Neptune is the eighth planet in order of distance from the Sun and the most distant known from the Solar System. It orbits the Sun at a distance of about 30.1 au (4.5 billion kilometers), with an orbital eccentricity half that of the Earth and a period of revolution of 164.79 years. It is the third most massive planet in the Solar System and the fourth largest in size — a little more massive but a little smaller than Uranus. In addition, it is the densest giant planet.
| Orbital characteristics | |
|---|---|
| Semi-major axis | 4,498,400,000 km (30,069 9 au) |
| Aphelion | 4,537,000,000 km (30,328 au) |
| Perihelion | 4,459,800,000 km (29,811 6 au) |
| Orbital circumference | 28,263,700,000 km (188.931 au) |
| Eccentricity | 0,008 59 |
| Period of revolution | 60 216.8 days (≈ 164.86 a) |
| Synodic period | 367.429 days |
| Average orbital speed | 5.432 48 km/s |
| Maximum orbital speed | 5.479 5 km/s |
| Minimum orbital speed | 5.386 1 km/s |
| Tilt on the ecliptic | 1,77° |
| Ascending node | 131,784° |
| Perihelion argument | 273,2° |
| Known satellites | 14, including Triton. |
| Known rings | 5 main ones |
| Physical characteristics | |
| Equatorial radius | 24,764 ± 15 km (3,883 Earths) |
| Polar radius | 24,341 ± 30 km (3,829 Earths) |
| Volumetric mean radius |
24,622 km (3,865 Earths) |
| Flattening | 0,0171 |
| Equatorial perimeter | 155,597 km |
| Area | 7,640 8 × 109 km2 (14,98 Earths) |
| Volume | 6,252 6 × 1013 km3 (57,74 Earths) |
| Mass | 1,024 3 × 1026 kg (17,147 Earths) |
| Global density | 1,638 kg/m3 |
| Surface gravity | 11.15 m/s2 (1.14 g) |
| Release speed | 23.5 km/s |
| Rotation period (sidereal day) |
0.671 25 days (16 h 6.6 min) |
| Rotational speed (at the equator) |
9,660 kph |
| Axis tilt | 28,32° |
| Right ascension of the North Pole | 299,36° |
| Declination of the North Pole | 43,46° |
| Visual geometric albedo | 0,41 |
| Bond’s Albedo | 0,29 |
| Solar irradiance | 1.51 W/m2 (0.001 Earth) |
| Blackbody equilibrium temperature |
46.6 K (−226.4 °C) |
| Surface temperature | |
| • Temperature at 10 kPa | 55 K (−218 °C) |
| • Temperature at 100 kPa | 72 K (−201 °C) |
| Characteristics of the atmosphere | |
| Density at 100 kPa |
0.45 kg/m3 |
| Ladder height | 19.1 to 20.3 km |
| Average molar mass | 2.53 to 2.69 g/mol |
| Dihydrogen H2 | 80 ± 3.2% |
| Helium He | 19 ± 3.2% |
| Methane CH4 | 1.5 ± 0.5% |
| HD hydrogen deuteride | 190 ppm |
| Ammonia NH3 | 100 ppm |
| Ethane C2H6 | 2.5 ppm |
| Acetylene C 2h2 | 100 ppb |
| History | |
| Discovered by | Urbain Le Verrier, Johann Gottfried Galle on the indications of Urbain Le Verrier. |
| Discovered on | August 31, 1846 September 23, 1846 |
Not visible to the naked eye, Neptune is the first celestial object and the only one of the eight planets in the Solar System to have been discovered by deduction rather than empirical observation. Indeed, the French astronomer Alexis Bouvard had noted unexplained gravitational perturbations in the orbit of Uranus and conjectured in the early nineteenth century that an eighth planet, more distant, could be the cause. The British astronomer John Couch Adams in 1843 and the French astronomer Urbain Le Verrier in 1846 independently calculated the predicted position of this hypothetical planet.
Thanks to the latter’s calculations, it was finally observed for the first time on September 23, 1846, by the Prussian astronomer Johann Gottfried Galle, to a degree from the predicted position. Although Galle used Le Verrier’s calculations to discover the planet, the authorship of the discovery between Adams and Le Verrier was long disputed. Its largest moon, Triton, was discovered 17 days later by William Lassell. As of 2013, 14 natural satellites of Neptune are known. The planet also has a weak and fragmented ring system and a magnetosphere.
The distance of the planet from the Earth gives it a very small apparent size, its study is difficult with telescopes located on Earth. Neptune was visited only once during the Voyager 2 mission, which flew by it on August 25, 1989. The advent of the Hubble Space Telescope and large ground-based adaptive optics telescopes then allowed for additional detailed observations.
Like those of Jupiter and Saturn, Neptune’s atmosphere is composed mainly of hydrogen and helium as well as traces of hydrocarbons and possibly nitrogen, although it contains a higher proportion of “ice” in the astrophysical sense, i.e. volatile substances such as water. ammonia and methane. However, like Uranus, its interior is mainly composed of ice and rocks, hence their name “ice giants”. In addition, methane is partially responsible for the blue hue of Neptune’s atmosphere, although the exact origin of this azure blue remains unexplained.
Moreover, unlike the foggy and relatively featureless atmosphere of Uranus, Neptune’s atmosphere has active and visible weather conditions. For example, at the time of the flyby of Voyager 2 in 1989, the southern hemisphere of the planet had a Great Dark Spot comparable to the Great Red Spot on Jupiter. These weather patterns are driven by the strongest winds known in the Solar System, reaching speeds of 2,100 km/h. Due to its great distance from the Sun, its outer atmosphere is one of the coldest places in the Solar System, with cloud tops temperatures approaching 55 K (−218.15 °C).
The planet is named after Neptune, god of the seas in Roman mythology, and has as its astronomical ♆ symbol, a stylized version of the god’s trident.
Discovery of Neptune
First appearances
Neptune is not visible to the naked eye. It, therefore, took the invention of the telescope to be able to observe it. However, this discovery differs from that of the other planets because it is above all mathematical: it is made by calculation from the trajectory and characteristics of Uranus. Thus, the telescope was then only used to confirm the discovery.
Several astronomers, before its discovery in the nineteenth century, observe it without noting that it is a planet. Thus, Galileo’s astronomical drawings show that he observed Neptune on 28 December 1612 while it appears in conjunction with Jupiter. The planet is then listed as a simple fixed star. He noticed her again in the sky a month later, on January 28, 1613, and a 2009 study suggests that he even finds that it has moved relative to a nearby star.
Thus, it cannot be a fixed star, but Galileo does not draw any conclusions and does not evoke it afterward. As he thought he had observed only one star, he is not credited with his discovery. Neptune is also observed by Joseph Jérôme Lefrançois de Lalande (1732 – 1807) in 1795 and by John Herschel, son of William Herschel, who previously discovered Uranus, in 1830, without them noting anything in particular, also taking it for a star.
Mathematicians began in 1788 to observe that the planet Uranus, recently discovered, does not have an orbit that seems to conform to existing models. Also, the more time passes, the more the error between the announced position of the celestial body and that recorded increases. Jean-Baptiste Joseph Delambre tries to explain the anomalies by adding the gravitational influence of Jupiter and Saturn in his calculations. His tables are then more accurate, but still do not allow to predict the movement of the planet in the long term. In 1821, the French astronomer Alexis Bouvard published new tables using 17 observations spread over the 40 years since its discovery to attempt, in vain, to explain the orbit of Uranus. Subsequent observations revealed substantial deviations from the tables, leading Bouvard to hypothesize that an unknown body would disrupt the orbit by gravitational interaction.
Search for a transuranic planet
At a meeting of the British Science Association, George Biddell Airy reported that Bouvard’s tables were erroneous in the order of one minute of a degree. Mainly two hypotheses are then opposed: that of Bouvard on the existence of another planet still unknown, which could affect the movements of Uranus, and that of a questioning of the universal law of gravitation, proposed by Airy – according to him, the law of gravitation would lose its validity as one moves away from the Sun. However, the existence of a new trans-uranic planet is a consensus among most astronomers to explain the perturbations in Uranus motion.
As a student at Cambridge, John Couch Adams found on June 26, 1841, Airy’s report on the problem of the orbit of Uranus and is interested in the question. In 1843, once his studies were finished, he set to work and relied on the Titius-Bode law to obtain a first approximation of the distance of this new planet from the Sun. Since most planets — except Mercury — have a weakly eccentric orbit, he also assumes that its orbit is circular in order to simplify calculations. He completed his work two years later, having determined Neptune’s position with an error of less than two degrees to the actual position, but he still had to confirm by observation. James Challis, director of the Cambridge Observatory, referred him to the Astronomer Royal Sir George Biddell Airy. He initially expressed doubts about the work of his young colleague.
At the same time, in France, François Arago – then director of the Paris Observatory – encouraged the mathematician Urbain Le Verrier, specialized in celestial mechanics, to determine the characteristics of this eighth planet. He began his work on Uranus in 1845 and, totally ignoring Adams’ work, used a different and independent method, and published his first results on November 10, 1845, in First dissertation on the theory of Uranus, then in Research on the movements of Uranus on June 1, 1846.
Airy, noticing the work of the French astronomer, made the parallel with those of Adams and came into contact with Le Verrier. The latter in turn asks him to carry out the research of the planet using the calculations he has just published, but Airy refuses. Finally, under pressure from George Peacock, Airy asked James Challis for July 12, 1846, to undertake the search for the new celestial body through a telescope.
Adams, informed by the director of Cambridge, provides new coordinates to Challis, specifying that the object would be magnitude 9, but Airy proposes to Challis to observe a large portion of the sky and up to a magnitude. This method requires Challis much more observation time, especially since he does not have reliable maps of the area to be observed. Challis began his research on August 1, 1846then travels the sky in August and September, without being able to identify it.
Discovery of the planet
Le Verrier communicates its final results to the French Academy of Sciences on August 31, 1846. Faced with the lack of enthusiasm of French astronomers, he decided to call on one of his acquaintances, the Prussian astronomer Johann Gottfried Galle, of the Berlin Observatory. Heinrich d’Arrest, a student at the observatory, suggested that Galle compare a recently drawn sky map in the region of the location predicted by Le Verrier with the current sky, to look for the characteristic displacement of a planet, as opposed to a star.
On September 23, 1846, Galle receives by letter the position of the planet. He discovered Neptune the same evening, pointing his telescope at the indicated place; it is then only one degree from the location calculated by Le Verrier. He observed it again the next day, to check if the star had moved, before confirming that it was indeed the planet sought. Triton, its largest natural satellite, was discovered by William Lassell 17 days after Neptune.
Across the Channel, the disappointment is great. Challis, reviewing his notes, discovers that he has observed Neptune twice, on the 4th and August 12, but did not recognize it as a planet because it lacked an up-to-date star map and was distracted by its simultaneous work on comet observations. In addition, a fierce nationalist rivalry began between the French and the British in order to attribute the paternity of the discovery. The British put forward Adams’ papers while the French refuted by recalling that only an official publication could validate the discovery, and refused on principle that Adams’ name appear next to that of Le Verrier in history books. In June 1847, Adams and Le Verrier first met at the British Association for the Advancement of Science and subsequently maintained a friendly relationship.
Finally, an international consensus emerged that Le Verrier and Adams should have joint credit. However, since 1966, Dennis Rawlins has questioned Adams’ claim to co-discovery and the question is re-evaluated by historians with the return in 1998 of the “Neptune papers” to the Royal Observatory of Greenwich. After studying the documents, the account suggests that “Adams does not deserve the same credit as Le Verrier for the discovery of Neptune. This credit belongs only to the person who managed both to predict the position of the planet and to convince astronomers to look for it”.
Name
Shortly after its discovery, Neptune is simply called “the planet outside Uranus” or “planet Le Verrier”. The first name suggestion comes from Johann Galle, who proposed the name “Janus”, from the Roman god of beginnings and ends, choices and gates. In England, Challis proposed the name “Oceanus”, a Titan son of Ouranos (Greek equivalent of Uranus).
Claiming the right to name his discovery, Le Verrier quickly proposed the name “Neptune” for this new planet, while falsely claiming that it had been officially approved by the Bureau des Longitudes. In October 1848, he changed his mind and sought to name the planet “Le Verrier”, after him, having the faithful support of the director of the observatory, François Arago. This suggestion, however, met with strong resistance outside France. French almanacs reintroduced the name “Herschel” for Uranus, after the discoverer of this planet, Sir William Herschel, and “Leverrier” for the new planet.
Wilhelm von Struve spoke in favor of the name “Neptune” on December 29, 1846, at the St. Petersburg Academy of Sciences. Also, “Neptune” quickly became an internationally accepted name. In Roman mythology, Neptune is the god of the sea, identified with the Greek god Poseidon. The request for a mythological name is also in accordance with the nomenclature of the other planets, which, with the exception of Earth, are named after Roman mythology.
Most languages today use a variant of the name “Neptune” for the planet. In Chinese, Vietnamese, Japanese, and Korean, the planet’s name is translated as “Star of the Sea King” (海王星). In modern Greek, the planet is called “Poseidon” (Ποσειδώνας / Poseidónas). In Hebrew, רהב (Rahab), named after a mythical sea monster mentioned in the Book of Psalms, was selected in a vote administered by the Academy of the Hebrew Language in 2009 as the official name of the planet — although the name נפטון (Neptun) remains commonly used. Finally, in Maori, Nahuatl and Gujarati, the planet takes the names respectively of the Maori sea god Tangaroa, the rain god Tlāloc and the Hindu god of the ocean Varun.
After the discovery of Neptune
Neptune is the only one of the eight known planets to have been discovered by mathematical calculation rather than empirical observation. Unlike the other seven planets, Neptune is never visible to the naked eye: its apparent magnitude is between 7.6 and 8.0 and makes it a star about four times less bright than the faintest stars visible to the naked eye whose apparent magnitude is 6. It appears as a blue-green disc only through a telescope.
During the nineteenth and early twentieth centuries, astronomers believed that Neptune was, like Uranus, a terrestrial planet. In 1909, scientists believed they were observing the green band in the spectrum of Neptune, characteristic of the presence of chlorophyll, and the hypothesis of plant life on this planet was put forward. However, a few years later, we realize that this band actually comes from the use of orthochromatic plates.
At the end of the nineteenth century, it is suggested that the irregularities observed in the motion of Uranus and Neptune stem from the presence of another planet even more distant. After extensive research, Pluto was discovered on February 18, 1930 at the coordinate point provided by the calculations of William Henry Pickering and Percival Lowell for Planet X.
However, the new planet is too far away to generate the irregularities observed in the motion of Uranus, while those observed for Neptune resulted from an error in estimating the mass of the planet (which was identified with the Voyager 2 mission). The discovery of Pluto is therefore rather fortuitous. Because of its great distance, knowledge of Neptune remained low at least until 1949, when Gerard Kuiper discovered its second moon Nereid. In the 1970s and 1980s, clues were obtained about the probable presence of planetary rings or at least fragments around Neptune. In 1981, a team led by Harold Reitsema observed a third of his satellites, Larissa.
Status
From its discovery in 1846 until the discovery of Pluto in 1930, Neptune was the most distant planet known. With this discovery, Neptune became the penultimate planet, except for a period of 20 years between 1979 and 1999, when Pluto’s elliptical orbit made it closer to the Sun than Neptune. Finally, the discovery of the Kuiper belt in 1992 led many astronomers to debate the question of Pluto and whether it should still be considered a planet or as part of the Kuiper belt. In 2006, the International Astronomical Union defined the word “planet” for the first time, reclassifying Pluto as a “dwarf planet” and making Neptune again the farthest planet from the Sun.
Neptune physical characteristics
Mass and diameter of Neptune
With a mass of 1.024 × 1026 kg, Neptune is an intermediate body between Earth and gas giants such as Jupiter or Saturn. Indeed, the Neptunian mass is 17 times larger than that of the Earth but 1/19th of the Jovian mass. The planet’s equatorial radius is 24,764 km, about four times that of Earth. Its gravity at 1 bar is 11.15 m/s2, or 1.14 times the surface gravity on Earth, surpassed only by Jupiter in the Solar System.
Because of gravitational compression, Neptune is smaller than Uranus (49,528 km in diameter for Neptune, against 51,118 km for Uranus) because it is more massive than the latter (Uranus has a mass of 8,681×1025kg;).
In addition, Neptune and Uranus are often considered subclass of giant planets, called “ice giants” because of their smaller size and higher concentration of volatile substances compared to Jupiter and Saturn. In the search for exoplanets, Neptune is used as a metonymy: discovered bodies with a similar mass are indeed referred to as “Neptunes”, for example, hot or cold Neptunes.
Internal structure
The internal structure of Neptune would be similar to that of Uranus. Also, although its density is three times lower than that of Earth, it is the densest giant planet in the Solar System. This implies that a larger percentage of its interior is composed of molten ice and rocky material. Thus, it probably has a solid core composed of iron, nickel and silicates, with a mass of about 1.2 times that of the Earth. The pressure at the center would be about 8 Mbar (800 GPa) — about twice as high as at the center of the Earth — and the temperature about 8,100 K (7,826.85 °C) — more than that prevailing in the Earth’s inner core and on the surface of the Sun.
Above this core, like Uranus, Neptune could have a fairly uniform composition (different ices, hydrogen and helium) and not a “layered” structure like Jupiter and Saturn. However, several current models of the structure of Uranus and Neptune propose the existence of three layers: a rocky core, a median layer ranging from icy to liquid and formed of water, methane and ammonia, and an atmosphere of hydrogen and helium, although reality can be more complex.
In 1981, theoretical studies and experiments carried out by laser compression led Marvin Ross, of Lawrence Livermore National Laboratory, to propose that this layer be totally ionized and that methane be pyrolyzed into carbon in the form of metal or diamond. Methane breaks down into carbon and hydrocarbons. Then, carbon precipitation releases energy—gravitational potential energy converted into heat—causing convection currents that release hydrocarbons into the atmosphere. This model would explain the presence of various hydrocarbons in Neptune’s atmosphere.
In 2017, new experiments simulating the conditions presumed to prevail around 10,000 km below the surface of Uranus and Neptune reinforced this model by producing nano-cut diamonds. These conditions of high temperature and pressure cannot be maintained for more than a nanosecond on Earth but, under conditions in the atmospheres of Neptune or Uranus, nano-diamonds would have time to grow to give rains of diamonds. It is also hypothesized that this type of diamond shower occurs on Jupiter and Saturn. Also, the top of the mantle can be an ocean of liquid carbon where solid “diamonds” float.
The mantle is equivalent to between 10 and 15 Earth masses and is rich in water, ammonia and methane. As is customary in planetary science, this mixture is called icy even though it is a hot and dense fluid. This fluid, which has a high electrical conductivity, is sometimes referred to as the water-ammonia ocean. The mantle may consist of a layer of ionic water in which water molecules break down into hydrogen and oxygen ions, and deeper into superionic water, in which oxygen crystallizes but hydrogen ions float freely in the oxygen network.
Internal heat
Neptune’s varied climate compared to Uranus is partly due to its higher internal heat. The upper regions of Neptune’s troposphere reach a low temperature of 55 K (−218.15°C). At a depth where the atmospheric pressure is equal to 1 bar (100 kPa), the temperature is 72 K (−201.15°C). Deeper inside the gas layers, the temperature rises steadily.
As with Uranus, the source of this warming is unknown. However, the gap is greater on Neptune: if Uranus radiates 1.1 times more energy than it receives from the Sun, Neptune radiates about 2.61 times more energy than it receives. Thus, even though Neptune is 50% farther from the Sun than Uranus and therefore receives only 40% of its sunlight, its internal heat is sufficient to generate the fastest planetary winds in the Solar System.
Depending on the thermal properties of its interior, the heat resulting from the formation of the planet may be sufficient to explain this current heat flux, although it is difficult to simultaneously explain Uranus’ lack of internal heat while observing the apparent similarity between the two planets. It is also possible that atmospheric activities on the two icy giants are more dependent on solar irradiation than on the amount of heat escaping from their interiors.
However, the temperature of Neptune is far from being stabilized. Thanks to nearly a hundred images taken with the Very Large Telescope, a team of researchers obtained results that cannot be explained by the current theory. On the one hand, between 2003 and 2018, the average temperature would have dropped by 8 °C, despite the arrival of the austral summer. On the other hand, a rapid warming is observed at the South Pole of Neptune from 2018 to 2020, with nearly 11 °C increase during these two years alone.
Atmosphere
Neptune’s atmosphere, more than 8,000 km thick, is composed by a volume of about 80% hydrogen and 19% helium with about 1.5% methane CH4 — the fact that the sum is more than 100% is due to uncertainties about these proportions. Traces of ammonia (NH3), ethane (C 2 H6) and acetylene (C 2 H 2) were also detected. Its atmosphere forms about 5% to 10% of its mass and represents 10% to 20% of its radius.
Neptune’s blue color comes mainly from methane, which absorbs light in red wavelengths. Indeed, significant methane absorption bands exist at wavelengths of the electromagnetic spectrum greater than 600 nm. However, the azure color of Neptune’s atmosphere cannot be explained by methane alone — which would give a color closer to the aquamarine of Uranus — and other chemical species, for the moment unidentified, are certainly at the origin of this particular hue. Indeed, the atmospheric methane content of Neptune being similar to that of Uranus, they would otherwise have the same color.
Neptune’s atmosphere is divided into two main regions: the lower troposphere, where temperature decreases with altitude, and the stratosphere, where temperature increases with altitude. The boundary between the two, the tropopause, is at a pressure of 0.1 bar (10 kPa). The stratosphere then gives way to the thermosphere towards pressures close to 10-5 to 10−4 bar (1 to 10Pa) and then gradually passes to the exosphere.
Models suggest that Neptune’s troposphere is surrounded by clouds of varying compositions depending on altitude. Upper-level clouds are found at pressures below one bar, where the temperature allows methane to condense. For pressures between one and five bars (100 and 500kPa), clouds of ammonia and hydrogen sulfide would form. Above a pressure of five bar, clouds can consist of ammonia, ammonium sulfide, hydrogen sulfide and water. Deeper, around 50 bar and where the temperature reaches 0 ° C, it would be possible to find clouds of water ice.
High-altitude clouds over Neptune have been observed, casting shadows on the opaque cloud bridge below. There are also high-altitude cloud bands that surround the planet at a constant latitude. These circumferential bands are 50 and 150 km wide and are about 50 and 110km above the cloud bridge. These altitudes correspond to the troposphere, where climatic phenomena occur.
Images of Neptune suggest that its lower stratosphere is cloudy due to condensation of methane’s ultraviolet photolysis products, such as ethane and ethine. The stratosphere also contains traces of carbon monoxide and hydrogen cyanide. Neptune’s stratosphere is warmer than that of Uranus due to its high concentration of hydrocarbons.
For reasons that remain unclear, the thermosphere is at an abnormally high temperature of about 750 K (476.85 °C), the planet being too far from the Sun for this heat to be generated by ultraviolet radiation. The heating mechanism could be the atmospheric interaction with ions in the planet’s magnetic field. It could also be the result of gravity waves dissipating into the atmosphere. The thermosphere contains traces of carbon dioxide and water, which may have been deposited from external sources such as meteorites and dust. An ionosphere consisting of several layers has also been discovered between 1,000 and 4,000km above level 1 bar.
The temperature measured in the upper layers of the atmosphere is of the order of 55K (−218 °C), the lowest average measured on a planet in the Solar System, after Uranus.
Climate on Neptune
The climate on Neptune is characterized by large storm systems, with winds exceeding 2,000 km/h (about 550 m/s), almost a supersonic flow in the planet’s atmosphere — where the speed of sound is twice as great as on Earth. These winds are also the fastest in the Solar System. By following the movement of persistent clouds, it has been observed that wind speeds vary from 20 m/s when they go eastward to 325m/s when they go west. At cloud tops, prevailing winds vary in speed from 400 m/s along the equator to 250m/s at the poles. Most winds on Neptune move in a direction opposite to the planet’s rotation. The general pattern of winds also shows a prograde rotation at high latitudes compared to a retrograde rotation at low latitudes. This difference in flow direction would be a kind of skin effect and not the result of deeper atmospheric processes.
Neptune differs greatly from Uranus in its typical level of meteorological activity. Indeed, no comparable phenomenon has been observed on Uranus according to the observations of Voyager 2 in 1986.
The abundance of methane, ethane and acetylene at Neptune’s equator is 10 to 100 times greater than that of the poles. This is interpreted as evidence of phenomena similar to upwelling at the equator caused by winds and then a water dive near the poles. Indeed, photochemistry otherwise cannot explain the distribution without meridian circulation.
In 2007, it was discovered that the upper troposphere at Neptune’s south pole is about 10 degrees warmer than the rest of its atmosphere, which has an average temperature of about 73 K (−200.15 °C). The temperature differential is sufficient to allow methane, which is frozen elsewhere in the troposphere, to escape into the stratosphere near the pole. This relative hot spot is due to Neptune’s axial tilt, which exposes the south pole to the Sun during the last quarter of Neptune’s year, or about 40 Earth years. As Neptune slowly moves to the opposite side of the Sun, the south pole becomes darkened and the north pole illuminated, causing this hot spot to move toward the north pole.
Due to seasonal changes, as the planet enters its spring in its southern hemisphere, the cloud bands of Neptune’s southern hemisphere increase in size and albedo. This trend was first observed in 1980, and is expected to last until the 2020s, due to the forty-year seasons on Neptune due to its long period of revolution.
Storms
During the passage of Voyager 2 in 1989, the most distinctive mark of the planet was the “Great Dark Spot”, which was about half the size of Jupiter’s “Great Red Spot”. This spot was a gigantic anticyclone covering 13,000 × 6,600 km that could travel at more than 1,000 km/h.
The Great Dark Spot generated large white clouds, just below the tropopause. Unlike clouds in Earth’s atmosphere, which are composed of water ice crystals, Neptune’s clouds are made up of methane crystals. Also, while cirrus clouds on Earth form and then disperse in a few hours, those of the Great Spot were still present after 36 hours (two rotations of the planet).
In November 1994, the Hubble Space Telescope detects that it has disappeared completely, telling astronomers that it had been covered or dissipated. The persistence of the accompanying clouds proves that some old spots can remain in the form of cyclones. However, an almost identical spot had then appeared in the northern hemisphere of Neptune. This new spot, called the Great Northern Dark Spot (NGDS), remained visible for several years. In 2018, a similar new spot was detected by Hubble.
The Scooter is a group of white clouds further south of the Great Dark Spot. This nickname first appeared in the months leading up to the flyby of Voyager 2 in 1989 because it was then observed to travel at faster speeds than the Great Dark Spot. The Little Dark Spot is a cyclone even further south, the second most intense storm observed during the 1989 flyby. In early images, it is completely dark but as Voyager 2 approached the planet, a bright core developed and can be seen in most high-resolution images. These two spots had also disappeared during the 1994 observation by Hubble.
These dark spots occur in the troposphere at lower altitudes than brighter clouds, so they appear as holes in upper cloud bridges. Since these are stable features that can persist for several months, it is assumed that they have vortex structures. Brighter, more persistent methane clouds that form near the tropopause are often associated with dark spots. Dark spots can dissipate when they migrate too close to the equator or possibly by some other unknown mechanism.
Magnetosphere
Neptune’s magnetosphere resembles that of Uranus, with a magnetic field strongly inclined 47° relative to its axis of rotation and offset by at least 0.55 radius from the physical center of the planet (about 13,500 km).
Before Voyager 2 arrived on Neptune, it was assumed that Uranus’ tilted magnetosphere was the result of its sideward rotation. However, comparing the magnetic fields of the two planets, it is now assumed that this extreme tilt may be characteristic of magnetic fluxes coming from the planets’ interiors and is not the result of its physical shift or polarity reversal. This field would then be generated by convective fluid movements in a thin spherical layer of electrically conductive liquids (probably a combination of ammonia, methane and water), creating a dynamo effect. However, its characteristics suggest that it could be generated by a different mechanism than those of Earth, Jupiter or Saturn.
The field has a rotation period of 16.11 hours. The dipole component of the magnetic field at Neptune’s magnetic equator is about 14 microtesla (0.14G). Neptune’s dipole magnetic moment is about 2.2 T·m3 (or 14 μT. Rn3, where Rn is the radius of Neptune). Neptune’s magnetic field has a complex geometry that includes relatively large contributions of non-dipole components, including a strong quadrupole moment that can exceed the dipole moment in intensity. Conversely, planets such as Earth, Jupiter and Saturn have only relatively weak quadrupole moments and their fields are less inclined with respect to the polar axis. Neptune’s great quadrupole moment may be the result of its shift from the center of the planet and the geometric constraints of the dynamo generator of the field. In addition, auroras are discovered on the planet by Voyager 2.
Neptune’s bow shock — where the magnetosphere begins to affect the solar wind—occurs at a distance of 34.9 times the planet’s radius. The magnetopause — where the pressure of the magnetosphere counterbalances the solar wind — is at a distance of 23 to 26.5 times the radius of Neptune. The tail of the magnetosphere extends to at least 72 times the radius of Neptune, and probably much further.
Orbital characteristics
Neptune orbit
The semi-major axis between Neptune and the Sun is 4.5 billion kilometers (about 30.1 astronomical units) and it completes an orbit on average every 164.79 ± 0.1 years. The perihelion distance is 29.81 au and 30.33 au aphelion, corresponding to an orbital eccentricity of 0.008,678. Also, Neptune’s orbit is inclined by 1.77° relative to that of the Earth and the plane of the ecliptic.
On July 11, 2011, Neptune completes its first full orbit since its discovery in 1846. However, due to the motion of the Sun relative to the barycenter of the Solar System, Neptune was not on July 11 at the position where it was discovered relative to the Sun. Thus, in the usual heliocentric coordinate system, the longitude of discovery was reached on July 12.
Rotation
The axial inclination of Neptune is 28.32°, which is similar to the inclinations of Earth (23°) and Mars (25°). As a result, Neptune undergoes the same seasonal changes as those known on Earth. Neptune’s long orbital period, however, means that these seasons last forty Earth years, with the planet being in the 2020s in its spring for the southern hemisphere.
Its sidereal day is about 16 hours 7 minutes, defined by the rotation period of the planet’s magnetic field. Indeed, sometime before its flyby of the planet, Voyager 2 detects at regular intervals radio waves, signs of its magnetic field. The latter being generated by electric currents internal to the planet, it was deduced that the period of internal rotation was equal to the time interval between these puffs. This rotation induces a flattening of the planet: the polar radius is 24,341 km while the equatorial radius is 24,764 km (pressure level at 1 bar).
However, since Neptune is not a solid body, its atmosphere undergoes differential rotation. Thus, its equatorial zone rotates with a period of about 18 hours while the period of rotations to the polar regions is 12 hours. This differential rotation is the most pronounced of all the planets in the Solar System and results in strong wind shear in latitude.
Orbital resonances
Neptune’s orbit has a strong impact on the region beyond, known as the Kuiper belt. This is a ring of small icy bodies, similar to the asteroid belt but much larger, extending from the orbit of Neptune at 30 AU to about 55 AU from the Sun. In the same way that Jupiter’s gravity dominates the asteroid belt, shaping its structure, Neptune’s gravity dominates the Kuiper belt. During the evolution of the Solar System, some regions of the Kuiper belt were destabilized by Neptune’s gravity, creating gaps in the structure of the Kuiper belt—for example in the region between 40 and 42 AU.
Orbital resonances occur when the fraction formed by the orbital period of Neptune and that of the object is a rational number, such as 1:2 or 3:4. The most populated resonance in the Kuiper belt, with more than 200 known objects, is the 2:3 resonance. Objects in this resonance make two orbits around the Sun for three of Neptune and are known as plutinos, as the largest known Kuiper belt object, Pluto, is one of them. Although Pluto regularly passes through Neptune’s orbit, the 2:3 resonance ensures that the two objects can never collide. The 3:4, 3:5, 4:7, and 2:5 resonances are less populated in comparison.
Neptune has at least twenty Trojans occupying the two stable Lagrangian points L4 and L5 of the Sun-Neptune system, one leading and the other dragging Neptune into its orbit. Neptune’s Trojan asteroids can be considered to be in 1:1 resonance with Neptune. Some Trojans are remarkably stable in their orbits and probably formed at the same time as Neptune rather than captured.
Training and migration
The formation of the ice giants, Neptune and Uranus, is difficult to model accurately. Current models suggest that the density of matter in the outer regions of the Solar System is too low to account for the formation of such large bodies from the traditionally accepted method of core accretion, also known as the core accretion model. Thus, various hypotheses have been put forward to explain their appearance.
The first is that the ice giants were not formed by accretion of the core, but from instabilities in the original protoplanetary disk that then saw their atmosphere blown by radiation from a massive OB association nearby.
Another is that they formed closer to the Sun, where the density of matter was higher, and then made a planetary migration to their current orbits after the withdrawal of the gaseous protoplanetary disk. This post-formation migration hypothesis is now preferred because of its ability to better explain the occupation of populations of small objects observed in the trans-Neptunian region. The most widely accepted stream of explanations of the details of this hypothesis is known as the Nice model, which explores the effect of a migration of Neptune and the other giant planets on the structure of the Kuiper belt.
Procession of Neptune
Moons
Neptune has 14 known natural satellites.
The most massive is Triton, discovered by William Lassell just 17 days after the discovery of Neptune, on October 10, 1846. It is the 8th by increasing distance to the latter and comprises more than 99.5% of the mass orbiting the planet. This makes it the only one massive enough to undergo sufficient gravitational compression to be spheroidal. Moreover, its diameter of just over 2,700 km makes it the 7th natural satellite of the Solar System by decreasing size — and a celestial body larger than Pluto.
It is also the only known large satellite in the Solar System to have a retrograde orbit — that is, reverse to the direction of rotation of its planet — suggesting that it is an ancient dwarf planet from the Kuiper belt captured by Neptune. Triton orbits Neptune in 5 days and 21 hours on an almost circular trajectory with a semi-major axis of 354,759 km. It is close enough to Neptune to be locked in a synchronous rotation and slowly spirals inward due to tidal acceleration. It will eventually break up in about 3.6 billion years when it reaches its Roche limit.
The inclination of its axis is 156.865° on the Laplace plane of the system, and up to 129.6° (-50.4°) on the orbital plane of its planet. This gives it very marked seasons throughout the Neptunian year, 164.79 Earth years long; The southern hemisphere thus passed its summer solstice in 2000. In 1989, Triton was the coldest object ever measured in the Solar System, with estimated temperatures of 38 K (−235.15 °C).
Nereid, the second satellite of Neptune to be discovered, was not discovered until 1949, more than a century after Triton. Very irregular, it is the third most massive moon in the Neptunian system and has one of the most eccentric orbits of all the satellites of the Solar System — only surpassed by Bestla, a satellite of Saturn. Also, its orbital eccentricity of 0.751 gives it an apoapsis seven times greater than its periapsis (minimum distance to Neptune).
Before the arrival of the Voyager 2 probe in the planet’s system, a single other moon was discovered: Larissa, in 1981, thanks to a star occultation; however, this third moon is only observed again during the flyby of Neptune by the space probe.
Then, the analysis of the photographs transmitted by Voyager 2 in 1989 makes it possible to discover five new satellites: Naïade, Thalassa, Despina, Galatea and Proteus. The first four, the innermost, orbit close enough to be located in the rings of Neptune. Proteus, on the other hand, is a moon of irregular and remarkable shape because it is of the maximum size that an object of its density can reach without being transformed into a spherical shape by its own gravity. Although the second most massive Neptunian moon, it accounts for only 0.25% of Triton’s mass.
Five new irregular moons were discovered between 2002 and 2003 and named Psamathea, Halimède, Sao, Laomediae and Neso in February 2007. In 2013, the last moon discovered is the smallest known to date, Hippocampus, is obtained by combining several Hubble images. Because Neptune is the Roman god of the sea, Neptune’s moons are named after inferior sea gods.
Planetary rings
Neptune has a system of planetary rings, although much less substantial than that of Saturn. The rings are dark and their composition and origin are uncertain: they may consist of ice particles covered with silicates, dust or carbon-based material, which most likely gives them a reddish hue.
William Lassell first mentions the existence of rings in 1846, however, this could have been an aberration of light. The first reliable detection of a ring was made in 1968 but went unnoticed until 1977, after the discovery of the rings of Uranus which pushed scientists to look for them around Neptune. From this, evidence of the presence of rings is reported. During a stellar occultation in 1984, the rings obscure a star during immersion but not during emersion, which then suggests that they may have gaps.
It is the images of Voyager 2 in 1989 that reveal its existence, that they are indeed “whole” and that there are several. One of them, the Adams ring, has “arcs” — that is, parts brighter than the rest of the ring — which are named counterclockwise Liberty, Equality (1 and 2 because it is a double arc), Brotherhood and Courage at the time of their first observation during the stellar occultation; the first three names having been named after the French motto by André Brahic.
The three main rings are Galle, 41,900 km from the center of Neptune, Le Verrier, 53,200 km, and Adams, 62,932 km. A faint outward extension of the Le Verrier ring is named Lassell. The latter is bounded at its outer edge by the Arago ring at 57,600 km. Le Verrier, Arago and Adams are narrow with widths of about 100 km maximum while Galle and Lassell, on the other hand, are very wide — between 2,000 and 5,000 km.
Four small moons have orbits inside the ring system: Naiad and Thalassa have their orbits in the interval between the rings of Galle and Le Verrier. Despina is just inside the Le Verrier ring and Galatea is towards the inside of the Adams ring. Moreover, if the existence of arcs was previously difficult to explain because the laws of motion predict that arcs would spread out in a uniform ring on short time scales, astronomers now believe that arcs are enclosed in their current shape by the gravitational effects of Galatea.
| Rings of Neptune | ||
|---|---|---|
| Name | Average distance (km) | Width (km) |
| Gall | 41 900 | 2 000 |
| The Verrier | 53 200 | 110 |
| Lassell | 55 200 | 4 000 |
| Arago | 57 200 | 100 |
| Adams | 62 932 | 15 to 50 |
Neptune’s rings contain a large amount of dust whose size is of the order of a micrometer: the fraction of dust depending on the slice considered varies from 20% to 70%. In this respect, they are similar to the rings of Jupiter, which have a dust share of 50% to 100%, and are very different from the rings of Saturn and Uranus, which contain little dust (less than 0.1%) and are therefore less bright. Taken together, Neptune’s rings resemble Jupiter’s, both systems consist of faint, narrow dust rings, and wide, even more tenuous dust rings.
The rings of Neptune, like those of Uranus, are considered relatively young; their age is probably much lower than that of the Solar System. On the other hand, as with Uranus, Neptune’s rings probably formed as a result of the fragmentation of ancient inner moons during collisions. Indeed, these collisions result in the formation of belts of small moons, which are all sources of dust for the rings. Earth observations announced in 2005 seem to show that Neptune’s rings are unstable and images taken at the WM Keck observatory in 2002 and 2003 show considerable degradation of the rings compared to Voyager 2 images; in particular, it seems that the Liberty arc was endangered. By 2009, the Liberty and Courage arcs had disappeared.
Others entourage of Neptune
Like Earth, Mars, Jupiter and Uranus, Neptune has Trojan asteroids sharing its orbit around the Sun.
In 2020, there are twenty at the Lagrange point L4 (early) and three at point L5 (late). 2001 QR322 is the first observed in August 2001 by Marc William Buie’s team on the 4 m Blanco telescope at the Cerro Tololo Observatory. Its relative position oscillates around point L4 and along the Neptunian orbit with a period of about 10,000 years. Its orbit is very stable, so it is located in a region that guarantees that it will still co-orbit with Neptune for billions of years.
In 2004 and 2005, three new Trojans were discovered by Scott S. Sheppard and Chadwick Trujillo. One of them, 2005 TN53, has the same orbital period as Neptune and orbits at the Lagrangian point L4 of Neptune with an inclination of 25 degrees. The other two are named (385571) Otréré and (385695) Clété, after two Amazons. 2008 LC18 is the first Trojan discovered at point L5 of Neptune.
Studies have shown that it would be possible for a theoretical quasi-satellite of Uranus or Neptune to remain so for the lifetime of the Solar System under certain conditions of eccentricity and inclination. Such objects have not yet been discovered, but Neptune does have a temporary quasi-satellite, (309239) 2007 RW10. It has been a quasi-satellite of Neptune for about 12,500 years and is expected to remain in this dynamic state for at least as long.
Observation of Neptune
Due to the evolution of its orbit, Neptune has brightened considerably since 1980. Its apparent magnitude varies in the 2020s between 7.67 and 8.0 with an average of 7.78 whereas before 1980, the planet had an average magnitude of about 8.0. The visual limiting magnitude of the naked eye being 6, Neptune remains invisible without an instrument. A telescope or powerful binoculars will show Neptune as a small blue disk, similar in appearance to Uranus.
Due to Neptune’s distance from Earth, ranging from 4.31 to 4.69 billion kilometers, its apparent size varies only from 2.2 to 2.4 arcseconds, the smallest variation for a planet in the Solar System. Its small apparent size made it difficult to study visually, so most of the knowledge about it was limited — such as the value of its rotation period — until the flyby of Voyager 2 and the advent of the Hubble Space Telescope and large ground-based telescopes with adaptive optics (AO).
The first scientifically exploitable observation of Neptune from ground-based telescopes using adaptive optics was made in 1997 in Hawaii. The southern hemisphere of Neptune has been in its spring season since the 1980s — which will last about 40 years because of the 165-year period of revolution — and it has been observed that it is warming, with atmospheric activity and luminosity increased accordingly. Combined with advances in technology, ground-based telescopes with adaptive optics are recording increasingly detailed images.
From Earth, Neptune undergoes an apparent retrograde motion every 367 days, resulting in a loop-like motion in front of the fixed stars during each opposition. These loops carried it close to the coordinates of the discovery of 1846 in April and July 2010 and again in October and November 2011. Its longitude of discovery is reached on the 11th or July 12, 2011, marking its first full orbit since Johann Galle’s first sighting.
The observation of Neptune in the radio wave band shows that it is a source of both continuous emission and irregular bursts. These two sources would come from its rotating magnetic field. In the infrared part of the spectrum, Neptune’s storms appear bright against the cooler background, making it easy to track the size and shape of these features.
Exploration of Neptune
Voyager 2 flyover
Voyager 2 is the first and only space probe to have visited Neptune and the source of much of the current knowledge about the planet. The trajectory through the Neptunian system is developed once the flyby of Uranus and its moons is completed. As this must be the last passage of Voyager 2 near a planet, there are no constraints on how to leave the planetary system and several choices are possible: the scientific team, therefore, opts for a short passage from the north pole of Neptune which will make it possible to use the gravitational assistance of the planet to plunge the probe under the ecliptic for a close flyby of Triton, Neptune’s main moon, regardless of the consequences of the trajectory, in a similar way to what had been done for Voyager 1 with Saturn and its moon Titan.
The distance from Neptune decreases the theoretical flow allowed by the radio link. Also, several measures were taken in the years leading up to the flyby to strengthen the terrestrial antenna array, including increasing the size of existing receiving antennas, commissioning a new antenna at Usuda, Japan, and using the Very Large Array in New Mexico.
The first observations are made from March 1989, 90 days before the closest passage to Neptune and nearly three years after the flyby of Uranus. They make it possible to discover the rings of Neptune, whose existence had never been proven until then: they are composed of very fine particles that do not allow their observation from Earth. A magnetic field is detected and measured, one being shifted from the geological center and inclined like that of Uranus, but nevertheless of much weaker intensity.
During the crossing of the Neptunian system, five new moons — or 6 if you count Larissa — were discovered. Given the remoteness of Voyager 2, it is difficult to send new instructions in time for the observation of these new celestial bodies. Only Proteus (400 km in diameter) was discovered early enough to schedule detailed observations. Indeed, the signals from the probe took 246 minutes to reach Earth and, therefore, the mission of Voyager 2 relied on preloaded commands.
The flyby of Neptune takes place on August 25, 1989: Voyager 2 passes 4,950 km from the north pole of the planet. Neptune’s atmosphere is analyzed. Despite the little solar energy received due to its remoteness (3% of what Jupiter receives), atmospheric dynamics are observed with manifestations such as the “Great Dark Spot” and clouds. Winds moving at more than 2,000 km/h are measured. The study of the magnetic field makes it possible to determine that the duration of a rotation is 16.11 hours. The flyby also provides the first accurate measurement of Neptune’s mass, which was found to be 0.5% lower than previously calculated. This new value then helped to refute the hypothesis that an undiscovered planet X acted on the orbits of Neptune and Uranus. Voyager 2 footage was broadcast live on a PBS night program, Neptune All Night.
Voyager 2 passes 39,790 km from Triton and can collect very precise data on this moon. The scientific community estimated at the time that the diameter of Triton was between 3,800 and 5,000km; the probe reduces this figure to 2,760 km. Very few craters are observed, which is explained by volcanism whose manifestations in the form of traces left by geysers are observed at the poles. A tenuous atmosphere (pressure of 10 to 14 millibars or 1% to 1.4% of that of the Earth), probably resulting from this activity, is detected by Voyager 2. Triton’s surface temperature, 38 kelvins, is the coldest ever detected on a celestial body in the Solar System.
After Voyager
After the Voyager 2 flyby mission, the next stage of scientific exploration of the Neptunian system is considered integrated into the Flagship Program. Such a hypothetical mission should be possible in the late 2020s or early 2030s. Also, an ongoing proposal for the Discovery program, Trident, would conduct a flyby of Neptune and Triton.
However, there have already been discussions to launch missions to Neptune earlier. In 2003, a project for a Neptune Orbiter probe with objectives similar to those of Cassini was proposed then, in 2009, the Argo mission that would have visited Jupiter, Saturn, Neptune and a Kuiper belt object. In addition, New Horizons 2 — which was later abandoned — could also have performed a close flyby of the Neptunian system.
Neptune in culture
Historical references
The chemical element neptunium was discovered by Edwin McMillan and Philip Abelson in 1940. The discovery was made at the Berkeley Radiation Laboratory — now Lawrence Berkeley National Laboratory — at the University of California, Berkeley, where the team produced the isotope 239 of neptunium, with a half-life of 2.4 days, by bombarding uranium-238 (referring to Uranus) with neutrons. This is the intermediate step leading to the production of plutonium-239 (referring to Pluto).
After Operation Uranus, Operation Neptune was the code name given to the landing in Normandy of Allied troops in June 1944 during the Second World War. It precedes the Battle of Normandy.
Music
“Neptune, the Mystic” is the 7th and last movement of the work for the large orchestra Les Planètes, composed and written by Gustav Holst between 1914 and 1916.
Jimi Hendrix writes and records for the first time in September 1969 Valleys of Neptune, a song that was only released (officially) in March 2010 in the eponymous album Valleys of Neptune, forty years after the artist’s death.
Science fiction
Since its discovery, Neptune has appeared in many science fiction works. It is notably the last resting place of the human race at the end of the Solar System in Olaf Stapledon’s novel The Last and the First (1930) or the main setting of James Gray’s films Ad Astra (2019) and Event Horizon, Paul W’s ship from beyond S. Anderson (1997).
She was also portrayed in the animated series Futurama, the pilot episode of Star Trek: Enterprise and the episode In the Arms of Morpheus of Doctor Who.
Symbolism around Neptune
The astronomical 
symbol of Neptune is a stylized version of the trident of the god Neptune, from which it takes its name. In modern times, it is still used as an astronomical symbol for the planet, although its use is discouraged by the International Astronomical Union.
There is an alternative 
symbol representing the initials of Le Verrier, who discovered the planet, more common in ancient (especially French) literature.
References (sources)
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