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Neptune is the eighth, or—due to Pluto's eccentric orbit—ninth planet farthest from the Sun, and the outermost gas giant in our solar system. It is fourth largest by diameter and third largest by mass, but it is the smallest of the gas giants. It is slightly smaller than Uranus, but more massive, this is due to a higher density and abundance of heavier elements within Neptune.


Discovered by Urbain Le Verrier
John Couch Adams
Johann Galle
Discovered on September 23, 1846
Orbital characteristics (Epoch J2000)
Semi-major axis 4,498,252,900 km
30.068 963 48 AU
Orbital circumference 28.263 Tm
188.925 AU
Eccentricity 0.008 585 87
Perihelion 4,459,631,496 km
29.810 795 27 AU
Aphelion 4,536,874,325 km
30.327 131 69 AU
Orbital period 60,223.3528 d
(164.88 a)
Synodic period 367.49 d
Avg. Orbital Speed 5.432 km/s
Max. Orbital Speed 5.479 km/s
Min. Orbital Speed 5.385 km/s
Inclination 1.769 17°
(6.43° to Sun's equator)
Longitude of the
ascending node
131.721 69°
Argument of the
273.249 66°
Number of satellites 13
Physical characteristics
Equatorial diameter 49,528 km [1]
(3.883 Earths)
Polar diameter 48,681 km
(3.829 Earths)
Oblateness 0.0171
Surface area 7.619×109 km2
Volume 6.254×1013 km3
(57.74 Earths)
Mass 1.0243×1026 kg
(17.147 Earths)
Mean density 1.638 g/cm3
Equatorial gravity
(At 1 bar)
11.15 m/s2
(1.14 g)
Escape velocity 23.5 km/s
Rotation period 16.11 h (16 h 6 min 36 s) 1
Rotation velocity 2.68 km/s = 9660 km/h

(at the equator)

Axial tilt 28.32°
Right ascension
of North pole
299.33° (19 h 57 min 20 s)
Declination 42.95°
Albedo 0.41
Surface temp.
min mean max
50 K 53 K N/A K
Adjective Neptunian
Atmospheric characteristics
Surface pressure ≫100 MPa
Hydrogen - H2 80% ±3.2%
Helium - He 19% ±3.2%
Methane - CH4 1.5%±0.5%
Hydrogen Deuteride - HD 192 ppm
Ethane - C2H6 1.5 ppm

Faint dark rings have been detected around the blue planet, but are less substantial than those of Saturn. When these rings were discovered, it was thought that they might not be complete, but this was disproved by Voyager 2. Neptune also has 2,000 km/h winds of hydrogen, helium, and methane that gives it its blue appearance. In its southern hemisphere, it has a Great Dark Spot comparable to the Great Red Spot on Jupiter. It possesses eight confirmed moons and five awaiting confirmation, the largest of which, Triton, is known for its subzero temperatures and methane production.

Neptune is named after the Roman god of the sea. Its symbol is a stylised representation of the god's trident (Unicode: ♆). Discovered on September 23, 1846, Neptune has been visited by only one spacecraft, Voyager 2, which flew by the planet on August 25, 1989.

Physical characteristics

Orbiting so far from the sun, Neptune receives very little heat and in fact the uppermost regions of the atmosphere is −218 °C (55 K). There is no solid surface due to the fact that Neptune is a gas giant. Atmospheric temperatures steadily rise as you go deeper inside Neptune due to an internal source of heat. It is thought that this may be leftover heat generated by infalling matter during the planet's birth, now slowly radiating away into space. Neptune's atmosphere has the highest wind speeds in the solar system, up to 2000 km/h, thought to be powered by this flow of internal heat.

The internal structure resembles that of Uranus. There is likely to be a core consisting of (molten) rock and metal, surrounded by a mixture of rock, water, ammonia, and methane. The atmosphere, extending perhaps 10 to 20 percent of the way towards the centre, is mostly hydrogen and helium at high altitudes, but has increasing concentrations of methane, ammonia, and water as it approaches and finally blends into the liquid interior. The pressure at the centre of Neptune is millions of times more than that on the surface of Earth. Comparing its rotational speed to its degree of oblateness indicates that it has its mass less concentrated towards the centre than does Uranus.

Neptune also resembles Uranus in its magnetosphere, with a magnetic field strongly tilted relative to its rotational axis at 47° and offset at least 0.55 radii (about 13,500 kilometres) from the planet's physical centre. Comparing the magnetic fields of the two planets, scientists think the extreme orientation may be characteristic of flows in the interior of the planet and not the result of Uranus' sideways orientation.

One difference between Neptune and Uranus is the level of meteorological activity. Uranus is visually quite bland, while Neptune's high winds come with notable weather phenomena. The Great Dark Spot, an Earth-sized dark marking resembling the Great Red Spot of Jupiter, disappeared in 1994 but another reappeared later. Unique among the gas giants is the presence of high clouds casting shadows on the opaque cloud deck below.

Discovery of Neptune

Galileo's astronomical drawings show that he had first observed Neptune on December 27, 1612, and again on January 27, 1613; on both occasions Galileo mistook Neptune for a fixed star when it appeared very close (in conjunction) to Jupiter in the night sky. Believing it to be a fixed star, he cannot be credited with its discovery. At the time Galileo first observed Neptune on December 28, 1612, it was stationary in the sky because it had just turned retrograde that very day; because it was stationary in the sky and only beginning the planet's yearly retrograde cycle, its motion was far too slight to be noticed with the naked eye, even with the help that a telescope provided. Had Neptune been moving at its regular/average speed when Galileo first observed it in 1612 and 1613, he would have most likely realised that it was a planet and not a fixed star because of Neptune's relatively rapid normal motion along the ecliptic compared to the extremely slow motion of any random fixed star found in the night sky.

In 1821, Alexis Bouvard published astronomical tables of the orbit of Uranus. Subsequent observations revealed substantial deviations from the tables, leading Bouvard to hypothesise some perturbing body. In 1843, John Couch Adams calculated the orbit of an eighth planet that would account for Uranus' motion. He sent his calculations to Sir George Airy, who asked Adams for a clarification; Adams began to draft a reply but never sent it.

In 1846, Urbain Le Verrier, independently of Adams, produced his own calculations but also experienced difficulties in encouraging any enthusiasm in his compatriots. However, in the same year, John Herschel started to champion the mathematical approach and persuaded James Challis to search for the planet.

After much procrastination, Challis began his reluctant search in July 1846. However, in the mean time, Le Verrier had convinced Johann Gottfried Galle to search for the planet. Though still a student at the Berlin Observatory, Heinrich d'Arrest suggested that a recently drawn chart of the sky, in the region of Le Verrier's predicted location, could be compared with the current sky to seek the displacement characteristic of a planet, as opposed to a stationary star. Neptune was discovered that very night, September 23, 1846, within 1° of where Le Verrier had predicted it to be, and about 10° from Adams' prediction. Challis later realised that he had observed the planet twice in August, failing to identify it owing to his casual approach to the work.

In the aftermath of the discovery, there was much nationalistic rivalry between the French and the British over who had priority and who should get credit for the discovery. Eventually an international consensus emerged that both Le Verrier and Adams jointly deserved credit. However, the issue is now being re-evaluated by historians with the rediscovery in 1998 of the "Neptune papers" (historical documents from the Royal Greenwich Observatory), which had apparently been misappropriated by astronomer Olin Eggen for nearly three decades and were not rediscovered (in his possession) until immediately after his death. After reviewing the documents, some historians now suggest that Adams did not in fact deserve equal credit with Le Verrier. [2]

Naming of Neptune

Shortly after its discovery, Neptune was referred to simply as "the planet exterior to Uranus" or as "Le Verrier's planet". The first suggestion for a name came from Galle. He proposed the name Janus. In England, Challis put forth the name Oceanus, particularly appropriate for a seafaring people. In France, Arago suggested that the new planet be called Leverrier, a suggestion which was met with stiff resistance outside France. French almanacs promptly reintroduced the name Herschel for Uranus and Leverrier for the new planet.

Meanwhile, on separate and independent occasions, Adams suggested altering the name Georgian to Uranus, while Leverrier (through the Board of Longitude) suggested Neptune for the new planet. Struve came out in favour of that name on December 29, 1846, to the Saint Petersburg Academy of Sciences [3]. Soon Neptune became internationally accepted nomenclature. In Roman mythology Neptune was the god of the sea, identified with the Greek Poseidon. The demand for a mythological name seemed to be in keeping with the nomenclature of the other planets, all of which, except Uranus, were named in antiquity.

Visibility from Earth

Neptune is never visible with the naked eye. With the use of a telescope it appears as a blue-green disk, similar in appearance to Uranus; the blue-green colour comes from the methane in its atmosphere.

The brightness of Neptune is between magnitudes +7.7 and +8.0, so a telescope or binoculars are required to observe it. It can be also photographed as a faint star with a normal camera and high-sensitivity film.

With an orbital period of 165 years, Neptune will soon return to the approximate position where Galle discovered it, on three different dates. These are April 11, 2009, when it will be in direct motion; July 17, 2009, when it will be in retrograde motion; and finally for the last time for the next 165 years, on February 7, 2010.


Stationary, retrograde Opposition Distance
to Earth (AU)
Brightness (mag)
Diameter Stationary, prograde Conjunction to Sun
May 17, 2004 August 6, 2004 29.05513 7.8 2.31" October 24, 2004 February 2, 2004
May 20, 2005 August 8, 2005 29.04701 7.8 2.31" October 26, 2005 February 3, 2005
May 22, 2006 August 11, 2006 29.03948 7.8 2.31" October 29, 2006 February 6, 2006
May 25, 2007 August 13, 2007 29.03193 7.8 2.31" October 31, 2007 February 8, 2007
May 26, 2008 August 15, 2008 29.02412 7.8 2.31" November 2, 2008 February 11, 2008
May 29, 2009 August 17, 2009 29.01536 7.8 2.31" November 4, 2009 February 12, 2009
June 1, 2010 August 20, 2010 29.00558 7.8 2.31" November 7, 2010 February 14, 2010
June 3, 2011 August 22, 2011 28.99475 7.8 2.31" November 9, 2011 February 17, 2011
June 5, 2012 August 24, 2012 28.98352 7.8 2.31" November 11, 2012 February 19, 2012
June 7, 2013 August 27, 2013 28.97248 7.8 2.31" November 13, 2013 February 21, 2013
June 10, 2014 August 29, 2014 28.96213 7.8 2.31" November 16, 2014 February 23, 2014
June 12, 2015 September 1, 2015 28.95303 7.8 2.31" November 18, 2015 February 26, 2015
June 14, 2016 September 2, 2016 28.94509 7.8 2.31" November 20, 2016 February 28, 2016
June 16, 2017 September 5, 2017 28.93843 7.8 2.32" November 22, 2017 March 2, 2017
June 19, 2018 September 7, 2018 28.93252 7.8 2.32" November 25, 2018 March 4, 2018
June 22, 2019 September 10, 2019 28.92724 7.8 2.32" November 27, 2019 March 7, 2019
June 23, 2020 September 11, 2020 28.92194 7.8 2.32" November 29, 2020 March 8, 2020

The rings of Neptune

Main article: Rings of Neptune

Neptune's rings

Neptune has a faint planetary ring system of unknown composition. The rings have a peculiar "clumpy" structure, the cause of which is not currently understood but which may be due to the gravitational interaction with small moons in orbit near them.

Evidence that the rings are incomplete first arose in the mid-1980s, when stellar occultation experiments were found to occasionally show an extra "blink" just before or after the planet occulted the star. Images by Voyager 2 in 1989 settled the issue, when the ring system was found to contain several faint rings, the outermost of which, Adams, contains three prominent arcs now named Liberté, Egalité, and Fraternité (Liberty, Equality, and Fraternity). The existence of arcs is very difficult to understand because the laws of motion would predict that arcs spread out into a uniform ring over very short timescales. The gravitational effects of Galatea, a moon just inward from the ring, are now believed to confine the arcs.

Several other rings were detected by the Voyager cameras. In addition to the narrow Adams Ring 63,000 km from the centre of Neptune, the Leverrier Ring is at 53,000 km and the broader, fainter Galle Ring is at 42,000 km. A faint outward extension to the Leverrier Ring has been named Lassell; it is bounded at its outer edge by the Arago Ring at 57,000 km.[4]

New Earth-based observations announced in 2005 appeared to show that Neptune's rings are much more unstable than previously thought. In particular, it seems that the Liberté ring might disappear in as little as one century. The new observations appear to throw our understanding of Neptune's rings into considerable confusion. [5]

Name of ring Radius (km) Width (km) Notes
1989 N3R ('Galle') 41,900 15 Named after Johann Galle
1989 N2R ('Leverrier') 53,200 15 Named after Urbain Le Verrier
1989 N4R ('Lassell') 55,400 6 Named after William Lassell
Arago Ring 57,600 - Named after François Arago
Liberté Ring Arc 62,900 - "Leading" arc
Égalité Ring Arc 62,900 - "Equidistant" arc
Fraternité Ring Arc 62,900 - "Trailing" arc
Courage Ring Arc 62,900 -
1989 N1R ('Adams') 62,930 <50 Named after John Couch Adams

The moons of Neptune

Main article: Neptune's natural satellites

Neptune has 13 known moons. The largest by far is Triton, discovered by William Lassell just 17 days after the discovery of Neptune itself. Unlike all other large planetary moons, it has a retrograde and synchronous orbit. Triton is the coldest object that has been measured in our solar system, and it is slowly spiraling toward Neptune. Neptune's second satellite, Nereid, has one of the most eccentric orbits of any satellite in the solar system.

From July to September 1989, Voyager 2 discovered six new Neptunian moons. Of these, the irregularly shaped 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. Neptune's first four moons, Naiad, Thalassa, Despina, and Galatea orbit close enough to be within Neptune's rings. The next farthest out, Larissa was originally discovered in 1981 when it had blocked a star. This was attributed to ring arcs, but when Voyager 2 observed Neptune in 1989, it was found to have been caused by the moon.

Five new irregular moons were announced in 2004.They were discovered in 2002 and 2003. (For a timeline of discovery dates, see Timeline of natural satellites.)

Trojan Asteroids of Neptune

There are two known Trojan asteroids of Neptune which have the same orbital period as Neptune and lie in the elongated, curved region around the L4 Lagrangian point 60° ahead of Neptune. These are 2001 QR322 and 2004 UP10.

Neptune in fiction and film

In Olaf Stapledon's epic novel Last and First Men (1930), Neptune is the final home of the highly evolved human race.

In H. P. Lovecraft's Cthulhu Mythos (1928–), Neptune is known as Yaksh and is inhabited by curious fungoid creatures. (Clark Ashton Smith's The Family Tree of the Gods, 1944)

Neptune is the intended destination of the mining ship Red Dwarf (1988–) in the books based on the BBC sitcom of that name, but an accident on board sends it into deep space instead.

Neptune is the setting of the sci-fi/horror film Event Horizon (1997), although it is used purely as a backdrop.

In the animated TV series Futurama (1999–2003), Neptunians are a purple-skinned, four-armed race of humanoids that coexist peacefully with humans across the Solar System. Robot Santa's base is also located on Neptune.

In the anime Bishoujo Senshi Sailor Moon Sailor Neptune is, obviously, the soldier representing Neptune. She is a Pisces, she has wavy deep green hair and her image colors represent those of neptune. On her forehead is the symbol of neptune, which resembles a trident, Poseidon's weapon. Her talisman is the Deep Aqua Mirror, also bearing the glyph and colors of Neptune.

In Samuel R. Delany's Triton (novel) (1976), humanity has colonized several parts of the Solar System including Neptune's largest moon.

Our Solar System's Planets: Neptune

Neptune's natural satellites

Naiad | Thalassa | Despina | Galatea | Larissa | Proteus | Triton | Nereid

S/2002 N 1 | S/2002 N 2 | S/2002 N 3 | S/2003 N 1 | S/2002 N 4

see also: The Solar System

Our Solar System

Sun | Mercury | Venus | Earth (Moon) | Mars | Asteroid belt

Jupiter | Saturn | Uranus | Neptune | Pluto (Charon) | Kuiper belt | Scattered disc | Oort cloud

See also astronomical objects and the solar system's list of objects, sorted by radius or mass

Astronomy Encyclopedia

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