makes it so that 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 |date=1999 |volume=118 |issue=5 |pages=2526–31 |bibcode=1999AJ....118.2526V |doi=10.1086/301088|doi-access=free }}</ref> The 3:4, 3:5, 4:7 and 2:5 resonances are less populated.<ref>{{Cite book |last=Davies |first=John |url=https://archive.org/details/beyondplutoexplo00davi/page/104 |title=Beyond Pluto: Exploring the outer limits of the solar system |date=2001 |publisher=[[Cambridge University Press]] |isbn=978-0-521-80019-8 |location=Cambridge |page=[https://archive.org/details/beyondplutoexplo00davi/page/104 104]}}</ref>

Neptune has a number of known [[Trojan (celestial body)|trojan objects]] occupying both the [[Sun]]–Neptune {{L4}} and {{L5}} [[Lagrangian point]]s—gravitationally stable regions leading and trailing Neptune in its orbit, respectively.<ref>{{Cite journal |last1=Chiang |first1=E. I. |last2=Jordan |first2=A. B. |last3=Millis |first3=R. L. |last4=Buie |first4=M. W. |last5=Wasserman |first5=L. H. |last6=Elliot |first6=J. L. |last7=Kern |first7=S. D. |last8=Trilling |first8=D. E. |last9=Meech |first9=K. J. |last10=Wagner |first10=R. M. |display-authors=4 |date=July 2003 |title=Resonance Occupation in the Kuiper Belt: Case Examples of the 5:2 and Trojan Resonances |journal=[[The Astronomical Journal]] |language=en |volume=126 |issue=1 |pages=430–443 |arxiv=astro-ph/0301458 |bibcode=2003AJ....126..430C |doi=10.1086/375207 |issn=0004-6256 |s2cid=54079935}}</ref> [[Neptune trojan]]s can be viewed as being in a 1:1 resonance with Neptune. Some Neptune trojans are remarkably stable in their orbits, and are likely to have formed alongside Neptune rather than being [[Gravitational capture|captured]]. The first object identified as associated with Neptune's trailing {{L5}} Lagrangian point was {{mpl|2008 LC|18}}.<ref name="Sheppard">{{Cite journal |last1=Sheppard |first1=Scott S. |author-link1=Scott S. Sheppard |last2=Trujillo |first2=Chadwick A. |date=10 September 2010 |title=Detection of a Trailing (L5) Neptune Trojan |journal=[[Science (journal)|Science]] |volume=329 |issue=5997 |pages=1304 |bibcode=2010Sci...329.1304S |doi=10.1126/science.1189666 |issn=0036-8075 |pmid=20705814 |s2cid=7657932 |doi-access=free}}</ref> Neptune has a temporary [[quasi-satellite]], {{mpl|(309239) 2007 RW|10}}.<ref name="quasi">{{cite journal |name-list-style=amp |title=(309239) 2007 RW10: a large temporary quasi-satellite of Neptune |journal=Astronomy and Astrophysics Letters |volume=545 |issue=2012 |pages=L9 |date=2012 |arxiv=1209.1577 |bibcode=2012A&A...545L...9D |doi=10.1051/0004-6361/201219931 |last1=De La Fuente Marcos |first1=C. |last2=De La Fuente Marcos |first2=R.|s2cid=118374080 }}</ref> The object has been a quasi-satellite of Neptune for about 12,500 years and it will remain in that dynamical state for another 12,500 years.<ref name=quasi/>

== Moons ==
{{Main|Moons of Neptune}}
{{For|a timeline of discovery dates|Timeline of discovery of Solar System planets and their moons}}
[[File:New_Webb_Images_Capture_Rare_View_of_Neptune’s_Rings_(Labeled).png|alt=Shown in this image are Neptune and its moons: Triton, Galatea, Naiad, Thalassa, Despina, Proteus, and Larissa|thumb|An annotated picture of Neptune's many moons as captured by the [[James Webb Space Telescope]]. The bright blue [[Diffraction spike|diffraction star]] is [[Triton (moon)|Triton]], Neptune's largest moon.]]
Neptune has 16 known [[Natural satellite|moons]].<ref name="Sheppardmoons2024"/> [[Triton (moon)|Triton]] is the largest Neptunian moon, accounting for more than 99.5% of the mass in orbit around Neptune,<ref group="lower-alpha">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%. The mass of the rings is negligible.</ref> and is the only one massive enough to be [[spheroid]]al. Triton was 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 and prograde motion|retrograde]] orbit, indicating that it was captured rather than forming in place; it was probably once a [[dwarf planet]] in the Kuiper belt.<ref>{{Cite journal |last1=Agnor |first1=Craig B. |last2=Hamilton |first2=Douglas P. |date=May 2006 |title=Neptune's capture of its moon Triton in a binary–planet gravitational encounter |journal=[[Nature (journal)|Nature]] |volume=441 |issue=7090 |pages=192–194 |bibcode=2006Natur.441..192A |doi=10.1038/nature04792 |issn=0028-0836 |pmid=16688170 |s2cid=4420518}}</ref> It is close enough to Neptune to be locked into a [[synchronous rotation]], and it is slowly spiralling inward because of [[tidal acceleration]]. It will eventually be torn apart, in about 3.6&nbsp;billion years, when it reaches the [[Roche limit]].<ref>{{cite journal |first1=Christopher F. |last1=Chyba |last2=Jankowski |first2=D.G. |last3=Nicholson |first3=P.D. |title=Tidal evolution in the Neptune-Triton system |journal=Astronomy and Astrophysics |date=1989 |volume=219 |issue=1–2 |pages=L23–L26 |bibcode=1989A&A...219L..23C }}</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. |newspaper=[[The New York Times]] |date=29 August 1989 |title=Triton May Be Coldest Spot in Solar System |url=https://query.nytimes.com/gst/fullpage.html?res=950DE4DC1138F93AA1575BC0A96F948260 |access-date=29 February 2008 |archive-url=https://web.archive.org/web/20081210100148/http://query.nytimes.com/gst/fullpage.html?res=950DE4DC1138F93AA1575BC0A96F948260 |archive-date=10 December 2008 |url-status=live }}</ref> with estimated temperatures of {{cvt|38|K|C|0}}.<ref>{{cite web | url=https://science.nasa.gov/neptune/moons/triton/#hds-sidebar-nav-1 | title=Triton - NASA Science | access-date=7 January 2024 | archive-date=7 January 2024 | archive-url=https://web.archive.org/web/20240107182347/https://science.nasa.gov/neptune/moons/triton/#hds-sidebar-nav-1 | url-status=live }}</ref><ref>{{Cite journal |last1=Nelson |first1=Robert M. |last2=Smythe |first2=William D. |last3=Wallis |first3=Brad D. |last4=Horn |first4=Linda J. |last5=Lane |first5=Arthur L. |last6=Mayo |first6=Marvin J. |display-authors=4 |date=19 October 1990 |title=Temperature and Thermal Emissivity of the Surface of Neptune's Satellite Triton |journal=[[Science (journal)|Science]] |volume=250 |issue=4979 |pages=429–431 |bibcode=1990Sci...250..429N |doi=10.1126/science.250.4979.429 |issn=0036-8075 |pmid=17793020 |s2cid=20022185}}</ref> This very low temperature is due to Triton's very high albedo which causes it to reflect a lot of sunlight instead of absorbing it.<ref>{{cite web | url=https://phys.libretexts.org/Bookshelves/Astronomy__Cosmology/Astronomy_1e_(OpenStax)/12%3A_Rings_Moons_and_Pluto/12.03%3A_Titan_and_Triton | title=12.3: Titan and Triton | date=7 October 2016 | access-date=7 January 2024 | archive-date=7 January 2024 | archive-url=https://web.archive.org/web/20240107175520/https://phys.libretexts.org/Bookshelves/Astronomy__Cosmology/Astronomy_1e_(OpenStax)/12:_Rings_Moons_and_Pluto/12.03:_Titan_and_Triton | url-status=live }}</ref><ref>{{cite web | url=https://sos.noaa.gov/catalog/datasets/triton-neptunes-moon/ | title=Triton: Neptune's Moon | date=January 2010 | access-date=7 January 2024 | archive-date=7 January 2024 | archive-url=https://web.archive.org/web/20240107175520/https://sos.noaa.gov/catalog/datasets/triton-neptunes-moon/ | url-status=live }}</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 group="lower-alpha"><math>\tfrac{r_{a}}{r_{p}} = \tfrac{2}{1-e} - 1 = 2/0.2488 - 1 \approx 7.039.</math></ref>

From July to September 1989, ''Voyager 2'' discovered six moons of Neptune.<ref name="science4936-2">{{Cite journal |last1=Stone |first1=E. C. |last2=Miner |first2=E. D. |date=15 November 1989 |title=The Voyager 2 Encounter with the Neptunian System |journal=[[Science (journal)|Science]] |volume=246 |issue=4936 |pages=1417–1421 |bibcode=1989Sci...246.1417S |doi=10.1126/science.246.4936.1417 |issn=0036-8075 |pmid=17755996 |s2cid=9367553}}</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 |author-link=Michael E. Brown |publisher=California Institute of Technology, Department of Geological Sciences |access-date=9 February 2008 |archive-url=https://web.archive.org/web/20110719164447/http://web.gps.caltech.edu/~mbrown/dwarfplanets/ |archive-date=19 July 2011 |url-status=dead }}</ref> Although the second-most-massive Neptunian moon, it is only 0.25% the mass of Triton. Neptune's innermost four moons—[[Naiad (moon)|Naiad]], [[Thalassa (moon)|Thalassa]], [[Despina (moon)|Despina]] and [[Galatea (moon)|Galatea]]—orbit close enough to be within Neptune's rings. The next-farthest out, [[Larissa (moon)|Larissa]], was originally discovered in 1981 when it had occulted a star. This occultation had been attributed to ring arcs, but when ''Voyager 2'' observed Neptune in 1989, Larissa was found to have caused it. Five new irregular moons discovered between 2002 and 2003 were announced in 2004.<ref name="HolmanKavelaarsGrav2004">{{cite journal |doi=10.1038/nature02832 |last1=Holman |first1=M.J. |author-link=Matthew J. Holman |last2=Kavelaars |first2=J.J. |author-link2=John J. Kavelaars |last3=Grav |first3=T. |display-authors=3 |year=2004 |last4=Gladman |first4=B.J. |author-link4=Brett J. Gladman |last5=Fraser |first5=W.C. |last6=Milisavljevic |first6=D. |last7=Nicholson |first7=P.D. |last8=Burns |first8=J.A. |last9=Carruba |first9=V. |title=Discovery of five irregular moons of Neptune |journal=Nature |volume=430 |issue=7002 |pages=865–67 |pmid=15318214 |bibcode=2004Natur.430..865H |s2cid=4412380 |url=https://www.cfa.harvard.edu/~mholman/nature_final.pdf |access-date=24 October 2011 |archive-url=https://web.archive.org/web/20131102023048/https://www.cfa.harvard.edu/~mholman/nature_final.pdf |archive-date=2 November 2013 |url-status=live }}</ref><ref>{{cite news |url=http://news.bbc.co.uk/2/hi/science/nature/3578210.stm |title=Five new moons for planet Neptune |date=18 August 2004 |publisher=BBC News|access-date=6 August 2007 |archive-url=https://web.archive.org/web/20070808234419/http://news.bbc.co.uk/2/hi/science/nature/3578210.stm |archive-date=8 August 2007 |url-status=live }}</ref> A new moon and the smallest yet, [[Hippocamp (moon)|Hippocamp]], was found in 2013 by combining multiple Hubble images.<ref>{{Cite web|url=https://www.theverge.com/2019/2/20/18233029/neptune-moon-hippocamp-proteus-hubble-space-telescope|title=Neptune's newly discovered moon may be the survivor of an ancient collision|last=Grush|first=Loren|date=20 February 2019|website=The Verge|access-date=22 February 2019|archive-url=https://web.archive.org/web/20190221130219/https://www.theverge.com/2019/2/20/18233029/neptune-moon-hippocamp-proteus-hubble-space-telescope|archive-date=21 February 2019|url-status=live}}</ref> Because Neptune was the Roman god of the sea, Neptune's moons have been named after lesser sea gods.<ref name=USGS/>

== Planetary rings ==
{{Main|Rings of Neptune}}
[[File:New Webb Image Captures Clearest View of Neptune’s Rings in Decades.png|thumb|Neptune's rings and moons viewed in infrared by the [[James Webb Space Telescope]]]]
Neptune has a [[planetary ring]] system, though one much less substantial than that of [[Rings of Saturn|Saturn]] and [[Rings of Uranus|Uranus]].<ref name="NYT-20220921">{{cite news |last=O"Callaghan |first=Jonathan |title=Neptune and Its Rings Come Into Focus With Webb Telescope - New images from the space-based observatory offer a novel view of the planet in infrared. |url=https://www.nytimes.com/2022/09/21/science/neptune-webb-telescope-photos.html |date=21 September 2022 |work=[[The New York Times]] |accessdate=23 September 2022 |archive-date=22 September 2022 |archive-url=https://web.archive.org/web/20220922183157/https://www.nytimes.com/2022/09/21/science/neptune-webb-telescope-photos.html |url-status=live }}</ref> The rings may consist of ice particles coated with silicates or carbon-based material, which most likely gives them a reddish hue.<ref>{{cite book |last=Cruikshank |first=Dale P. |title=Neptune and Triton |publisher=[[University of Arizona Press]] |date=1996 |isbn=978-0-8165-1525-7 |pages=703–804}}</ref> The three main rings are the narrow Adams Ring, 63,000&nbsp;km from the centre of Neptune, the Le Verrier Ring, at 53,000&nbsp;km, and the broader, fainter Galle Ring, at 42,000&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 57,000&nbsp;km.<ref>{{cite web |last=Blue |first=Jennifer |date=8 December 2004 |url=http://planetarynames.wr.usgs.gov/Page/Rings |title=Nomenclature Ring and Ring Gap Nomenclature |website=Gazetteer of Planetary Nomenclature |publisher=USGS |access-date=28 February 2008 |archive-url=https://web.archive.org/web/20100705150612/http://planetarynames.wr.usgs.gov/Page/Rings |archive-date=5 July 2010 |url-status=live }}</ref>

The first of these planetary rings was detected in 1968 by a team led by [[Edward Guinan]].<ref name=ring1>{{cite news |last=Wilford |first=John N. |date=10 June 1982 |title=Data Shows 2 Rings Circling Neptune |newspaper=[[The New York Times]] |url=https://query.nytimes.com/gst/fullpage.html?sec=technology&res=950DE3D71F38F933A25755C0A964948260&n=Top/News/Science/Topics/Space |access-date=29 February 2008 |archive-url=https://web.archive.org/web/20081210100701/http://query.nytimes.com/gst/fullpage.html?sec=technology&res=950DE3D71F38F933A25755C0A964948260&n=Top%2FNews%2FScience%2FTopics%2FSpace |archive-date=10 December 2008 |url-status=live }}</ref><ref>{{cite journal |last1=Guinan |first1=E.F. |last2=Harris |first2=C.C. |last3=Maloney |first3=F.P. |title=Evidence for a Ring System of Neptune |journal=Bulletin of the American Astronomical Society |date=1982 |volume=14 |page=658 |bibcode=1982BAAS...14..658G}}</ref> In the early 1980s, analysis of this data along with newer observations led to the hypothesis that this ring might be incomplete.<ref>{{Cite journal |last1=Goldreich |first1=P. |last2=Tremaine |first2=S. |last3=Borderies |first3=N. |date=August 1986 |title=Towards a theory for Neptune's arc rings |url=https://authors.library.caltech.edu/37437/1/1986AJ_____92__490G.pdf |url-status=live |journal=[[The Astronomical Journal]] |volume=92 |pages=490 |bibcode=1986AJ.....92..490G |doi=10.1086/114178 |archive-url=https://web.archive.org/web/20210929080319/ |archive-date=29 September 2021 |access-date=12 June 2019}}</ref> Evidence that the rings might have gaps first arose during a stellar [[occultation]] in 1984 when the rings obscured a star on immersion but not on emersion.<ref name="Nicholson90">{{Cite journal |last1=Nicholson |first1=Philip D. |last2=Cooke |first2=Maren L. |last3=Matthews |first3=Keith |last4=Elias |first4=Jonathan H. |last5=Gilmore |first5=Gerard |display-authors=etal |date=September 1990 |title=Five stellar occultations by Neptune: Further observations of ring arcs |journal=[[Icarus (journal)|Icarus]] |volume=87 |issue=1 |pages=1–39 |bibcode=1990Icar...87....1N |doi=10.1016/0019-1035(90)90020-A |doi-access=free}}</ref> Images from ''Voyager 2'' in 1989 settled the issue by showing several faint rings.

The outermost ring, Adams, contains five prominent arcs now named ''Courage'', ''Liberté'', ''Egalité&nbsp;1'', ''Egalité&nbsp;2'' and ''Fraternité'' (Courage, Liberty, Equality and Fraternity).<ref>{{cite book |first=Arthur N. |last=Cox |date=2001 |title=Allen's Astrophysical Quantities |publisher=Springer |isbn=978-0-387-98746-0|title-link=Clabon Allen }}</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 short timescales. Astronomers now estimate that the arcs are corralled into their current form by the gravitational effects of [[Galatea (moon)|Galatea]], a moon just inward from the ring.<ref>{{Cite web |last1=Munsell |first1=Kirk |last2=Smith |first2=Harman |last3=Harvey |first3=Samantha |date=13 November 2007 |title=Planets: Neptune: Rings |url=http://solarsystem.nasa.gov/planets/profile.cfm?Object=Neptune&Display=Rings |url-status=live |archive-url=https://web.archive.org/web/20120704160507/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Neptune&Display=Rings |archive-date=4 July 2012 |access-date=29 February 2008 |website=Solar System Exploration |publisher=NASA}}</ref><ref>{{Cite journal |last1=Salo |first1=Heikki |last2=Hänninen |first2=Jyrki |date=1998 |title=Neptune's Partial Rings: Action of Galatea on Self-Gravitating Arc Particles |journal=Science |volume=282 |issue=5391 |pages=1102–04 |bibcode=1998Sci...282.1102S |doi=10.1126/science.282.5391.1102 |pmid=9804544}}</ref>

Earth-based observations announced in 2005 appeared to show that Neptune's rings were 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=https://www.newscientist.com/article/mg18524925.900 |title=Neptune's rings are fading away |date=26 March 2005 |website=New Scientist |access-date=6 August 2007 |archive-url=https://web.archive.org/web/20081210072747/http://www.newscientist.com/article/mg18524925.900 |archive-date=10 December 2008 |url-status=live }}</ref>
{{clear left}}

== Observation ==
[[File:Neptunlauf September Oktober2022.png|thumb|right|Movement of Neptune in front of the stars of [[Aquarius (constellation)|Aquarius]] in 2022]]
Neptune brightened about 10% between 1980 and 2000 mostly due to the changing of the seasons.<ref name="Schmude_et_al"/> Neptune may continue to brighten as it approaches perihelion in 2042. The [[apparent magnitude]] currently ranges from 7.67 to 7.89 with a mean of 7.78 and a standard deviation of 0.06.<ref name="Mallama_and_Hilton"/> Prior to 1980, the planet was as faint as magnitude 8.0.<ref name="Mallama_and_Hilton"/> Neptune is too faint to be visible to the [[naked eye]]. It 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|Moore]] (2000):207.</ref>

Because of the distance of Neptune from Earth, its [[angular diameter]] only ranges from 2.2 to 2.4&nbsp;[[arcsecond]]s,<ref name="fact"/><ref name=ephemeris/> the smallest of the Solar System planets. Its small apparent size makes it challenging to study visually. Most telescopic data was fairly limited until the advent of the [[Hubble Space Telescope]] and large ground-based telescopes with [[adaptive optics]] (AO).<ref>In 1977, for example, even the rotation period of Neptune remained uncertain. {{cite journal |last=Cruikshank |first=D.P. |title=On the rotation period of Neptune |journal=Astrophysical Journal Letters |date=1 March 1978 |volume=220 |pages=L57–L59 |bibcode=1978ApJ...220L..57C |doi=10.1086/182636}}</ref><ref>{{cite journal |last1=Max |first1=C. |title=Adaptive Optics Imaging of Neptune and Titan with the W.M. Keck Telescope |journal=Bulletin of the American Astronomical Society |date=1999 |volume=31 |page=1512 |bibcode=1999AAS...195.9302M |display-authors=4 |last2=MacIntosh |first2=B. |last3=Gibbard |first3=S. |last4=Roe |first4=H. |last5=De Pater |first5=I. |last6=Ghez |first6=A. |last7=Acton |first7=S. |last8=Wizinowich |first8=P. |last9=Lai |first9=O.}}</ref><ref>{{Cite APOD |date=18 February 2000 |title= Neptune through Adaptive Optics}}</ref> The first scientifically useful observation of Neptune from ground-based telescopes using adaptive optics was commenced in 1997 from Hawaii.<ref>{{Cite journal |last1=Roddier |first1=F. |last2=Roddier |first2=C. |last3=Brahic |first3=A. |last4=Dumas |first4=C. |last5=Graves |first5=J. E. |last6=Northcott |first6=M. J. |last7=Owen |first7=T. |date=1 August 1997 |title=First ground-based adaptive optics observations of Neptune and Proteus |url=https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&doi=7f3f9d9e7129d47c25a3494f1609a8164e6f3cbe |journal=Planetary and Space Science |volume=45 |issue=8 |pages=1031–1036 |doi=10.1016/S0032-0633(97)00026-3 |bibcode=1997P&SS...45.1031R |access-date=1 February 2024 |archive-date=1 February 2024 |archive-url=https://web.archive.org/web/20240201215839/https://citeseerx.ist.psu.edu/document?repid=rep1&type=pdf&doi=7f3f9d9e7129d47c25a3494f1609a8164e6f3cbe |url-status=live |citeseerx=10.1.1.66.7754 }}</ref> Neptune is currently approaching perihelion (closest approach to the Sun) and has been shown to be heating up, with increased atmospheric activity and brightness as a consequence. Combined with technological advancements, ground-based telescopes with adaptive optics are recording increasingly more detailed images of it. Both ''Hubble'' and the adaptive-optics telescopes on Earth have made many new discoveries within the Solar System since the mid-1990s, with a large increase in the number of known satellites and moons around the outer planet, among others. In 2004 and 2005, five new small satellites of Neptune with diameters between 38 and 61 kilometres were discovered.<ref>{{Cite book |last=Engvold |first=Oddbjorn |url=https://books.google.com/books?id=8s1JV-TrXacC&q=Neptune&pg=PA147 |title=Reports on Astronomy 2003-2005 (IAU XXVIA): IAU Transactions XXVIA |date=10 May 2007 |publisher=Cambridge University Press |isbn=978-0-521-85604-1 |pages=147f |language=en |access-date=15 March 2023 |archive-date=11 May 2023 |archive-url=https://web.archive.org/web/20230511214821/https://books.google.com/books?id=8s1JV-TrXacC&q=Neptune&pg=PA147 |url-status=live }}</ref>

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

Neptune's 164-year orbital period means that the planet takes an average of 13&nbsp;years to move through each constellation of the zodiac. In 2011, it completed its first full orbit of the Sun since being discovered and returned to where it was first spotted northeast of [[Iota Aquarii]].<ref name="auto"/>

Observation of Neptune in the radio-frequency band shows that it is a source of both continuous emission and irregular bursts. Both sources are thought to originate from its rotating magnetic field.<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 |last1=Gibbard |first1=S. G. |last2=Roe |first2=H. |last3=de Pater |first3=I. |last4=Macintosh |first4=B. |last5=Gavel |first5=D. |last6=Max |first6=C. E. |last7=Baines |first7=K. H. |last8=Ghez |first8=A. |display-authors=4 |date=March 2002 |title=High-Resolution Infrared Imaging of Neptune from the Keck Telescope |url=https://zenodo.org/record/1229846 |url-status=live |journal=[[Icarus (journal)|Icarus]] |volume=156 |issue=1 |pages=1–15 |bibcode=2002Icar..156....1G |doi=10.1006/icar.2001.6766 |issn=0019-1035 |archive-url=https://web.archive.org/web/20181023120338/https://zenodo.org/record/1229846 |archive-date=23 October 2018 |access-date=12 June 2019}}</ref>

== Exploration ==
{{Main|Exploration of Neptune}}
[[File:Animation_of_Voyager_2_trajectory.gif|thumb|upright=1.25|Animation of ''Voyager 2''{{'s}} trajectory from 20 August 1977 to 30 December 2000

{{legend2|magenta|''Voyager 2''}}{{·}}{{legend2|Royalblue|[[Earth]]}}{{·}}{{legend2|Lime|[[Jupiter]]}} {{·}}{{legend2|Cyan|[[Saturn]]}}{{·}}{{legend2|Gold|[[Uranus]]}}{{·}}{{legend2|OrangeRed|Neptune}}{{·}}{{legend2|Yellow|[[Sun]]}}]]
''[[Voyager 2]]'' is the only spacecraft that has visited Neptune. The spacecraft{{'s}} closest approach to the planet occurred on 25 August 1989. Because this was the last major planet the spacecraft could visit, it was decided to make a close flyby of the moon [[Triton (moon)|Triton]], regardless of the consequences to the trajectory, similarly to what was done for ''[[Voyager 1]]''{{'}}s encounter with [[Saturn]] and its moon [[Titan (moon)|Titan]]. The images relayed back to Earth from ''Voyager 2'' became the basis of a 1989 [[PBS]] all-night program, ''[[Neptune All Night]]''.<ref>{{cite web |last=Phillips |first=Cynthia|author-link=Cynthia B. Phillips |date=5 August 2003 |url=http://www.seti.org/about-us/voices/phillips-080503.php |title=Fascination with Distant Worlds |access-date=3 October 2007 |publisher=[[SETI Institute]] |archive-url=https://web.archive.org/web/20071103094424/http://www.seti.org/about-us/voices/phillips-080503.php |archive-date=3 November 2007}}</ref>

During the encounter, signals from the spacecraft required 246&nbsp;minutes to reach Earth. Hence, for the most part, ''Voyager 2''{{'s}} mission relied on preloaded commands for the Neptune encounter. The spacecraft performed a near-encounter with the moon [[Nereid (moon)|Nereid]] before it came within 4,400&nbsp;km of Neptune's atmosphere on 25 August, then passed close to the planet's largest moon Triton later the same day.<ref name=burgess>[[#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. Neptune's rotation period was determined using measurements of radio emissions and ''Voyager 2'' showed that Neptune had a surprisingly active weather system. Six new moons were discovered, and the planet was shown to have more than one ring.<ref name=science4936-2/><ref name=burgess/> The flyby provided the first accurate measurement of Neptune's mass which was found to be 0.5 per cent less than previously calculated. The new figure disproved the hypothesis that an undiscovered [[Planet X]] acted upon the orbits of Neptune and Uranus.<ref>{{harvnb|Standage|2000|p=188}}</ref><ref>{{Cite web |last1=Gebhardt |first1=Chris |last2=Goldader |first2=Jeff |date=20 August 2011 |title=Thirty-four years after launch, Voyager 2 continues to explore |url=http://www.nasaspaceflight.com/2011/08/thirty-four-years-voyager-2-continues-explore/ |url-status=live |archive-url=https://web.archive.org/web/20160219211545/http://www.nasaspaceflight.com/2011/08/thirty-four-years-voyager-2-continues-explore/ |archive-date=19 February 2016 |access-date=22 January 2016 |website=NASASpaceflight}}</ref>

Since 2018, the [[China National Space Administration]] has been studying a concept for a pair of ''Voyager''-like interstellar probes tentatively known as ''[[Shensuo (spacecraft)|Shensuo]]''.<ref name="WuEtAl2019">{{cite journal |last1=Wu |first1=Weiren |last2=Yu |first2=Dengyun |last3=Huang |first3=Jiangchuan |last4=Zong |first4=Qiugang |last5=Wang |first5=Chi |last6=Yu |first6=Guobin |last7=He |first7=Rongwei |last8=Wang |first8=Qian |last9=Kang |first9=Yan |last10=Meng |first10=Linzhi |last11=Wu |first11=Ke |last12=He |first12=Jiansen |last13=Li |first13=Hui |title=Exploring the solar system boundary |journal=Scientia Sinica Informationis |date=9 January 2019 |volume=49 |issue=1 |pages=1 |doi=10.1360/N112018-00273 |language=en |doi-access=free }}</ref> Both probes would be launched in the 2020s and take differing paths to explore opposing ends of the [[heliosphere]]; the second probe, ''IHP-2'', would fly by Neptune in January 2038, passing only 1,000&nbsp;km above the cloud tops, and potentially carry an atmospheric impactor to be released during its approach.<ref name="SpaceNews IHP">{{cite news |last1=Jones |first1=Andrew |title=China to launch a pair of spacecraft towards the edge of the solar system |url=https://spacenews.com/china-to-launch-a-pair-of-spacecraft-towards-the-edge-of-the-solar-system/ |access-date=29 April 2021 |website=SpaceNews |date=16 April 2021 |archive-date=15 May 2021 |archive-url=https://archive.today/20210515103459/https://spacenews.com/china-to-launch-a-pair-of-spacecraft-towards-the-edge-of-the-solar-system/ |url-status=live }}</ref> Afterward, it will continue its mission throughout the [[Kuiper belt]] toward the heliosphere tail, which is so far unexplored.

After ''Voyager 2'' and ''IHP-2''{{'}}s flybys, the next step in scientific exploration of the Neptunian system is considered to be an orbital mission; most proposals have been by [[NASA]], most often for a [[Flagship Program|Flagship]] orbiter.<ref name="Flagship">{{cite news |last=Clark |first=Stephen |url=http://spaceflightnow.com/2015/08/25/uranus-neptune-in-nasas-sights-for-new-robotic-mission/ |title=Uranus, Neptune in NASA's sights for new robotic mission |website=Spaceflight Now |date=25 August 2015 |access-date=7 September 2015 |archive-url=https://web.archive.org/web/20150906075245/http://spaceflightnow.com/2015/08/25/uranus-neptune-in-nasas-sights-for-new-robotic-mission/ |archive-date=6 September 2015 |url-status=live }}</ref> In 2003, there was a proposal in NASA's "Vision Missions Studies" for a "Neptune Orbiter with Probes" mission that does ''[[Cassini–Huygens|Cassini]]''-level science.<ref>{{cite journal |last1=Spilker |first1=T.R. |last2=Ingersoll |first2=A.P. |title=Outstanding Science in the Neptune System From an Aerocaptured Vision Mission |journal=Bulletin of the American Astronomical Society |date=2004 |volume=36 |page=1094 |bibcode=2004DPS....36.1412S}}</ref> A subsequent proposal, that was not selected, was for ''[[Argo (NASA spacecraft)|Argo]]'', a flyby spacecraft to be launched in 2019, that would visit Jupiter, Saturn, Neptune, and a Kuiper belt object. The focus would have been on Neptune and its largest moon Triton to be investigated around 2029.<ref name="argho1">{{Cite web |last=Hansen |first=Candice |display-authors=etal |title=Argo – A Voyage Through the Outer Solar System |url=http://www.spacepolicyonline.com/pages/images/stories/PSDS%20GP1%20Hansen_Argo_Neptune%20Mission%20Concept.pdf |url-status=dead |archive-url=https://web.archive.org/web/20150924104222/http://www.spacepolicyonline.com/pages/images/stories/PSDS%20GP1%20Hansen_Argo_Neptune%20Mission%20Concept.pdf |archive-date=24 September 2015 |access-date=5 August 2015 |website=SpacePolicyOnline.com |publisher=Space and Technology Policy Group, LLC}}</ref> The proposed ''[[New Horizons 2]]'' mission might have done a close flyby of the Neptunian system, but it was later scrapped. Currently a pending proposal for the [[Discovery Program]], the [[Trident (spacecraft)|''Trident'']] spacecraft would conduct a flyby of Neptune and Triton;<ref name="Trident_Description">{{cite web |url=https://www.hou.usra.edu/meetings/lpsc2019/pdf/3188.pdf |title=Exploring Triton With Trident: A Discovery-Class Mission |date=23 March 2019 |website=Universities Space Research Association |access-date=26 March 2019 |archive-date=2 August 2020 |archive-url=https://web.archive.org/web/20200802004250/https://www.hou.usra.edu/meetings/lpsc2019/pdf/3188.pdf |url-status=live }}</ref> however, the mission was not selected for Discovery 15 or 16. ''[[Neptune Odyssey]]'' is another concept for a Neptune orbiter and atmospheric probe that was studied as a possible [[large strategic science mission]] by NASA; it would have launched between 2031 and 2033, and arrive at Neptune by 2049.<ref name="Rymer">{{Cite web |last1=Rymer |first1=Abigail |last2=Clyde |first2=Brenda |last3=Runyon |first3=Kirby |date=August 2020 |title=Neptune Odyssey: Mission to the Neptune-Triton System |url=https://science.nasa.gov/science-pink/s3fs-public/atoms/files/Neptune%20Odyssey.pdf |url-status=live |archive-url=https://web.archive.org/web/20201215003151/https://science.nasa.gov/science-pink/s3fs-public/atoms/files/Neptune%20Odyssey.pdf |archive-date=15 December 2020 |access-date=18 April 2021 |publisher=NASA Science}}</ref> However, for logistical reasons the [[Uranus Orbiter and Probe]] mission was selected as the [[ice giant]] orbiter mission recommendation, with top priority ahead of the [[Enceladus Orbilander]].<ref name="2023decadal">{{cite book |title=Origins, Worlds, and Life: A Decadal Strategy for Planetary Science and Astrobiology 2023-2032 |date=19 January 2023 |publisher=National Academies Press |isbn=978-0-309-47578-5 |pages=800 |doi=10.17226/26522 |s2cid=248283239 |edition=Prepublication |url=https://nap.nationalacademies.org/catalog/26522/origins-worlds-and-life-a-decadal-strategy-for-planetary-science |access-date=30 April 2022}}</ref> Two notable proposals for a Triton-focused Neptune orbiter mission that would be costed right between the ''Trident'' and ''Odyssey'' missions (under the [[New Frontiers program]]) are ''Triton Ocean World Surveyor'' and ''Nautilus'', with cruise stages taking place in the 2031–47 and 2041–56 time periods, respectively.<ref>{{Cite web|url=https://science.nasa.gov/wp-content/uploads/2023/10/t-ows-triton-ocean-worlds-surveyor.pdf|date=7 June 2021|title=Triton Ocean Worlds Surveyor concept study|website=NASA|last1=Hansen-Koharcheck|first1=Candice|last2=Fielhauer|first2=Karl|access-date=11 January 2024|archive-date=9 November 2023|archive-url=https://web.archive.org/web/20231109122553/https://science.nasa.gov/wp-content/uploads/2023/10/t-ows-triton-ocean-worlds-surveyor.pdf|url-status=live}}</ref><ref name="Steckel Conrad Dekarske Dolan 2023">{{cite journal | last1=Steckel | first1=Amanda | last2=Conrad | first2=Jack William | last3=Dekarske | first3=Jason | last4=Dolan | first4=Sydney | last5=Downey | first5=Brynna Grace | last6=Felton | first6=Ryan | last7=Hanson | first7=Lavender Elle | last8=Giesche | first8=Alena | last9=Horvath | first9=Tyler | last10=Maxwell | first10=Rachel | last11=Shumway | first11=Andrew O | last12=Siddique | first12=Anamika | last13=Strom | first13=Caleb | last14=Teece | first14=Bronwyn | last15=Todd | first15=Jessica | last16=Trinh | first16=Kevin T | last17=Velez | first17=Michael A | last18=Walter | first18=Callum Andrew | last19=Lowes | first19=Leslie L | last20=Hudson | first20=Troy | last21=Scully | first21=Jennifer E. C. | title=The Science Case for Nautilus: A Multi-Flyby Mission Concept to Triton | journal=AGU - Agu23 | publisher=AGU | date=12 December 2023 | url=https://agu.confex.com/agu/fm23/meetingapp.cgi/Paper/1425806 | access-date=11 January 2024 | archive-date=11 January 2024 | archive-url=https://web.archive.org/web/20240111010546/https://agu.confex.com/agu/fm23/meetingapp.cgi/Paper/1425806 | url-status=live }}</ref> Neptune is a potential target for China's ''Tianwen-5'', which could arrive in 2058.<ref>{{cite web |title=China's plans for outer Solar System exploration |url=https://www.planetary.org/articles/chinas-plans-for-outer-solar-system-exploration |date=21 December 2023 |access-date=18 April 2024 |work=The Planetary Society}}</ref>

{{Clear}}

== See also ==
{{div col|colwidth=20em}}
* [[Outline of Neptune]]
* [[Hot Neptune]]
* [[Planets in astrology#Neptune|Neptune in astrology]]
* [[Neptune in fiction]]
* [[Neptunium]]
* ''[[Neptune, the Mystic]]'' – one of the seven movements in [[Gustav Holst]]'s ''Planets'' suite
* [[Timeline of the far future]]
* [[List of gravitationally rounded objects of the Solar System#Planets|Stats of planets in the Solar System]]
{{div col end}}

== Notes ==
{{notelist}}

== References ==
{{reflist|refs=

<ref name="Encrenaz 2003">
{{cite journal
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<ref name="Herbert & Sandel 1999">
{{cite journal
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<ref name="Lunine 1993">
{{cite journal
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<ref name="Podolak Weizman et al. 1995">
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<ref name="Seidelmann Archinal A'hearn et al. 2007">
{{cite journal
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<ref name="Unsöld & Baschek 2001">
{{cite book
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<ref Name="Mallama_and_Hilton">
{{cite journal
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 |doi=10.1016/j.ascom.2018.08.002  |arxiv=1808.01973
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}}
</ref>

<ref Name="Schmude_et_al">
{{cite report
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<ref name="Mallama_et_al">
{{cite journal
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<ref name="Pearl_et_al_Neptune">
{{cite journal
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<ref name="Sheppardmoons2024">{{cite web
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|accessdate   = 23 February 2024
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|archive-url  = https://web.archive.org/web/20240223160326/https://sites.google.com/carnegiescience.edu/sheppard/home/newuranusneptunemoons
 |url-status   = live
}}</ref>

}} <!-- end "refs=" -->

== Bibliography ==
* {{Cite book
 |last=Burgess |first=Eric
 |url=https://archive.org/details/farencounternept00burg
 |title=Far Encounter: The Neptune System
 |publisher=[[Columbia University Press]]
 |year=1991
 |isbn=978-0-231-07412-4
 |location=New York
 |ref=Burgess
}}
* {{Cite book
 |last=Moore |first=Patrick |author-link=Patrick Moore
 |title=The Data Book of Astronomy
 |publisher=[[Taylor & Francis Group]]
 |year=2000
 |isbn=978-0-7503-0620-1
 |location=Baton Rouge |ref=Moore
}}

== Further reading ==
* {{Cite book
 |last1=Miner |first1=Ellis D.
 |last2=Wessen |first2=Randii R.
 |title=Neptune: The Planet, Rings, and Satellites
 |year=2002
 |publisher=[[Springer Science+Business Media|Springer]]
 |isbn=978-1-85233-216-7
 |series=Springer-Praxis books in astronomy and space sciences
 |location=London; New York
}}
* {{Cite book
 |last=Standage |first=Tom
 |url=https://archive.org/details/neptunefilestory00stan
 |title=The Neptune File: A Story of Astronomical Rivalry and the Pioneers of Planet Hunting
 |date=2000
 |publisher=[[Walker Publishing Company|Walker]]
 |isbn=978-0-8027-1363-6
 |location=New York
}}

== External links ==
{{commons}}
* [http://nssdc.gsfc.nasa.gov/planetary/factsheet/neptunefact.html NASA's Neptune fact sheet]
* [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 No. 63, includes full transcript.
* [https://web.archive.org/web/20021115171710/http://solarsystem.nasa.gov/planets/profile.cfm?Object=Neptune Neptune Profile] (archived 15 November 2002) at [http://solarsystem.nasa.gov NASA's Solar System Exploration site]
* [https://gravitysimulator.org/solar-system/the-neptunian-system Interactive 3D gravity simulation of Neptune and its inner moons] {{Webarchive|url=https://web.archive.org/web/20200922021820/https://gravitysimulator.org/solar-system/the-neptunian-system/ |date=22 September 2020 }}

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