Neptune (planet)

Neptune from Voyager 2
Click image for description
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 60,224.9036 d
(164.89 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 kg/m3
Equatorial gravity (At 1 bar) 11.15 m/s2
(1.14 gee)
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
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

Neptune is the eighth planet 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. Due to Pluto's eccentric orbit, Neptune is sometimes the furthest planet from the Sun. Neptune is named after the Roman god of the sea. Its symbol is a stylized representation of the god's trident (Unicode: ♆).

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. Its surface temperature is −218 °C (55 K). However, the planet seems to have 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 center, 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. Comparing its rotational speed to its degree of oblateness indicates that it has its mass less concentrated towards the center 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 kilometers) from the planet's physical center. 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 28, 1612, and again on January 27, 1613; on both occasions Galileo mistook Neptune for a fixed star when it appeared very close (in conjunction) 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

As for a name for the new planet, the first suggestion came from Galle. He proposed the name Janus. In England, Challis put forth the name Oceanus, particularly appropriate for a sea-faring 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 suggested Neptune for the new planet. 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 sensitive film. From Earth it has a disk of 2" diameter.

With an orbital period of 165 years, Neptune will soon return to the approximate position where it was discovered by Galle, 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, retrorad Opposition Stationary, prograd Conjunction to sun
May 17th, 2004 August 6th, 2004 October 24th, 2004 February 2nd, 2004
May 20th, 2005 August 8th, 2005 October 26th, 2005 February 3rd, 2005
May 22nd, 2006 August 11th, 2006 October 29th, 2006 February 6th, 2006
May 25th, 2007 August 13th, 2007 October 31st, 2007 February 8th, 2007
May 26th, 2008 August 15th, 2008 November 2nd, 2008 February 11th, 2008
May 29th, 2009 August 17th, 2009 November 4th, 2009 February 12th, 2009
June 1st, 2010 August 20th, 2010 November 7th, 2010 February 14th, 2010

The rings of Neptune

Main article: Rings of Neptune

Neptune's rings
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 Libert駧, Egalit駧, and Fraternit駧 (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 center 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. [3] (

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 Libert駧 ring might disappear in as little as one century. The new observations appear to throw our understanding of Neptune's rings into considerable confusion. [4] (

Name Radius (km) Width (km)
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 Fran篩s Arago
Libert頒ing Arc 62,900 - "Leading" arc
ɧalit頒ing Arc 62,900 - "Equidistant" arc
Fraternit頒ing 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. 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.

Neptune in fiction and film

  • In Olaf Stapledon's epic novel Last and First Men, Neptune is the final home of the highly evolved human race.
  • Neptune is the setting of the sci-fi/horror film Event Horizon, although it is used purely as a backdrop.
  • Neptune is the intended destination of the mining ship Red Dwarf in the books based on the BBC sitcom of that name, but an accident on board sends it into deep space instead.

Neptune in astrology

Main article: Planets in astrology

See also


  • Neptune: The Planet, Rings, and Satellites, Ellis D. Miner et Randii R. Wessen, 2002. ISBN 1-852-33216-6
  • Neptune and Triton, Dale P. Cruikshank, 1995. ISBN 0-816-51525-5
  • The case of the pilfered planet - Did the British steal Neptune?, William Sheehan, Nicolas Kollerstrom and Craig B. Waff, Scientific American December 2004.

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Neptune's natural satellites

edit  (

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

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Jupiter | Saturn | Uranus | Neptune | Pluto | Kuiper belt | Oort cloud
See also astronomical objects and the solar system's list of objects, sorted by radius or mass

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