Neptune (planet)

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Neptune  Astronomical symbol of Neptune
Neptune - Voyager 2 (29347980845) flatten crop.jpg
Neptune (recorded by Voyager 2 , August 25, 1989)
Properties of the orbit
Major semi-axis 30,047  AU
(4,495 million km)
Perihelion - aphelion 29,709-30,385 AU
eccentricity 0.00859
Inclination of the orbit plane 1.7692 °
Sidereal period of rotation 164.79  a
Synodic period 367.49  d
Mean orbital velocity 5.43 km / s
Smallest - largest distance to earth 28,783-31,332 AU
Physical Properties
Equatorial diameter ≈4 earth diameter
49,528 km
Pole diameter 48,682 km
Dimensions ≈17 earth masses
1.024 · 10 26  kg
Medium density 1.638 g / cm 3
Main components
(proportion of fabric in the upper layers)
Gravitational acceleration 11.15 m / s 2
Escape speed 23.5 km / s
Rotation period 15 h 57 min 59 s
Inclination of the axis of rotation 28.32 °
Geometric albedo 0.41
Max. Apparent brightness +7.67 m
Min. - Average - Max.
72  K  (−201  ° C )
related to the zero level of the planet
Moons 14 + ring system
Neptune Earth Comparison.png
Size comparison between Earth (left) and Neptune

The Neptune is the eighth and outermost known planet of our solar system . It was discovered in 1846 by the French mathematician Urbain Le Verrier by the German astronomer Johann Gottfried Galle on the basis of calculations from orbital disturbances of Uranus . Neptune is on average 4.5 billion kilometers from Earth and shows a disc of 2 arc seconds . With a diameter of almost 50,000 kilometers, Neptune has almost four times the diameter of the earth and around 58 times the volume of the earth. After Jupiter , Saturn and Uranus , Neptune is the fourth largest planet in the solar system.

Together with Uranus, Neptune forms the subgroup of the ice giants . Due to its size, Neptune dominates the outer zone of the planetary system, which can be seen, for example, in the orbital period of some " Transneptunes " such as Pluto and the Plutino group, which is about 1.5 times the orbital period of Neptune (a 3: 2 orbital resonance ). Of the 14 known moons of Neptune, Triton is by far the largest with a diameter of 2700 kilometers.

The giant planet is named after Neptune , the Roman god of the sea and rivers. Its symbol ♆ is a stylized trident , the weapon of the sea god. When searching for exoplanets , objects with a mass similar to Neptune are sometimes referred to by astronomers as "Neptune-class" or " Hot Neptune " planets, analogous to the extrasolar "Jupiters" or " Hot Jupiters " . The chemical element neptunium , discovered in 1940, was named after the planet .

As the only planet in the solar system, Neptune cannot be seen from Earth with the naked eye. With the help of a telescope he can currently be observed in the autumn sky . His opposition was on September 2nd in 2016 and shifts backwards by about 2 days a year .

Orbit and rotation


Neptune's orbit around the Sun is almost circular with an eccentricity of 0.00859. Its point closest to the Sun, the perihelion , is 29.709  AU and its point furthest from the Sun , the aphelion , is 30.385  AU . This makes it the outermost planet in the solar system . Its orbit plane is only slightly inclined towards the ecliptic ( orbit plane of the earth) at 1.7692 ° . It takes Neptune about 165 years to orbit the sun.

In the outer area of ​​the solar system, due to its relatively large mass, Neptune influences the orbits of many smaller bodies such as those of the Plutinos and the Transneptunes . Pluto's orbit is so eccentric that it is closer to the Sun in its perihelion than Neptune. From the perspective of the north pole of the ecliptic - perpendicular to the plane of the ecliptic - their orbits therefore appear to intersect. However, Pluto's orbit is inclined more than 17.1 ° to the plane of the ecliptic. At the time of Pluto's proximity to the Sun, Pluto is almost at its northernmost point above the ecliptic plane and therefore does not intersect Neptune's orbit. In addition, Neptune forces Pluto to have a 3: 2 orbit resonance . While Neptune makes three orbits of the sun, Pluto only circles the sun twice. The orbits are synchronized in such a way that Neptune is always far away from him when it appears to cross Pluto's orbit. From February 7, 1979 to February 11, 1999, Pluto was closer to the sun than Neptune.

On July 12, 2011, Neptune returned to the point in its orbit at which it was when it was discovered on September 23, 1846.


With a rotation period of 15 hours, 57 minutes and 59 seconds, Neptune rotates very quickly like the other three giant planets. The consequence of this rapid rotation is a flattening of 1.7%. Thus, the diameter at the poles is about 1000 km less than at the equator. The inclination of the equator in relation to its orbit is 28.32 °. The inclination of its axis of rotation is thus slightly greater than that of the earth.

Physical Properties

With a diameter of almost 50,000 km, Neptune is one of the giant planets. With a density of 1.64 g / cm³, it is the most compact giant planet. Even if Neptune is slightly smaller than Uranus , Neptune is 17 times more massive with earth's mass. Jupiter is more than 18 times the mass of Neptune. The equatorial acceleration due to gravity at zero level among the planets of the solar system is only greater at Jupiter than at Neptune (23.12 m / s² compared to 11.15 m / s²).

Upper layers

Neptune in natural colors with three moons

The upper layers of the atmosphere consist mainly of hydrogen (80 ± 3.2 vol%) and helium (19 ± 3.2 vol%), some methane (1.5 ± 0.5 vol%), and deuterated hydrogen HD (192 vol ppm ) and traces of ethane (1.5 vol ppm). Neptune's blue color, like Uranus, is caused by the methane that absorbs red light. Distinctive absorption bands of methane occur in the red and infrared part of the spectrum at wavelengths above 600 nm. However, its blue color appears much stronger than that of the blue-green Uranus, whose atmosphere is structured similarly. Presumably, another component of the atmosphere is responsible for Neptune's more intense color. The upper layers have an extension of about 10 to 20% of the planet's radius. Higher concentrations of methane, ammonia, and water are present in the lower parts of the atmosphere.

Since Neptune orbits the sun at a great distance, he receives little heat from it. Its temperature is around −218 ° C (55 K) at the depth at which there is a pressure of 0.1  bar , and at 1 bar −201 ° C (72 K).

Due to the inclination of the axis, it is currently midsummer at the South Pole. This has been exposed to the sun for over 40 years (the quarter of a Neptune year), the next equinox is not until 2038. Despite the great distance to the sun , the energy received is enough to make these areas up to 10 K warmer than the rest of Neptune .

It is not possible to define an atmosphere that is clearly delimited below, because the gas exceeds the critical pressure above the critical temperature with increasing depth . Therefore there is no phase transition into the liquid state, so that there is no well-defined surface of the planet.

internal structure

The inner structure of Neptune:
upper atmosphere, uppermost cloud layer
atmosphere (hydrogen, helium, methane gas)
mantle (water, ammonia, methane ice)
core (rock, ice)

Uranus and Neptune are "ice giants". They have a larger solid core than Jupiter and Saturn. Like Uranus, it could be more or less uniform in composition. In contrast, Jupiter and Saturn have separate inner layers.

It is assumed that in the center there is a solid core about 1 to 1½ times the mass of the earth. This consists of rock and metal and is no bigger than the earth. The temperature in its center is around 7000 ° C and the pressure is a few million bar.

The center is surrounded by a mantle or ocean made of a mixture of rock, water, ammonia and methane , which corresponds to a mass of 10 to 15 times the mass of the earth (this mixture of water, methane or ammonia is called ice by the planetologists , even if in reality they are hot and very dense liquids and these substances normally occur in the solid state in the outer solar system). The upper layer surrounding the mantle has a share of about one to two masses of earth.

If one compares the speed of rotation with the factor of the flattening , it turns out that the mass in the interior of Neptune is more evenly distributed than in Uranus. With Uranus the mass towards the center becomes much denser than with Neptune.

Like Jupiter and Saturn, Neptune has an internal heat source. It emits about 2.7 times the energy that it absorbs from solar radiation. One reason for this could be radioactive processes that heat up the planet's core. Another possibility would be the radiation of the heat that was still present, which was formed during the formation by incident matter on the planet. Gravity waves breaking over the tropopause could also be the cause of this heat release.



Changes in brightness of Neptune between 1996 and 2002 (images from the Hubble telescope )

Scientists from the University of Wisconsin – Madison and NASA's Jet Propulsion Laboratory examined one full revolution of Neptune in 1996, 1998 and 2002. They noticed an increasing brightness and a higher cloud density in the southern hemisphere , while hardly any changes seemed to take place near the equator. In doing so, they confirmed the reports from the 1980 Lowell Observatory , from which the phenomenon was first observed. Just like on Earth, during a Neptune year, the inclination of the axis of Neptune causes a change in solar radiation and thus leads to seasons. However, unlike Earth, they last more than 40 years.


One difference between Neptune and Uranus is the level of meteorological activity. When the Voyager 2 spacecraft passed Uranus in 1986, this planet was practically structureless, while Neptune exhibited remarkable weather phenomena when Voyager 2 approached in 1989. Long bright clouds resembling Earth's cirrus clouds were spotted high in Neptune's atmosphere. Due to the rapid rotation, its high cloud layers also have a strip- like structure.

One might expect that the greater the distance from the sun, there would be less and less energy available to drive winds. Winds of up to several hundred km / h occur on Jupiter. However, Neptune only absorbs three percent of the solar energy of Jupiter or one thousandth of the solar radiation on Earth per unit area. Nevertheless, instead of slower winds on Neptune, the scientists discovered dynamic storms with over 1600 km / h (peak values ​​up to 2100 km / h). The highest wind speed ever recorded in the solar system was thus reached in Neptune's atmosphere. Since relatively little solar energy reaches Neptune, it is assumed that once winds have started, they are hardly slowed down. With enough energy available, turbulence would have to arise that oppose resistance to the winds (as is the case with Jupiter). This does not seem to happen with Neptune, which means that extremely high speeds can be observed. Another theory suggests that internal heat sources drive the winds.

It looks like Neptune's atmosphere is changing very quickly. Even small temperature differences between the upper frosty cloud ceiling and the lower cloud layer, reinforced by Neptune's strong internal heat source, could be responsible for the instabilities in the atmosphere. In Neptune's cold atmosphere with temperatures of −218 ° C (55 K), the cirrus clouds are composed of frozen methane and less of water ice crystals (as on Earth).


Storms in the Neptune atmosphere (1989):
 ● Great Dark Spot  (above)
 ● Scooter  (middle white cloud)
 ● Small Dark Spot  (below)
The "Great Dark Spot" (as seen from Voyager 2)

In 1989 Voyager 2 discovered the so-called "Great Dark Spot" in the southern hemisphere of Neptune. This cyclone system , which is similar to the “Little Red Spot” and “Big Red Spot” of Jupiter and represents a high pressure area, extended over an area the size of Eurasia. Originally it was thought that the structure was itself a cloud. Later they agreed on a hole in the visible cloud cover. The “Great Dark Spot” ( GDS ) was at 22 ° south latitude and orbited Neptune in 18.3 hours. The shape of the system suggests that the storm system rotates counterclockwise. The bright clouds east and south of the GDS changed their appearance within a few hours. However, the GDS was not found again on November 2, 1994 by the Hubble Space Telescope . The reason for the disappearance of the GDS is unknown. According to one theory, the heat from the planet's core could have disturbed the equilibrium of the atmosphere and disrupted existing, circumferential structures. It could also have simply disintegrated or been obscured by other parts of the atmosphere. Instead, a new storm similar to the GDS was discovered in the northern hemisphere.

The "Small Dark Spot" (D2) is a southern cyclone storm that rotates clockwise. It was the second strongest storm during the 1989 encounter. At first it was completely dark. However, as Voyager 2 approached the planet, a bright core developed, which can be seen in most high-resolution images.


The "Scooter" is another storm that was discovered in 1989 in the months leading up to Voyager 2's arrival at Neptune. It forms white cloud groups south of the GDS and moves once around Neptune in 16 hours and is therefore much faster than the GDS moved. The structure could be a plume of smoke rising from the lower layers. The following pictures showed clouds that were moving even faster than the "scooter".

Magnetic field

Neptune and Uranus only have a thin layer of conductive, metallic material and therefore do not generate a dipole but a quadrupole field with two north and two south poles. The magnetic field is strongly inclined at 47 ° with respect to the axis of rotation. The field strength at the equator is about 1.4  µT and is therefore about 1300 of the equatorial field of Jupiter (420 µT) and 120 of the equatorial earth field (30 µT). The magnetic dipole moment , which is a measure of the strength of the magnetic field at a given distance from the center of the planet, is 2.2 · 10 17  T · m³ 28 times stronger than the earth's magnetic field (7.9 · 10 15  T · m³). The center of the magnetic field is shifted about 13,500 km from the center of the planet, so it is likely that the magnetic field originates in higher layers than Earth, Jupiter or Saturn. The cause of the alignment of the field could be the flow movements in the interior of the planet. It may be in a phase of polarity reversal. Voyager 2 also detected faint complex auroras at the magnetic poles.

Ring system

Neptune has a very fine azure-colored ring system, which consists of several distinct rings and the unusual ring arcs in the outer Adams ring. The rings, like the rings of Uranus and Jupiter, are unusually dark and contain a high proportion of microscopic dust that could have come from the impacts of tiny meteorites on Neptune's moons.

When the rings were discovered in the 1980s by a team led by Edward Guinan using star darkening, it was assumed that they were not complete. Voyager 2's observations refuted this assumption. The cause of this phenomenon are bright lumps in the ring system. The reason for the “lumpy” structure has not yet been clarified. The gravitational interaction with small moons in the ring environment could contribute to this accumulation.

The rings are named after astronomers who made significant contributions to the exploration of Neptune. The four moons Naiad , Thalassa , Despina and Galatea orbit Neptune within the ring region.

Complete rings
Surname Orbital radius
named after
bile 041,900 02000 00.08 40… 5% Johann Galle
uncertain ≲50,000   wide
LeVerrier 053,200 00110 02 40 ... 80% Urbain Le Verrier
Lassell 053,200 ... 57,200 04000 00.15 13… 45% William Lassell
Arago 057,200 < 0100 François Arago
not named 061,950  narrow
Adams 062,933 00050 04.5 17… 55% John Couch Adams
Ring arcs in the Adams ring
Surname Width
length strength Remarks
1989 1989 2003 1989 2003 1989 2003
Liberté 15th ≈26 ° ≈25 ° 04 ° ≈4 ° strong weak leading  ring arch
Égalité 15th ≈11 ° ≈13 ° ≈5 ° ≈8 ° strong strong equidistant  ring arch
Fraternity 15th 00 ° 00 ° 10 ° ≈8 ° strong strong subsequent  ring arch
courage 15th ≈33 ° ≈41 ° ≈2 ° ≈4 ° weak weak
All ring arcs have optical depths of 0.12 (120 ) and dust proportions of 40 ... 80%

Inner rings

Neptune's Ring System (from Voyager 2)

The inner ring system consists of the following ring structures from the outside to the inside:

  • An unnamed , indistinct, lumpy ring of dust in Galatea's orbit.
  • The wide Lassell Ring (1989 N4R) is a dull arch that extends with a radius of 59,200 km and 4,000 km in the direction of Neptune. It's dusty, but not as much as the other rings and is more like the contiguous part of the Adams ring. There is a lighter extension on the outer edge called the Arago ring (1989 N5R). The inner edge of the Lassel ring is adjacent to the LeVerrier ring.
  • The narrow LeVerrier ring (1989 N2R) is the second most conspicuous of the Neptune rings and lies just outside the orbit of the moon Despina at a distance of 700 km .
  • The innermost bile ring (1989 N3R) is dull and not fully understood. It lies clearly within the orbit of the innermost Neptune moon Naiad .

LeVerrier and Galle rings, like the ring arches, are very dusty. Small shepherd moons in the narrower rings prevent the rings from drifting apart and thus becoming more diffuse.

The images from Voyager 2 still suggest a broad slice of diffuse material. It appears to extend within 50,000 km of the Galle ring. Because of Neptune's sheen, this disk is not easy to spot and its existence is therefore not considered certain.

The Adams Ring and the Ring Arches

The Adams Ring and the Leverrier Ring. In the Adams ring, the ring arcs Egalité, Fraternité and Liberté emerge from the outside inwards. (Voyager 2 Aug 1989)

The most noticeable ring is the narrow outer Adams ring, although it still appears very faint compared to the rings of Saturn and Uranus . Its original name was 1989 N1R. As a special feature, it contains several elongated arched sections, each spanning 4 to 10 ° of the total length of the ring.

These ring arcs are much brighter and more opaque than the rest of the ring, and bear a distant resemblance to the G-ring of Saturn. The existence of the ring arcs is physically difficult to explain. Due to the laws of motion, it must be expected that the arc segments will distribute themselves into complete rings within a short period of time. The Adams ring has 42 radial links with an amplitude of about 30 km. These structures and the delimitation of the ring arcs are probably caused by the gravitational influence of the moon Galatea , which rotates only 1000 km within the ring. The value of the amplitude was used to determine Galatea's mass.

The three main arcs are called Liberté , Égalité and Fraternité ( freedom, equality and fraternity according to the motto of the French Revolution ). This name was suggested by the original explorers who discovered it during the stellar occultations in 1984 and 1985. All ring arcs are close together and together span a length of less than 40 °.

The high-resolution images from Voyager 2 revealed a decidedly lumpy structure in the arches. The typical distance between visible clumps is 0.1 ° to 0.2 °. This corresponds to 100 to 200 km along the ring. Since the chunks were not dissolved, it is not known whether they contain larger parts. However, they contain concentrations of microscopic dust, as evidenced by their increased brightness when backlit by the sun.

As with all of Neptune's rings, the fine dust is an important component. While there is already a lot of dust in the connected background ring, it plays an even greater role for the ring arcs. There it is responsible for most of the light that is scattered. This is in contrast, for example, to the main rings of Saturn, whose main ring contains less than one percent dust. The "Adams" ring has an intense red color and the diffuse background ring varies in brightness along its length. The ring is about 50% darker on the opposite side.

Dynamics of the arcs

When the Hubble telescope and terrestrial telescopes with adaptive optics began to operate, the arcs were observed several times again, beginning in 1998. It was noticed that the arches were surprisingly dynamic and changed considerably over a number of years. Fraternité and Égalité have swapped their material and noticeably changed their lengths. Earthbound studies published in 2005 show that Neptune's rings are significantly more unstable than previously thought. The Liberté ring arch, in particular, is weary and could be gone in less than a century. Its brightness in 2003 was only 30% of its original brightness from 1989 and can hardly be seen in the images of the Hubble space telescope from June 2005.

In the meantime, the arch seems to have got a split, doubly curved profile and moved several degrees of arc closer to the more stable Égalité . In 1998, a brightening was observed in the Courage ring arch, which looked very dull during the flyby of Voyager 2. Lately it has been as dark again as it was when it was discovered and has moved an additional 8 ° over the other arcs. There was some evidence that the arches were generally fading more and more. Observations in the visible range, however, show that the total amount of matter in the arcs remained roughly the same, but the arcs in the infrared range became darker compared to previous recordings.

This dynamics of the ring arcs is not yet understood and the new observations call into question the current state of knowledge about Neptune's ring system.

Discovery and observation of the rings

Ring system with some lunar orbits (true to scale)

The first signs of the rings around Neptune were observations of star occultations. Although about 50 of them were observed by Voyager 2 prior to the visit, in the early 1980s only five of the observations indicated signs of rings. Indications of incomplete rings were found in the mid-1980s, when observations of star occultation by Neptune also showed occasional flashes before or after the star was obscured by the planet. This was evidence that the rings were incomplete (or not continuous).

The 1989 flyby of Neptune by Voyager 2 contributed much to the current state of knowledge about the rings. Pictures from the space probe showed the structure of the ring system, which consists of several thin, faint rings. Various other rings were discovered by the probe's cameras. In addition to the narrow Adams Ring, which is 62,930 km from the center of Neptune, the LeVerrier Ring was discovered at 53,200 km and the wider, darker Galle Ring at 41,900 km. The pale extension of the LeVerrier ring to the outside was named after Lassell and is bounded on its outer edge by the Arago ring at 57,600 km.

Voyager 2's images of the arches answered the question of their incompleteness. The amount of dust was estimated by comparing the brightness of the rings with front and rear sunlight. Microscopic dust appears brighter when illuminated by the sun from the background. In contrast, larger particles become darker, since only their “night side” is visible. Of the outer planets, only spacecraft can provide such a backlit view that is necessary for this type of analysis.

The brightest parts of the ring (the arcs of the Adams ring) could be examined with earth-based telescopes in 2005. The recordings were made in the infrared range at wavelengths in which the sunlight is strongly absorbed by methane in the Neptune atmosphere, whereby the brightness of the planet is comparatively low and the arcs in the recordings are just visible. The more indistinct rings are still well below the threshold of visibility.

Origin and Migration

A simulation based on the Nice model showing the outer planets and the Kuiper belt:
a) before the Jupiter / Saturn 2: 1 resonance, b) dispersion of the objects of the Kuiper belt into the solar system after the orbit of Neptune had shifted, c) after the ejection of objects of the Kuiper Belt by Jupiter

The formation and formation of the ice giants Neptune and Uranus is difficult to explain. According to current (as of 2014) models of planet formation , the density of matter in the outer regions of the solar system was too low to form such large bodies based on the traditionally accepted theory of core accretion . An alternative hypothesis suggests that the ice giants were not formed by core accretion of matter, but rather by instabilities within the original protoplanetary disk . Later, their atmospheres were driven away by radiation from a nearby massive star of spectral class O or B. Another suggestion is that the two planets formed much closer to the Sun, where the density of matter was higher, and that they then gradually migrated to their current orbits.

Migration theory is favored because it makes it possible to explain the current resonances of the orbits in the Kuiper Belt , especially the 2: 5 resonances. As Neptune wandered outward, it collided with original objects of the Kuiper Belt. This created new resonances and caused chaos in their orbits for other bodies. It is believed that the “ scattered disk objects ” were placed in their current positions through interactions with the resonances caused by Neptune's migration . In 2004, a computer model (the Nice model ) by Alessandro Morbidelli (Côte d'Azur Observatory in Nice) showed the possibility that Neptune's migration in the direction of the Kuiper belt was triggered by the formation of a 1: 2 orbital resonance between Jupiter and Saturn could. A gravitational thrust would have formed, which would have propelled both Uranus and Neptune. These would then have entered orbits further out and would even have swapped places. The resulting displacement of the objects of the original Kuiper Belt could also explain the Great Bombardment that occurred 600 million years after the formation of the Solar System and the appearance of Jupiter's Trojans .

According to other research, Neptune did not throw the Kuiper Belt objects out of their original orbits. Because double asteroids, which circle each other as partners at a large distance, would have been separated into single asteroids during swing-by by Neptune's strong gravity.


Neptune's moon Proteus (Voyager 2, 1989)
Neptune (above) and Triton (below) (Voyager 2, 1989)
Color photo of Triton (Voyager 2, 1989)

14 Neptune moons are known. By far the largest of them is Triton . It was discovered by William Lassell 17 days after the discovery of Neptune . Because of its close proximity to Neptune, it is forced into a bound rotation . It is possible that Triton was once an object of the Kuiper Belt and was captured by Neptune. In contrast to all other large moons in the solar system, it runs retrograde (retrograde, i.e. opposite to the rotation of the planet) around Neptune. It slowly approaches Neptune on a spiral path, only to be torn apart when it crosses the Roche boundary . With temperatures of −235 ° C (38 K), Triton is the coldest object ever measured in the solar system.

It took over 100 years until the discovery of Neptune's second moon, Nereid . Nereid has one of the most eccentric orbits of all the moons in the solar system.

The remaining 12 moons were discovered between 1989 and 2013 and are much smaller except for Proteus .

From July to September 1989, the Voyager 2 space probe discovered six Neptune moons. The irregularly shaped Proteus with its dark, soot-like appearance is striking. The four innermost Neptune moons Naiad , Thalassa , Despina and Galatea have orbits within the Neptune rings. The first indication of the next moon Larissa from within came in 1981 when it covered a star, where it was initially assumed that part of an arc of a ring was assumed. When Voyager 2 explored Neptune in 1989, it was found that this star occultation was caused by a moon.

Five other irregular moons of Neptune were discovered in 2002 and 2003 and announced in 2004. Two of the newly discovered moons, Psamathe and Neso , have the largest orbits of any natural moons in the solar system known to date. It takes you 25 years to orbit Neptune. Their average distance from Neptune is 125 times the distance from the moon to the earth.

In 2013, observations from the Hubble Space Telescope discovered another moon, named Hippocamp in 2019 . It has a diameter of almost 20 kilometers and orbits the planet in 23 hours. The moon discovered by Mark Showalter from the SETI Institute in Mountain View / California was given the provisional designation S / 2004 N 1.

Since Neptune was the Roman god of the sea, the moons of the planet were named after other, subordinate sea gods.

Formation of the moons

It is likely that the inner moons did not originate with Neptune, but were formed by fragments that developed when Triton was captured . Triton's original orbit, which it occupied after being captured by Neptune, was very eccentric. This caused chaotic disturbances of the original inner moons of Neptune, which collided and were crushed to a disk of rubble. It was only when Triton gradually assumed a circular orbit that the parts of the debris disk were able to join together again to form new moons.

The process of incorporating Triton as the moon has been the subject of a number of theories over the years. Today astronomers assume that he was bound to Neptune during an encounter of three objects. In this scenario Triton was the object of a double system 1 that had survived the violent encounter with Neptune.

Numerical simulations show that another moon discovered in 2002, Halimede , has had a high probability of colliding with Nereid since its formation. Since both moons appear to be of a similar gray color, they could be fragments of the moon Nereid.

1 Binary objects, gravitational connections between two bodies, are often found among trans-Neptunian objects (> 10%; the best known is Pluto - Charon ) and not as common with asteroids as with 243 Ida and Dactyl .

Irregular moons

Neptune's irregular moons

Irregular moons are captured satellites at a great distance, have a high orbital inclination, and are mostly retrograde.

The diagram illustrates the orbits of Neptune's irregular moons that have been discovered so far. The eccentricity of the orbits is represented by yellow segments (which cover the area from the pericenter to the apocenter ) and the inclination by the Y-axis. The satellites above the X-axis move progressively (clockwise), the satellites below it move retrograde (retrograde). The X-axis is labeled Gm (million km) and the relevant fraction of the Hill sphere . The gravitational influence, within which an orbit around the planet is possible, reaches about 116 million km into space at Neptune.

Due to the similarity of the orbits of Neso and Psamathe , these moons could have descended from a larger moon that broke apart in the past.

Triton cannot be seen here. It moves retrograde, but has an almost circular orbit. In the case of Nereid, who moves on a right-hand but very eccentric orbit, it is assumed that his orbit was severely disturbed during the "integration" of Triton into the Neptune system.

Railway resonances

Orbital resonances in the Kuiper belt, caused by Neptune: The 2: 3 resonances (Plutinos), the "classic belt" (Cubewano) with orbits that are not influenced by Neptune and the 1: 2 resonances (Twotinos, a group of Trans-Neptunians Objects ).

Neptune's orbit has a significant impact on the region immediately beyond, known as the Kuiper Belt . The Kuiper belt is a ring made of small icy objects. It is comparable to the asteroid belt , but much larger and extends from Neptune's orbit (30 AU distance from the sun) to 55 AU distance from the sun. Just as Jupiter's gravity rules the asteroid belt by forming the structure, so does Neptune's gravity influence the Kuiper belt. Over the age of the solar system, certain regions of the Kuiper Belt have been destabilized by Neptune's gravity, including holes were formed in the structure of the Kuiper belt. The range between 40 and 42 AU from the Sun is such an example.

However, there are orbits within these empty regions where objects beyond the age of the solar system can exist. These orbital resonances occur when an object's orbit around the sun is an exact fraction of Neptune's orbit, such as 1: 2 or 3: 4. Assuming that a body orbits the Sun once every two Neptune orbits, it will only complete half the orbit when Neptune returns to its previous position. This also happens on the other side of the sun. The most populated resonant orbit in the Kuiper Belt, with over 200 known objects, is the 2: 3 resonance. The objects in this orbit complete one orbit for every 1 12 orbits of Neptune. They are called Plutinos because Pluto is one of them; this group includes the largest known Kuiper belt objects. Although Pluto regularly crosses Neptune's orbit, the two can never collide due to the 2: 3 resonance. Other, more sparsely populated resonances exist on the 3: 4, 3: 5, 4: 7 and 2: 5 resonances.

Neptune has a number of Trojans (“Neptunian Trojans”) that occupy the Lagrange points L 4 and L 5 . There are gravitationally stable regions in front of and behind its orbit. Neptunian Trojans are often described as being in 11 resonance to Neptune. The Trojans are remarkably stable in their orbits and were likely not captured by Neptune but formed next to it.


Several Neptun Trojans are known, for example 2001 QR 322 , (385571) Otrera , 2005 TN 53 , (385695) Clete , 2006 RJ 103 , (309239) 2007 RW 10 and (527604) 2007 VL 305 . They are so called in analogy to the classic Trojans of Jupiter. The objects hurry ahead of the planet 60 ° on the Lagrangian point L 4 (the extended curved curve of the planet's orbit) and have the same orbital period as the planet.

On August 12, 2010, the Department of Terrestrial Magnetism (DTM) of the Carnegie Institution for Science in Washington, DC announced the discovery by Scott Sheppard and Chadwick Trujillo of a Trojan at long-range position L 5 : 2008 LC 18 . It is the first detected Neptune Trojan in this position.

The discovery of 2005 TN 53 with a large orbit inclination of over 25 ° is significant, as this could indicate a dense cloud of Trojans. It is believed that large (radius ≈ 100 km) Neptunian Trojans could exceed the number of Jupiter's Trojans by an order of magnitude.

It was also considered as part of the mission of the New Horizons space probe during its journey to Pluto , the Trojans 2008 LC 18 and possibly other subsequent (L 5 ) Trojans detected in the near future , provided they come close enough to the probe. One candidate in 2011 was HM 102 . However, since New Horizons approached this celestial body up to a maximum of 180 million km, which was not sufficient for a meaningful observation, an observation was finally dispensed with.


Because of its apparent brightness between +7.8 m and +8.0 m, Neptune is never visible to the naked eye. Even Jupiter 's Galilean moons , the dwarf planet (1) Ceres, and the asteroids (4) Vesta , (2) Pallas , (7) Iris , (3) Juno, and (6) Hebe are brighter than Neptune. In strong binoculars or a telescope , it appears as a blue disc, the appearance of which is similar to Uranus. The blue color comes from the methane in its atmosphere. The apparent diameter is about 2.5 arc seconds . Its small apparent size makes observation a challenge. Most of the data from telescopes was very limited until the Hubble Space Telescope and terrestrial telescopes with adaptive optics began operating .

Like all planets and asteroids beyond Earth, Neptune sometimes shows what appears to be a reverse motion . In addition to the beginning of retrograde, there are other events in a synodic period such as opposition , the return to right-hand motion, and the conjunction with the sun.

Discovery and naming

Urbain Le Verrier , the mathematician who helped discover Neptune

Even Galileo Galilei Neptune had seen on December 28, 1612 and again on 27 January 1613th From his notes of January 1613 an observation of the conjunction with Jupiter emerges, in which Galileo had mistaken Neptune for a Jupiter moon or a fixed star . At the time of its first observation in December 1612, the planet was stationary because that very day it began to move backwards. This was the beginning of the annual cycle of the retrograde movement. Neptune's movement was far too small to be seen with Galileo's small telescope . If he had observed Neptune just a few days earlier, his movement in the sky would have been much clearer.

Another sighting before the actual discovery was made by Michel Lefrançois de Lalande (1766-1839), the nephew of Jérôme Lalande . Michel Lalande was involved in the preparation of a star catalog and observed Neptune on May 8 and 10, 1795. He thought the luminous point was a star and first entered it on a map. Two days later he corrected the position because he was no longer sure about the entry. Thereby he took the opportunity to recognize this change of position as a sign of a planetary movement, so that the discovery escaped him.

In 1821 Alexis Bouvard published astronomical tables on the orbit of Uranus, which was discovered by chance in 1781 . Subsequent observations revealed significant discrepancies with the calculated values. The movement of Uranus around the sun showed disturbances and did not correspond to Kepler's laws . Astronomers like Bouvard therefore suspected that there must be another planet beyond Uranus, which due to its gravitational force disrupts the movement of Uranus. In 1843, John Adams calculated the orbit of this hypothetical further planet and sent his calculations to Sir George Airy , the then "Astronomer Royal". He asked Adams for a more detailed explanation. Adams began a letter of reply, but he never sent it.

Independently of this, in 1846 the French mathematician Urbain Le Verrier calculated the position at which the unknown planet should be, whereby Le Verrier's calculation was much more accurate than that of Adams. But this work did not arouse much interest either. John Herschel advocated the mathematical approach later that year and persuaded James Challis to track down the planet. In July 1846, after a long delay, Challis reluctantly began the search. Adams's calculation was used by Challis from Cambridge as a template for his observations on August 4 and 12, 1846. It was only later that Challis realized that he had observed the planet twice. The identification failed because of his sloppy attitude towards this work. Because Challis had not yet compared the observations of the various evenings, he did not yet recognize Neptune as a planet among the numerous stars, although it changed its position in the sky.

Meanwhile, Le Verrier asked in a letter to Johann Gottfried Galle , an observer at the Berlin observatory , which had a powerful lens telescope with an objective of 23 centimeters in diameter and 4.30 meters of focal length, to keep an eye out for the predicted planet: “I'm looking for one stubborn observer who would be willing to spend some time investigating a section of the sky in which there may be a planet to be discovered. ” He described the calculated position and pointed out that the planet, with an estimated diameter of just over three arc seconds in the telescope, was small disc recognizable and should be distinguishable from a fixed star. The letter arrived in Berlin on September 23, 1846, and Galle received permission from the director of the observatory, Franz Encke , to search for the planet. That same evening, Galle, together with the observatory assistant Heinrich d'Arrest, looked for a planetary disk in the region of the sky in question, but was initially unsuccessful.

Finally, D'Arrest suggested comparing the stars with the Berlin academic star maps. The observatory actually owned the relevant sheet of the still very incomplete map series, namely the " Hora  XXI" , which was only recently completed by Carl Bremiker and not yet commercially available . Back at the telescope, Galle began to announce the stars visible in the telescope, while d'Arrest compared these stars with the map. It wasn't long before d'Arrest shouted: “This star is not on the map!” Together with the summoned Encke, they repeatedly measured the coordinates of the star 8th magnitude, which was found in the sky but not on the map, and believed it was moving slightly to see, but they could not yet determine with certainty. The suspicious star was only about one degree from the predicted position. The next evening, new position measurements left no doubt that the star had meanwhile moved, namely by the amount that was to be expected according to the orbit calculated by Le Verrier. Closer inspection showed a small disc, estimated to be a good two and a half arcseconds in diameter. Galle was able to report the success of the short search to Le Verrier: "The planet whose position you have calculated actually exists" . This made Neptune the first and only planet to be discovered not through systematic search, but through mathematical prediction.

After the background to the discovery became known, there was broad agreement that both Le Verrier and Adams, together with Galle, deserved the honor of the discovery. However, with the rediscovery of the "Neptune papers" (historical documents from the " Royal Greenwich Observatory ") this matter was reopened. They were in the possession of the astronomer Olin Eggen and were apparently suppressed by him for nearly three decades. They were rediscovered in 1998 immediately after his death. After reviewing the documents, some historians believed that Le Verrier deserved more credit as an explorer than Adams.

Shortly after its discovery, Neptune was referred to simply as "the planet outside Uranus" or "Le Verrier's planet". The first suggestion of a name came from Galle. He suggested the name " Janus " . In England, Challis advertised " Oceanus " . In France, François Arago suggested naming the new planet “LeVerrier” . This proposal was vehemently rejected outside France . French yearbooks immediately reintroduced the name "Herschel" for Uranus and "Leverrier" for the new planet.

Meanwhile, Adams independently proposed changing the name from Georgian to Uranus , while Le Verrier proposed the name "Neptune" for the new planet. Friedrich Struve supported the name on December 29, 1846 opposite the St. Petersburg Academy of Sciences. "Neptune" soon became the internationally accepted name. In Roman mythology, Neptunus was the god of the sea, who had his counterpart in the Greek god Poseidon . The name was in accordance with the mythological names of the other planets, all of which except Uranus were named in ancient times.

In East Asian languages ​​(Chinese, Japanese, Korean, Vietnamese) the name of the planet has been translated literally as Sea King Star .

In India the planet was named Varuna (Devanāgarī: वरुण), after the god of the sea in historical Vedic / Hindu mythology. This god corresponds to Poseidon in Greek and Neptune in Roman mythology.


Neptune 2 hours before Voyager 2's closest approach . It shows a vertical relief and light stripes of cloud. The clouds are 50–160 kilometers wide and thousands of kilometers long.

Voyager 2 was the first and so far only spacecraft that Neptune has visited. It flew over its north pole and passed the planet on August 25, 1989 at a distance of only 4950 kilometers. This was the closest approach to an object since the probe left Earth. Since this was the last major planet Voyager 2 was able to visit, it was decided, regardless of the consequences of its trajectory, that a close gravity deflection (fly-by) to the moon Triton should take place. This was also done when Voyager 1 met Saturn and its moon Titan .

Voyager 2 examined the atmosphere, rings, magnetosphere, and the moons of Neptune. The probe discovered the “Great Dark Spot”, the almond-shaped “Small Dark Spot” (D2) and a bright cloud moving rapidly high above the cloud cover, which was called the “Scooter”.

Because of the great distance, the sun appears over 1000 times weaker than on earth, although it still shines very brightly with a brightness of −21 m . Therefore it was astonished to find out that the strongest winds of all giant planets blow on Neptune.

Four rings were found by the probe and the ring arcs were detected. With the help of their “Planetary Radio Astronomy Instrument”, a Neptune day could be determined to be 16 hours and 7 minutes. There were auroras detected (Aurora), similar to the Earth, but were much more complex than this.

Voyager 2 discovered six moons. Three moons were photographed in detail: Proteus, Nereid, and Triton. Although Nereid was discovered as early as 1949, very little was known about the moon. The probe approached Triton within 40,000 km. The Trabant was Voyager 2's final mission target. Triton revealed remarkably active geysers and polar caps discovered . A very weak atmosphere with thin clouds was found on the satellite.

The images sent to Earth by Voyager 2 became the basis of a PBS night program called "Neptune All Night".

Possible future missions

In August 2015, NASA commissioned the Jet Propulsion Laboratory to design an orbiter for Uranus and Neptune. This could start in the late 2020s or early 2030s.

See also


  • Patrick Moore , Garry Hunt, Iain Nicholson, and Peter Cattermole: Atlas of the Solar System. Royal Astronomical Society and Herder-Verlag, 465 pages, Freiburg / Basel / Vienna 1986, ISBN 3-451-19613-1 .
  • Tom Standage : The Neptune Files. The adventurous story of the discovery of the 8th planet. Campus Verlag, Frankfurt / New York 2000, ISBN 3-593-36676-2 .
  • Morton Grosser: Discovery of the planet Neptune. Suhrkamp, ​​Frankfurt am Main 1970.
  • Ellis D. Miner et al .: Neptune - the planet, rings and satellites. Springer, London 2002, ISBN 1-85233-216-6 .
  • Garry E. Hunt et al .: Atlas of Neptune. Cambridge Univ. Press, Cambridge 1994, ISBN 0-521-37478-2 .
  • Patrick GJ Irwin: Giant planets of our solar system - atmospheres, composition, and structure. Springer, Berlin 2009, ISBN 978-3-540-85157-8 .
  • William Sheehan, Nicholas Kollerstrom, Craig B. Waff: The Neptune Affair . Spectrum of Science, April 2005, pp. 82-88 (2005), ISSN  0170-2971 .

Web links


Commons : Neptun  - album with pictures, videos and audio files
Wiktionary: Neptune  - explanations of meanings, word origins, synonyms, translations
Wikibooks: Neptune  - learning and teaching materials

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