# Neptune <!-- SOLSIM:BEGIN g31 — Solar System explorer state (hand-built on wt-core, specs/solar/); do not hand-edit inside --> **Microsim — three.js (Wikitube framework):** *Neptune in the Solar System explorer* <div class="wt-sim" data-src="https://wikitube-3d-microsims.netlify.app/solar/Solar_System.html?obj=Neptune&embed=1" data-title="Neptune in the Solar System explorer"></div> *The Solar System explorer locked on this article's state (`?obj=Neptune`); every object and population of the [[PORTAL_Solar_System|Solar System portal]] has its own state in the same scene.* <!-- SOLSIM:END --> *Try: set the speed to 100 years/s and watch Neptune take about a second and a half per lap along the outer edge of the planetary region; drag the year slider from 1846 to 2011 to see it complete its first full orbit since its discovery; then switch the scale to true to open Neptune's local frame and follow Triton circling the planet backwards.* **Neptune** is the eighth and outermost known planet of the [[Sun]], an [[Ice_giant|ice giant]] of 17.15 Earth masses orbiting at a mean distance of 30.1 [[Astronomical_unit|AU]] once every 164.8 years.[^nasa-fs-n] It is a little smaller than [[Uranus]] but more massive, and at 1,638 kg/m³ it is the densest of the four giant planets.[^nasa-fs-n] Beneath an atmosphere of [[Hydrogen|hydrogen]] and [[Helium|helium]] tinted blue by methane lies a hot fluid mantle of [[Water|water]], ammonia and methane around a rocky core.[^hubbard1997][^podolak1995] Neptune is the planet found by calculation. Irregularities in the motion of Uranus led Urbain Le Verrier and John Couch Adams to predict an outer planet, and Johann Gottfried Galle saw it on 23 September 1846 within a degree of Le Verrier's position.[^galle1846][^airy1846] Despite receiving only about 40% as much sunlight as Uranus, it has the most active weather of the ice giants, including dark vortices and winds of up to about 600 m/s, powered by internal heat.[^lunine1993][^suomi1991] Its largest moon, [[Triton_(moon)|Triton]], orbits backwards and was probably captured from the [[Kuiper_belt|Kuiper belt]].[^agnor2006] Voyager 2, which flew past on 25 August 1989, is the only spacecraft to have visited.[^stone1989] The explorer at the top of this page is locked on Neptune. It shows the planet at 30 AU, the outer edge of the planetary region, with its orbit drawn from JPL elements, and its moon Triton circling in the retrograde direction.[^jpl-t1] Where the scene draws the belt of icy bodies just beyond, which Neptune's [[Gravity|gravity]] has sculpted, its points are ILLUSTRATIVE samples, not a catalogue. ## History The story of Neptune's discovery is one of a planet inferred before it was seen, followed by a dispute over who deserved the credit. ### Discovery Galileo drew Neptune without knowing it. His sketches of Jupiter's moons on 28 December 1612 and 27 January 1613 include a "star" at the position Neptune then occupied; in December 1612 the planet had just begun its retrograde loop and was almost stationary, too slow for his small telescope to reveal.[^hirschfeld2001][^littmann2004] The planet was found through Uranus. Alexis Bouvard published tables of Uranus's motion in 1821, and when later observations departed from them he suspected the pull of an unknown body.[^bouvard1821][^airy1846] John Couch Adams worked on the problem from 1843 and produced several estimates of the unseen planet's position.[^adams1846] Urbain Le Verrier, independently, published a prediction in 1846; its closeness to Adams's figures prompted George Airy to have James Challis search from Cambridge, but Challis, who had in fact recorded the planet twice in August 1845, did not recognise it.[^challis1846][^airy1846] Le Verrier then wrote to Galle at the Berlin Observatory. On the night the letter arrived, 23 September 1846, Galle, helped by the student Heinrich d'Arrest and a new star chart, found the planet about 1° from the predicted place.[^galle1846][^levenson2015] French and British partisans argued over priority; joint credit to Le Verrier and Adams became the consensus, though historians re-examined Adams's claim after the "Neptune papers" returned to the Royal Observatory in 1998.[^sheehan2004][^kollerstrom2001] ### Naming Galle proposed Janus and Challis Oceanus.[^moore2000] Le Verrier first put forward Neptune, then sought to name the planet after himself, a proposal that met strong resistance outside France.[^littmann2004][^baum2003] Friedrich Georg Wilhelm von Struve backed "Neptune" before the Saint Petersburg Academy of Sciences on 29 December 1846, and the name of the Roman god of the sea, the counterpart of the Greek Poseidon, was soon accepted everywhere.[^hind1847][^usgs-names] In Chinese, Japanese, Korean and Vietnamese the planet is the "sea king star".[^nineplanets-ling] ### Status From 1846 until the discovery of [[Pluto]] in 1930, Neptune was the most distant known planet. Between 1979 and 1999 Pluto's eccentric orbit carried it inside Neptune's, making Neptune temporarily the farthest planet from the Sun.[^long2008] As Pluto's estimated mass fell and the Kuiper belt was discovered, its status was debated, and in 2006 the IAU's first formal [[IAU_definition_of_planet|definition of a planet]] made Pluto a [[Dwarf_planet|dwarf planet]], leaving Neptune once more the outermost planet.[^weissman1995][^iau-b5b6] ## Formation The ice giants are hard to form where they now orbit. The disc material at 20–30 AU was so thin, and orbital periods there so long, that growth by core accretion would have been too slow to build 15–17 Earth-mass planets before the gas was gone.[^boss2002] One proposal is that they formed by gravitational instability in the disc and later had their gas envelopes stripped by radiation from a nearby massive star.[^boss2002] The more favoured alternative is that Uranus and Neptune grew closer to the Sun, where material was denser, and migrated outward after the gas disc cleared.[^thommes2002] Migration also explains the populations of small bodies beyond Neptune, since an outward-moving planet sweeps [[Resonance|resonances]] across the disc and traps objects in them.[^hansen2005] The widely used [[Nice_model|Nice model]] develops this idea: interactions between the giant planets and a disc of planetesimals move Neptune outward, reshaping the [[Kuiper_belt|Kuiper belt]] as it goes.[^crida2009][^desch2007][^smith2009] ## Physical characteristics Neptune's mass is 1.024 × 10²⁶ kg, 17.15 times Earth's and about 1/19 of [[Jupiter]]'s. Its equatorial radius at the 1 bar level is 24,764 km, nearly four times [[Earth]]'s, and its gravity there, 11.27 m/s², is 1.15 times Earth's.[^nasa-fs-n][^seidelmann2007] It is classed with Uranus as an ice giant, a subclass of giant planet that is smaller than Jupiter and [[Saturn]] and richer in volatiles.[^boss2002] The name has become a unit of comparison: exoplanets of similar mass are often called "Neptunes".[^eso2006] ### Internal structure The interior resembles Uranus's. The hydrogen–helium atmosphere makes up about 5–10% of the mass and extends perhaps 10–20% of the way to the centre, reaching pressures of about 10 GPa at its base.[^hubbard1997] Beneath it lies a mantle of 10–15 Earth masses, rich in water, ammonia and methane: "icy" in planetary usage, but in fact a hot, dense, electrically conducting fluid, sometimes called a water–ammonia ocean.[^hamilton2001][^atreya2006] Laboratory results suggest that its water may be ionic, and deeper down superionic, with [[Oxygen|oxygen]] in a crystal lattice through which hydrogen ions move.[^shiga2010] Near 7,000 km depth methane may decompose, with the [[Carbon|carbon]] forming diamonds that sink through the mantle.[^kerr1999][^kraus2017] The core, probably iron, nickel and silicates, has a model mass of about 1.2 Earth masses; the central pressure is about 700 GPa and the temperature perhaps 5,400 K.[^podolak1995][^hubbard1997][^nettelmann] ### Internal heating Neptune emits about 2.61 times as much energy as it absorbs from the Sun, compared with only about 1.1 times for Uranus.[^pearl1991][^bagenal2004] Solar heating is feeble at 30 AU: the fact-sheet irradiance of 1.51 W/m² is about 41% of the 3.69 W/m² at Uranus (derived), yet Neptune's internal power drives the fastest winds of any planet.[^nasa-fs-n][^nasa-fs-u] At the 1 bar level the temperature is 72 K, falling to 51.8 K at the tropopause.[^lindal1992] Depending on the interior's thermal properties, heat left over from formation may be enough to explain Neptune's heat flow; the harder problem is why Uranus, so similar in other respects, has so little.[^depater2001] ## Rotation and magnetosphere Neptune's rotation and its magnetic field were both first measured properly by Voyager 2, which timed the planet's radio emission and mapped a field as oddly oriented as that of Uranus.[^stone1989][^ness1989] ### Rotation The axial tilt is 28.32°, not far from Earth's 23.44°, so Neptune has seasons of the familiar kind; each lasts about forty years.[^nasa-fs-n][^villard2003] The interior, tracked by the magnetic field, turns once every 16.11 hours.[^nasa-fs-n] The atmosphere rotates differentially, more strongly than on any other planet: the broad equatorial zone takes about 18 hours to go round, and the polar regions about 12 hours, producing strong wind shear with latitude.[^hubbard1991][^max2003] ### Magnetosphere The magnetic axis is tilted about 47° from the rotation axis, and the field's centre is offset by at least 0.55 planetary radii, about 13,500 km.[^ness1989] Before 1989 Uranus's skewed field had been blamed on its sideways spin; finding the same pattern on Neptune, whose tilt is moderate, pointed instead to how the field is made. Models generate such fields by [[Convection|convection]] in a thin shell of conducting fluid, probably water, ammonia and methane, rather than in a deep core.[^stanley2004] The dipole field at the magnetic equator is about 14 μT (0.14 gauss), and a strong quadrupole component may rival the dipole.[^connerney1991][^ness1989] Facing the [[Solar_wind|solar wind]], the bow shock stands at 34.9 Neptune radii and the magnetopause at 23–26.5 radii; the tail extends at least 72 radii.[^ness1989] Voyager 2 detected faint aurorae in ultraviolet and radio light.[^lamy2020] In 2023 the James Webb Space Telescope detected the trihydrogen cation (H₃⁺) at Neptune for the first time; its emission follows patterns expected of infrared aurorae, which on Neptune occur at mid-latitudes because of the tilted field.[^melin2025] ## Colour Neptune's colour comes mainly from methane, which absorbs red light above about 600 nm and leaves the reflected light bluish.[^crisp1995] Popular images long showed it a deep azure, much bluer than Uranus. That impression came largely from Voyager 2 composites whose contrast had been stretched to bring out clouds and bands.[^kuthunur2024] A 2024 reanalysis that put images of both planets on the same calibrated footing found that Neptune is only slightly bluer than Uranus, a pale greenish blue.[^irwin2024][^oxford2024] The small remaining difference is attributed to a thicker haze layer on Uranus, which whitens its colour.[^noirlab2022] ## Atmosphere At high altitudes the atmosphere is about 80% hydrogen and 19% helium by volume, with about 1.5% methane.[^nasa-fs-n][^hubbard1997] The troposphere, where temperature falls with height, is capped by the tropopause at 0.1 bar; above it the stratosphere warms upward and gives way to the thermosphere at pressures below 10⁻⁵–10⁻⁴ bar.[^lunine1993] Clouds are expected in layers set by condensation temperatures: methane above the 1 bar level, ammonia and hydrogen sulfide at 1–5 bar, and water ice near 50 bar, where the temperature reaches 273 K.[^elkins2006] Bands of high cloud 50–150 km wide float 50–110 km above the main deck and cast shadows on it.[^max2003] A study of three decades of images found that their abundance tracks the Sun's 11-year activity cycle rather than Neptune's seasons; by 2023 they had almost vanished.[^chavez2023][^nasa2023] The stratosphere is hazy with ethane and acetylene made from methane by ultraviolet light, and contains traces of carbon monoxide and hydrogen cyanide.[^lunine1993] Voyager 2 found the thermosphere at an anomalously high 750 K, too hot to be heated by sunlight at this distance; interaction with magnetospheric ions and gravity waves from below are candidate heat sources.[^broadfoot1989] JWST's H₃⁺ measurements in 2023 gave only about 450 K, implying a cooling of roughly 10 K per year that is not yet explained.[^melin2025] ### Climate Neptune's winds are the fastest measured on any planet, approaching 600 m/s.[^suomi1991] At cloud level they blow westward, against the rotation, at up to about 400 m/s near the equator, and eastward at high latitudes.[^elkins2006][^lunine1993] Hydrocarbons are 10–100 times more abundant at the equator than at the poles, a sign that gas rises at the equator and sinks near the poles.[^lunine1993] In 2007 the upper troposphere at the south pole, then in a decades-long summer, was found to be about 10 K warmer than the average of 73 K, warm enough for methane to leak into the stratosphere.[^orton2007] Voyager 2 found the Great Dark Spot, an anticyclone of 13,000 × 6,600 km comparable to Jupiter's Great Red Spot.[^pia02245] When the Hubble Space Telescope looked in November 1994 it was gone, and a similar spot had appeared in the northern hemisphere.[^hammel1995] Such dark spots lie deeper than the bright clouds, appear as holes in the upper deck and last months; bright methane "companion clouds" often accompany them.[^gibbard2003][^lebeau1998] New dark vortices were seen by Hubble in 2018, and in 2023 a dark spot was detected from the ground for the first time.[^wong2018][^eso2023] Voyager 2's instruments also recorded whistlers attributed to lightning.[^gurnett1990] ## Orbital motion and observation Neptune's [[Orbit|orbit]] has a mean radius of about 4.5 billion km (30.1 AU) and an eccentricity below 0.01, the second most circular planetary orbit after that of [[Venus]]; its distance from the Sun varies only between about 29.8 and 30.3 AU.[^nasa-fs][^jpl-t1] The inclination to the ecliptic is 1.77°.[^nasa-fs-n] The sidereal period of 164.79 years follows [[Kepler's_laws_of_planetary_motion|Kepler's third law]] for that distance: 30.07^1.5 ≈ 164.9 years (derived).[^nasa-fs-n][^jpl-t1] The planet returned to the ecliptic longitude of its discovery for the first time in July 2011.[^gaherty2011] ### Observation At magnitude 7.67–7.89 Neptune is too faint for the naked eye; a telescope or strong binoculars show it as a small bluish disc.[^mallama2018][^moore2000] Its apparent diameter is only 2.2–2.4 arcseconds, the smallest of the planets.[^nasa-fs-n] It brightened by about 10% between 1980 and 2000, largely as its seasons changed.[^schmude2016] Detailed study from Earth became possible only with the Hubble Space Telescope and ground-based adaptive optics, first applied to Neptune in 1997, and with them the tracking of cloud features in the infrared.[^roddier1997][^gibbard2002] Each year the planet traces a retrograde loop against the stars around opposition, a consequence of Earth overtaking it every 367.5 days (the synodic period).[^nasa-fs-n] ## Satellite system and resonance Neptune's gravitational reach extends well beyond its moons and rings: by [[Resonance|resonance]] it organises much of the [[Kuiper_belt|Kuiper belt]]. ### Planetary rings The rings are faint and dark, probably ice grains coated with silicate or carbon-rich material that makes them reddish.[^cruikshank1996] The three main rings are named after the discoverers: Galle at 42,000 km from the centre, Le Verrier at 53,000 km, and the narrow Adams ring at 63,000 km; a faint extension of Le Verrier, Lassell, reaches out to the Arago ring at 57,000 km.[^usgs-rings] A 1984 stellar occultation in which a star dimmed on one side of the planet but not the other suggested incomplete rings.[^nicholson1990] Voyager 2 showed that the Adams ring contains five bright arcs: Courage, Liberté, Egalité 1, Egalité 2 and Fraternité.[^cox2001] Arcs should spread into a uniform ring, and the moon Galatea, just inside, is thought to confine them.[^salo1998] Keck images in 2002–2003 showed the arcs decaying, with Liberté possibly gone within a century.[^newscientist2005] ### Moons Neptune has 16 known moons.[^sheppard2024] Triton, discovered by William Lassell in October 1846, weeks after the planet, holds more than 99.5% of the mass orbiting Neptune: 2.14 × 10²² kg against under 10²⁰ kg for all the other moons together (derived).[^usgs-names][^nasa-fs-nsat] Its orbit is retrograde, the mark of a captured body, possibly one member of a binary whose partner was ejected.[^agnor2006] Triton is also slowly spiralling inward by [[Tide|tidal]] interaction.[^wang2025] Nereid, the second moon found, has an orbital eccentricity of 0.7512, so its farthest point from Neptune is seven times its nearest (1.7512/0.2488 ≈ 7.0, derived).[^nasa-fs-nsat] Voyager 2 found six moons in 1989, including Proteus, about as large as a body of its density can be without being pulled round.[^stone1989][^brown-dwarfs] Five irregular moons were announced in 2004, and Hippocamp, found in Hubble images in 2013, is one of the smallest.[^holman2004][^grush2019] The moons are named after lesser sea deities.[^usgs-names] ### Orbital resonances Neptune's gravity has cleared unstable zones in the Kuiper belt, such as the range from 40 to 42 AU, while bodies whose periods form a simple ratio with Neptune's survive.[^petit1999] The most populated such resonance, holding more than 200 known objects, is 2:3: the [[Plutino|plutinos]], including Pluto, orbit the Sun twice while Neptune orbits three times.[^mpc-tno][^jewitt-plutinos] Pluto crosses Neptune's orbit, but the resonance keeps the two apart.[^varadi1999] The 3:4, 3:5, 4:7 and 2:5 resonances hold fewer [[Resonant_trans-Neptunian_object|resonant objects]].[^davies2001] Neptune trojans, in a 1:1 resonance, share its orbit near the L4 and L5 Lagrangian points; the first at L5 was 2008 LC18.[^chiang2003][^sheppard2010] ## Exploration Voyager 2 made its closest approach on 25 August 1989, passing about 4,400 km above the cloud tops and then flying close by Triton, since Neptune was the last planet it could reach.[^burgess1991] Signals took 246 minutes to reach Earth, so the encounter ran on pre-loaded commands.[^burgess1991] The flyby found six moons and confirmed complete rings, fixed the rotation period from radio emissions, mapped the tilted, offset magnetic field and revealed the active weather.[^stone1989] It also measured Neptune's mass to be 0.5% less than earlier estimates, removing the discrepancy that had been cited as evidence for a "Planet X" perturbing Uranus and Neptune.[^gebhardt2011] No other spacecraft has followed. The 2023–2032 US decadal survey chose a Uranus orbiter and probe over a Neptune mission for logistical reasons.[^nas2023] Concepts for Neptune include the Neptune Odyssey orbiter and probe, and the Triton-focused Trident, Triton Ocean Worlds Surveyor and Nautilus.[^rymer2020][^trident2019][^steckel2023] China has studied an interstellar probe, IHP-2, that would fly past Neptune in 2038, and Neptune is a candidate target for Tianwen-5.[^jones2021][^jones2023] ## See also - [[Uranus]] · [[Ice_giant]] · [[Gas_giant]] - [[Triton_(moon)]] · [[Kuiper_belt]] · [[Plutino]] - [[Nice_model]] · [[Planet_Nine]] - [[Neptunium]] - [[PORTAL_Solar_System|Solar System portal]] ## Notes Derived values: the ratio of solar irradiance at Neptune to that at Uranus, 1.508/3.69 ≈ 0.41, uses the NASA fact-sheet values; the Kepler's-third-law period uses JPL's semi-major axis of 30.07 AU. The time for one lap at 100 years/s in the explorer is about 1.65 s. Triton's share of the satellite mass and Nereid's apoapsis-to-periapsis ratio use the NASA satellite fact sheet masses and eccentricity. ## References [^nasa-fs-n]: Williams, D. R. "Neptune Fact Sheet". NASA NSSDCA (last updated 3 October 2024). https://nssdc.gsfc.nasa.gov/planetary/factsheet/neptunefact.html (fetched 2026-09-18). [^nasa-fs-u]: Williams, D. R. "Uranus Fact Sheet". NASA NSSDCA (last updated 2 October 2024). https://nssdc.gsfc.nasa.gov/planetary/factsheet/uranusfact.html (fetched 2026-09-18). [^nasa-fs-nsat]: Williams, D. R. "Neptunian Satellite Fact Sheet". NASA NSSDCA (last updated 11 March 2024). https://nssdc.gsfc.nasa.gov/planetary/factsheet/neptuniansatfact.html (fetched 2026-09-18). [^nasa-fs]: NASA NSSDCA. "Planetary Fact Sheet". https://nssdc.gsfc.nasa.gov/planetary/factsheet/ (fetched 2026-09-18). [^moore2000]: Moore, P. (2000). *The Data Book of Astronomy*. Institute of Physics Publishing, pp. 206–207. [^cox2001]: Cox, A. N. (ed.) 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Dover. ## External links - NASA Science: Neptune. https://science.nasa.gov/neptune/ - NASA NSSDCA Neptune fact sheet. https://nssdc.gsfc.nasa.gov/planetary/factsheet/neptunefact.html - JPL Photojournal: Neptune. https://photojournal.jpl.nasa.gov/target/neptune - ESA/Hubble: Neptune images. https://esahubble.org/images/archive/category/solarsystem/ ## Wikipedia : Wikitube **Strict pair:** [Wikipedia](https://en.wikipedia.org/wiki/Neptune) : [Wikitube](https://en.wikitube.io/wiki/Neptune) · pinned revision [1374076933](https://en.wikipedia.org/w/index.php?oldid=1374076933) · 2026-09-18 ## Previous hub tags Hubs: `Life_Physics`. Portals: [[PORTAL_Solar_System]]. --- *Solar System portal child articles, wave 1 · 2026-09-18 · drafted · row SOL-018 · explorer state `?obj=Neptune`.* <!-- hub_tags: Life_Physics · PORTAL_Solar_System -->