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Neptune is a large solar system planet found in 1846.

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Neptune is the eighth and outermost planet in our Solar System. Neptune is the fourth-largest planet by diameter and the third-largest by mass. Neptune is 17 times the mass of Earth and is slightly more massive than its near twin Uranus which is 14 Earth Masses, but slightly smaller due to its higher density. The planet Neptune is named after the Roman god of the sea. Its astronomical symbol is a stylized version of the god's trident.

Neptune Planet. Symbol of Neptune.

Neptune from Voyager 2.
Solar system planet 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 17º
(6.43º to Sun's equator)
Longitude of the
ascending node
131.721 69º
Argument of the
273.249 66º
Number of Satellites 13
Physical characteristics
Equatorial diameter 49,528 km
(3.883 Earths)
Polar diameter 48,681 km
(3.829 Earths)
Oblateness 0.0171
Surface area 7.619×109 km2
(14.94 Earths)
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)
Rotation velocity 2.68 km/s = 9660 km/h (at the equator)
axial tilt 28.32º
Right ascension
of North pole
299.33º (19 h 57 min 20 s)
Declination 42.95º
Albedo 0.41
Surface temp.
50 K 53 K N/A K
Adjective Neptunian
Atmospheric characteristics
Atmospheric pressure »100 kPa
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's atmosphere is primarily composed of Hydrogen and helium, with traces of methane that account for the planet's blue appearance. Neptune's blue colour is much more vivid than that of Uranus, which has a similar amount of methane, so an unknown component is presumed to cause Neptune's intense color. Neptune also has the strongest winds of any planet in the solar system, with estimates as high as 2,500 km/h or 1,500 mph. At the time of the 1989 Voyager 2 flyby, it had in its southern hemisphere a Great Dark Spot comparable to the Great Red Spot on Jupiter. Neptune's temperature at its cloud tops is usually close to -210? (-346ºF), one of the coldest in the solar system, due to its long distance from the sun. Neptune's center is about 7,000? (13,000ºF), however, hotter than the sun's surface. This is due to extremely hot gases and rock in the center.

Faint dark colored rings have been detected around the blue planet, but are much less substantial than those of Saturn. When these rings were discovered by a team led by Edward Guinan, it was thought that they might not be complete but this was disproved by Voyager 2. Neptune possesses thirteen confirmed moons. Neptune's largest moon, Triton, is notable for its retrograde orbit, extreme cold (38K), and extremely tenuous (14 microbar) nitrogen/methane atmosphere.

Discovered on September 23, 1846, Neptune is notable for being the first planet discovered based on mathematical prediction rather than regular observations. Perturbations in the orbit of Uranus led astronomers to deduce Neptune's existence. It has been visited by only one spacecraft, Voyager 2, which flew by the planet on August 25, 1989. In 2003, there was a proposal to NASA's "Vision Missions Studies" to implement a "Neptune Orbiter with Probes" mission that does Cassini-level science without fission-based electric power or propulsion. The work is being done in conjunction with JPL and the California Institute of Technology.

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 had mistaken 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 detected with Galileo's small telescope.

Neptune and Earth.
Size comparison of Neptune and Earth.

In 1821, Alexis Bouvard published astronomical tables of the orbit of Uranus. Subsequent observations revealed substantial deviations from the tables, leading Bouvard to hypothesize 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, the Astronomer Royal, 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 meantime, 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 fixed star. Neptune was discovered that very night, September 23, 1846, within 1º of where Le Verrier had predicted it to be, and about 10º from Adams' prediction. Challis later realized that he had observed the planet twice in August, failing to identify it owing to his casual approach to the work.

In the aftermath of the discovery, there was much nationalistic rivalry between the French and the British over who had priority and deserved credit for the discovery. Eventually an international consensus emerged that both Le Verrier and Adams jointly deserved credit. However, the issue is 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 only rediscovered (in his possession) immediately after his death. After reviewing the documents, some historians now suggest that Adams does not deserve equal credit with Le Verrier.

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.

Internal structure of Neptune.

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 favor of that name on December 29, 1846, to the Saint Petersburg Academy of Sciences. Soon Neptune became the 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 for Uranus, were named in antiquity.

The planet's name is translated literally as the sea king star in the Chinese, Korean, Japanese, and Vietnamese languages ( in Chinese characters, in Korean).

Physical characteristics of Neptune.

Neptune Great Dark Spot.
Neptune Great Dark Spot, as seen from Voyager 2.

Relative size of Neptune.

At 1.0243×1026 kg Neptune is an intermediate body between Earth and the largest gas giants: it is seventeen Earth masses but just 1/18th the mass of Jupiter. It and Uranus are often considered a sub-class of gas giant termed "ice giants", given their smaller size and important differences in composition relative to Jupiter and Saturn. In the search for extra-solar planets Neptune has been used as a metonym: discovered bodies of similar mass are often referred to as "Neptunes" just as astronomers refer to various extra-solar "Jupiters."

Composition of Neptune.

Orbiting so far from the sun, Neptune receives very little heat with the uppermost regions of the atmosphere at -218 ºC (55 K). Deeper inside the layers of gas, however, the temperature rises steadily. It is thought that this may be leftover heat generated by infalling matter during the planet's birth, now slowly radiating away into space.

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. There is no solid surface and the atmosphere, extending perhaps 10 to 20 percent of the way towards the center, is mostly hydrogen and helium at high altitudes (80% and 19%, respectively). Increasing concentrations of methane, ammonia, and water are found as the dark, hotter and lower regions atmosphere approaches and finally blends into the superheated liquid interior. The pressure at the center 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 center than does Uranus.

Magnetic field of Neptune.

Neptune also resembles Uranus in its magnetosphere, with a magnetic field strongly tilted relative to its rotational axis at 47º and offset at least 0.55 radii (about 13,500 kilometres) 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.

Weather on Neptune.

Neptune: Great Dark Spot (top), Scooter (middle white cloud), and the Wizard's eye (bottom).

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. Neptune's atmosphere has the highest wind speeds in the solar system, thought to be powered by the flow of internal heat, and its weather is characterized by extremely violent hurricanes, with winds reaching up to 2000 km/h.

In 1989, the Great Dark Spot, a cyclonic storm system the size of Eurasia, was discovered by NASA's Voyager 2 spacecraft. The storm resembled the Great Red Spot of Jupiter. However, on November 2, 1994 the Hubble Space Telescope did not see the Great Dark Spot on the planet. Instead, a new storm similar to the Great Dark Spot was found in the planet's northern hemisphere. The reason for the Great Dark Spot's disappearance is unknown. Many scientists believe heat transfer from the planet's core disrupted the atmospheric equilibrium and disrupted existing circulation patterns. The Scooter is another storm described as a white cloud south of the Great Dark Spot. The Wizard's eye (Great Dark Spot 2) is a southern hurricane, the second most intensive hurricane on the planet.

Unique among the gas giants is the presence of high clouds casting shadows on the opaque cloud deck below. Though Neptune's atmosphere is much more dynamic than that of Uranus, both planets are made of the same gases and ices. Uranus and Neptune are not strictly gas giants similar to Jupiter and Saturn, but are rather ice giants, meaning they have a larger solid core and are also made of ices. Neptune is very cold, with temperatures as low as -224ºC (-372ºF) recorded at the cloud tops in 1989.

Exploration of Neptune.

Voyager 2 image of Neptune.
Voyager 2 image of Neptune.

The closest approach of Voyager 2 to Neptune occurred on August 25, 1989. Since this was the last major planet the spacecraft could visit, it was decided to make a close flyby of the moon Triton, regardless of the consequences to the trajectory, similarly to what was done for Voyager 1's encounter with Saturn and its moon Titan.

The probe also discovered the Great Dark Spot, which has since disappeared, according to Hubble Space Telescope observations. Originally thought to be a large cloud itself, it was later postulated to be a hole in the visible cloud deck.

Neptune turned out to have the strongest winds of all the solar system's gas giants. In the outer regions of the solar system, where the Sun shines over 1000 times fainter than on Earth (still very bright with a magnitude of -21), the last of the four giants defied all expectations of the scientists.

One might expect that the farther one gets from The Sun, the less energy there would be to drive the winds around. The winds on Jupiter were already hundreds of kilometres per hour. Rather than seeing slower winds, the scientists found faster winds (over 1600 km/h) on more distant Neptune.

Scientists now know why this is the case -if enough energy is produced, turbulence is created, which slows the winds down (like those of Jupiter). At Neptune however, there is so little energy, that once winds are started, they meet very little resistance, and are able to maintain extremely high velocities.

Planetary rings of Neptune.

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.

Neptune's rings.
Neptune's rings.

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 ring, 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.

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.

Name of ringRadius (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

Natural satellites of Neptune.

Neptune has 13 known moons. The largest by far, and the only one massive enough to be spheroidal, is Triton, discovered by William Lassell just 17 days after the discovery of Neptune itself. Unlike all other large planetary moons, Triton has a retrograde orbit, indicating that it was captured, and probably represents a large example of a Kuiper belt object (although clearly no longer in the Kuiper Belt). It is close enough to Neptune to be locked into a synchronous orbit, and is slowly spiraling inward and eventually will be torn apart when it reaches the Roche limit. Triton is the coldest object that has been measured in the solar system, with temperatures of 38.15K (-235ºC, -392ºF).

Triton, compared to Earth's Moon

(Pronunciation key)

Orbital radius (km)Orbital period (days)
Triton trye'-t?n
(80% Luna)
(30% Luna)
(90% Luna)
(20% Luna)

Neptune's second known satellite, the irregular moon 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. Although the second most massive Neptunian moon, it is only one quarter of one percent of the mass of Triton. Neptune's innermost 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 occulted 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 discovered between 2002 and 2003 were announced in 2004. As Neptune was the Roman god of the sea, the planet's moons have been named after lesser sea gods.

For a timeline of discovery dates, see Timeline of discovery of Solar System planets and their natural satellites

Neptune's appearance and visibility from Earth.

Neptune is never visible with the naked eye. The brightness of Neptune is between magnitudes +7.7 and +8.0, so a telescope or binoculars are required to observe it. With the use of a telescope it appears as a small blue-green disk, similar in appearance to Uranus; the blue-green colour comes from the methane in its atmosphere. Its small apparent size has made it almost impossible to study visually; even observatory data was fairly poor until the advent of adaptive optics.

With an orbital period of 165 years, Neptune will soon return to the approximate position in the sky where Galle discovered it. This will happen three different times. These are April 11, 2009, when it will be in prograde 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. This is explained by the concept of retrogradation. Like all planets in the solar system beyond Earth, Neptune undergoes retrogradation at certain points during its Synodic period. In addition to the start of retrogradation, other events within the synodic period include Astronomical opposition, the return to prograde motion, and conjunction to the Sun.

In its orbit around the Sun, Neptune will return to its original point of discovery in August 2011.

Further reading about the solar system.
The Sun · Mercury · Venus · Earth · Mars · Ceres · Saturn · Uranus . Pluto · Eris
Planets · Dwarf planets · Moons: Terran · Martian · Asteroidal · Jovian · Saturnian · Uranian · Neptunian · Plutonian · Eridian
Small bodies:   Meteorites · Asteroids (Asteroid belt) · Centaurs · TNOs (Kuiper belt/Scattered disc) · Comets (Oort cloud)
Solar system related pages. astronomical objects and the solar system's list of solar system objects.

More About The Planet Neptune.

Hubble's Neptune Movies

Sep 2, 2005 - Photographs of Neptune taken by the Hubble Space Telescope have been assembled into time-lapse movies that show the planet's dynamic atmosphere and satellites. This natural view image is what you'd see if you saw the planet with your own eyes.

Mission to Neptune Under Study

Dec 9, 2004 - With Jupiter and now Saturn getting attention, NASA is setting its eyes further out in the solar system - on Neptune. A mission to this "ice giant" could launch in a decade, and arrive at the 8th planet by 2035. It would be powered by a nuclear-electric propulsion system, similar to the one being considered for the Jupiter Icy Moons Orbiter (JIMO) mission. Because it is so far from the Sun, Neptune has had less interaction with the solar wind, asteroids and comets, so studying it would give scientists a better understanding of the conditions that led to the formation of the Solar System.

Neptune Emptied the Kuiper Belts

Nov 27, 2003 - Researchers from the Southwest Research Institute believe they have a theory that could help explain why there are so few objects in the Kuiper belt - a band of objects outside the orbit of Neptune. According to theories of how planetary systems form, there should be 100 times more material in the Kuiper belt than astronomers have observed. The researchers believe that the gas giants, including Neptune, formed closer to the Sun, and have slowly drifted further out over time. As Neptune migrated out, it could have pushed the Kuiper objects out of the solar system.

Three new moons discovered for Neptune

Jan 14, 2003 - A team of astronomers from the Harvard-Smithsonian Center for Astrophysics have discovered three previously unknown moons orbiting the planet Neptune. Since they're only 30-40km across, the moons were a challenge to spot. The team had to digitally merge multiple exposures of the planet moving across a background of stars. Over time, the planets and their motions were picked up as points of light. This brings the gas giant's total to 11 known moons.

Neptune Has a Trojan

Jan 9, 2003 - Astronomers have discovered a new object which shares a very similar orbit with Neptune. Part of a classification of objects called Trojans, 2001 QR322 is 230 km across and requires 166 years to orbit the Sun. Although clusters of Trojans have been found following Jupiter's orbit, none have ever been found to share an orbit with any other giant planet; although, they've been predicted for years.

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