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Definition: Neptune (astronomy) from The Macmillan Encyclopedia

The most distant giant planet, orbiting the sun every 165 years at a mean distance of 4497 million km. It is somewhat smaller (48 600 km in diameter) and more massive (17.2 earth masses) than Uranus, exhibits a similar featureless greenish disc in a telescope, and is thought to be almost identical to Uranus in atmospheric and internal structure. It has eight satellites. Neptune's existence was predicted by John Couch Adams and Urbain Leverrier. It was discovered in 1846 by J. G. Galle, using Leverrier's predicted position. In 1989 it was circumnavigated by Voyager 2.

Summary Article: Neptune
From Encyclopedia of the History of Astronomy and Astrophysics

Alexis Bouvard tried to calculate an orbit for Uranus in 1820 using both pre-discovery and post-discovery observations. But he could not find a single orbit to fit them. The best that he could produce was an orbit based on only the post-discovery observations, but this implied that some of the pre-discovery observations were in error by up to 65″, which seemed very unlikely. Unfortunately, it did not take long for Uranus to deviate increasingly from even this orbit, so that by 1845 the longitude discrepancy had reached about 2′. One possible explanation was that Uranus was being disturbed by another planet, and in 1836 Friedrich Nicolai suggested, that if the Titius-Bode series was correct, the unknown planet would be about 38 AU from the Sun.

John Couch Adams, an English mathematician, set out in 1843 to try to calculate the orbit of the planet that seemed to be disturbing the orbit of Uranus. By September 1845, he had calculated its orbital elements and its expected position in the sky, assuming that it was in an elliptical orbit with a mean solar distance of 38.4 AU. Over the next year he progressively modified this orbit. Unfortunately, his predictions of its expected location varied wildly, making it impossible to use them for a telescopic search of the suspected planet.

In parallel, and unknown to Adams, Urbain Le Verrier, a French mathematician undertook the same task, starting work in June 1845. He published his final results on 31 August 1846 and three weeks later asked Johann Galle of the Berlin Observatory if he would undertake a telescopic search for it. Galle and his assistant Heinrich d'Arrest found the planet within an hour of starting their search on 23 September. It turned out to be less than 1° from the position predicted by Le Verrier. However, it was not until the following night that they could be sure that they had discovered the planet when they found that it had moved. Johann Encke, the director of the Berlin Observatory then announced its discovery, crediting Galle and himself, but ignoring the young d'Arrest.

Galle in his letter to Le Verrier of 25 September, notifying him of the discovery, suggested calling the new planet Janus. Le Verrier, in his reply of 1 October, said that the Bureau des Longitudes had already named the planet Neptune. There was then a brief attempt by Le Verrier to have it called after himself, but that failed, so the new planet was called Neptune.

The discovery of Neptune was followed by a heated argument between the English and French astronomical establishments on the priority of the orbital predictions. But much of the evidence on the English side was never published, and an ‘official line’ was agreed. However, that evidence has recently come to light, being found in Chile in 1999. Nevertheless, it had been clear in the 1840s that when Neptune's real orbit was calculated, it turned out to be quite different from the orbits finally predicted by either Le Verrier or Adams. At an average distance of about 30 AU it was appreciably closer to the Sun than either had assumed. So its discovery had been somewhat fortuitous.

Later it transpired that Michel de Lalande, Joseph de Lalande's nephew, had seen Neptune on 8 and 10 May 1795. In his original manuscript he had rejected the first observation and queried the second, as the position of the ‘star’ had moved slightly between the two nights. But it did not seem to have occurred to him that he might have been observing a planet. Amazingly, in 1980 Charles T. Kowal and Stillman Drake found, on looking through Galileo's notebooks, that he had also apparently observed Neptune on both 28 December 1612 and 28 January 1613, when it was near to Jupiter.

Less than a month after Neptune's discovery, William Lassell observed an object close to Neptune, which he thought may be a satellite. But bad weather, and the nearness of Neptune to the Sun's glare, meant that it was not until the following July that he was able to confirm that it was a satellite, now called Triton. It was later found to have a retrograde orbit. Triton was seen to be very bright, considering its distance from the Sun. As a result, it was thought that it may be the largest satellite in the solar system. It was also quite close to Neptune in its retrograde orbit, so it was thought probable that Neptune's axial spin would also be retrograde.

The discovery of Triton allowed the mass of Neptune to be determined as about 17.1 times the mass of the Earth. In 1895 Edward Emerson Barnard measured Neptune's diameter as 52,900 km, implying a density of about 1.32 g/cm3, similar to that of Jupiter and Uranus.

A number of astronomers tried to measure the rotation period of Neptune in the late nineteenth and early twentieth centuries, but they produced wildly different results. Then in 1928 Joseph Moore and Donald Menzel deduced an unambiguous rotation period of 15.8 ± 1 hours, prograde, by measuring its Doppler shift. So the idea that Neptune's axial spin may be retrograde, because of the retrograde orbit of Triton, was found to be incorrect. As a result it was clear that Triton was orbiting its planet in the opposite sense to the planet's spin, being the first major satellite in the solar system to be observed to do so. Then in 1949 Gerard Kuiper discovered Nereid, Neptune's second satellite, and found that it orbited Neptune prograde, or in the opposite sense to Triton. Triton's orbit was almost circular, with a radius of 355,000 km, but Nereid's was highly elliptical, with an apogee of 9.7 and a perigee of 1.3 million km.

The rotation period of 15.8 hours deduced by Moore and Menzel was generally accepted, until in 1977 Sethanne Hayes and Michael Belton deduced a period of 22 hours, based on Doppler shifts. Then in 1981 Robert Hamilton Brown, Dale Cruikshank and Alan Tokunaga found a period of 17.95 hours photometrically.

Bradford Smith, Harold Reitsema and S. M. Larson were able to record clear cloud features in Neptune's atmosphere for the first time in 1979. The images in the 890 nm methane absorption band showed a broad, dark equatorial band, where methane deep in Neptune's atmosphere absorbed sunlight, and bright features in both the northern and southern hemispheres due to high-altitude clouds. Four years later, images taken by Richard Terrile and Bradford Smith showed four clear atmospheric features, which allowed the planet's rotation period to be determined. The resulting period of 17.83 hours, prograde, was virtually the same as that measured photometrically two years earlier by Hamilton Brown and colleagues.

Heidi Hemmel confirmed the changing nature of Neptune's atmosphere, which had been first reported a hundred years before by Maxwell Hall, when she found in 1986 and 1987 that clouds, that had clearly been seen in the northern hemisphere just shortly before, had disappeared. In 1986 and 1987 she also noticed one very bright cloud at latitude 38° S that had a rotation period about the centre of Neptune of 17.83 hours. A year later, however, the only bright cloud was at 30° S, with a rotation period of 17.67 hours.

Dale Cruikshank and Peter Silvaggio detected the 2.3 μm methane absorption band on Triton in 1978, indicating that it had a tenuous methane atmosphere. If that was the case, Triton would be only the third satellite in the solar system known at the time to have an atmosphere, the others being Titan and Io. The 1.7 μm methane band was relatively weak, however, indicating that there was little or no methane frost or ice on the illuminated surface, which was assumed to be largely rocky.

Triton's orbit is inclined at 23° to Neptune's equator, and Neptune's equator is inclined at 29° to the plane of its orbit around the Sun. So the Sun is at the zenith on Triton at 52° latitude on midsummer's day. As Neptune's year is 165 years long, there is plenty of time for one hemisphere of Triton to heat up and the other to cool down, the poles each being without sunlight for 82 years. So Cruikshank and Silvaggio suggested that there may be methane ice or frost on Triton's unilluminated surface, even though they had not detected any elsewhere. Then in 1983 Apt, Carleton and Mackay analysed the visible spectrum of Triton, and concluded that there was clear evidence for methane frost or ice on its surface, although water ice appeared to be largely absent. In the same year Cruikshank, Clark and Hamilton Brown also detected nitrogen on the surface, implying that it would also be present in Triton's tenuous atmosphere, as nitrogen is highly volatile. So by the time that the first spacecraft, Voyager 2, arrived at Neptune in 1989, Triton was thought to possess an atmosphere of methane and nitrogen, with methane ice or frost and nitrogen on its surface. The exact amounts and condition of each constituent would depend critically on Triton's surface temperature, however, which was thought to be in the range 50 to 65 K, depending on albedo and latitude. Liquid nitrogen freezes at about 63 K at zero pressure, so it was thought that most of the nitrogen on Triton's surface would probably be in the solid form, unless Triton's temperature was very near the top end of its expected range.

In 1986 Cruikshank and colleagues found that the methane feature in Triton's spectrum was weakening compared with earlier years. Similarly, whilst observations in 1977 and 1981 had shown that the intensity of Triton varied by 6% as it rotated, those in 1987 showed that the intensity variation was less than 2%. So it was thought that the methane atmosphere was becoming more hazy as the Sun was gradually heating up the southern hemisphere, and there were fears that the atmosphere may be too hazy during the Voyager 2 intercept of 1989 for the spacecraft to image the surface.

The Voyager 2 spacecraft imaged a dark spot, called the Great Dark Spot or GDS, on Neptune, centred at about 22° S latitude and rotating around the planet once every 18.3 hours. Although it was physically smaller than Jupiter's Great Red Spot (GRS), it was about the same size as the GRS relative to the size of their respective planets. Also like the GRS, the GDS rotated counter-clockwise about its centre south of the equator, making it a high-pressure feature. Numerous other features were observed on Neptune during the Voyager encounter, including a second, smaller dark spot called D2 at about 53° S, which like the GDS also appeared to be a high-pressure system. When D2 was discovered it had a rotation period around Neptune of 16.0 hours. Its period then slowed to 16.3 hours as it moved north, before moving south with a period of 15.8 hours. It seemed to be less constrained in latitude than similar features on Jupiter and Saturn.

Voyager detected radio signals from Neptune that were varying with a period of 16.11 hours, which was assumed to be the spin rate of Neptune's interior. Relative to this, Neptune's clouds showed that there was an easterly equatorial jet, like that on Uranus, only those on Neptune were five times as fast at an incredible 1,800 km/h. These Neptune winds were, along with Saturn's equatorial jet, the fastest atmospheric winds in the solar system, which was surprising considering that Neptune receives such a small amount of radiation from the Sun. So the winds were thought to be driven by Neptune's significant internal heat source. Later images by the Hubble Space Telescope showed that both the GDS and D2 had disappeared by 1995.

Before the Voyager 2 intercept, Neptune was known to have two satellites, Triton and Nereid. Triton, which is the closer to Neptune, orbits the planet in a retrograde sense, suggesting that it may have been captured by Neptune. On the other hand its orbit is almost circular, whereas that of Nereid, which is prograde, is much further away from Neptune and is highly elliptical. The satellites’ orbits are inclined at 23° and 27°, respectively, to Neptune's equatorial plane, which are unusually large inclinations, so maybe both satellites have been captured, Triton because of its retrograde orbit and Nereid because of its large orbital eccentricity. It was hoped that the discovery of new satellites by Voyager 2 would help to clarify this.

Voyager 2 discovered six new satellites of Neptune, all of which were in circular, prograde orbits in Neptune's equatorial plane, inside of Triton's orbit. One of these satellites, Larissa, had previously been detected by Reitsema and colleagues during a stellar occultation in 1981. But at that time it was not clear whether they had detected a satellite or ring of Neptune (see later section). Proteus and Larissa were the only new satellites to be imaged at high resolution, showing highly-cratered surfaces with reflectivities of only 6%. Proteus, with a diameter of 420 km, was found to have a 160 km diameter crater on its relatively small surface. Contrary to expectations, the presence of these six small satellites in circular, prograde orbits did not help to resolve the question as to whether Triton is an original or captured satellite of Neptune.

Figure 2.19. This image of Triton, which is about 500 km across, shows a marked lack of impact craters implying a relatively young surface. It also shows two depression, possibly old impact basins, that have since been flooded by cryovolcanic fluids.(Courtesy NASA/JPL/Caltech.)

The Voyager 2 spacecraft found that Triton had a highly reflective surface with an average reflectivity of about 85%. As a result its measured diameter of about 2,700 km was near the low end of the expected range. Its density was about 2.05 g/cm3, greater than that of any of the satellites of Saturn or Uranus, and somewhat higher than generally expected. This indicated that Triton has less ice and more rock in its interior than anticipated. Interestingly, Triton's density and size are almost identical to those of Pluto, adding to the idea that Triton was captured by Neptune.

Because of Triton's highly reflective surface, its surface temperature was lower than anticipated at 38 K. This was far too cold for nitrogen to exist in liquid form on the surface, except in possible ‘hot’ spots. Because of the lower than expected temperature, the surface-level atmospheric pressure of 15 microbars was considerably less than expected, as most of the atmosphere was ‘frozen out’ on the surface. Fortunately, although there was a haze layer about ten kilometres above the surface, the surface could be clearly seen. The very thin atmosphere was found to consist almost completely of nitrogen, with only trace amounts of methane, carbon monoxide, carbon dioxide and water vapour.

Voyager arrived at Triton during early summer in its southern hemisphere. It revealed a very bright, pinkish-white, southern polar cap of nitrogen ice, with trace amounts of methane, carbon monoxide, carbon dioxide and water ice extending three-quarters of the way from pole to equator. The ice cap appeared to be slowly melting at its edges to reveal a darker, redder surface underneath.

The retreating and advancing polar caps have ensured that the surface of Triton is still subject to change. Dark streaks up to 150 km long were seen all over the south polar cap and, as the ice retreats every summer, those streaks near the edge of the cap must have been produced relatively recently. Their appearance suggested that nitrogen or methane had been ejected at one end of the streak in some sort of explosion, and carried by the wind in Triton's tenuous atmosphere.

Robert Hamilton Brown proposed a mechanism for these eruptions, likening them to geysers on Earth. He suggested that sunlight penetrated the almost-transparent nitrogen ice of the polar cap, where about two metres down it was absorbed by frozen methane, which had been darkened by exposure to ultraviolet light. The heat was trapped by the nitrogen ice, as it was a poor conductor, and as the heat built up the subsurface nitrogen ice was turned into gas. Eventually the gas pressure was too much to be resisted by the surface ice, and the nitrogen gas exploded through the surface, carrying with it the Sun-darkened methane, to produce a geyser-like eruption.

A month after the Voyager encounter, Lawrence Soderblom and Tammy Becker were stereoscopically examining images of the dark streaks on the south polar cap, when they were amazed to discover an image of an eruption in progress. It produced a plume 8 km high, which extended horizontally for about 150 km. They then found an image of a second eruption in progress. These two eruptions, and three other suspected eruptions, were all near to the sub-solar point, indicating that they were caused by solar heating, giving some support to Robert Hamilton Brown's theory of geyser-like eruptions.

There was only a relatively small number of craters seen on Triton (see Figure 2.19, for example), and the largest was only 27 km in diameter, indicating that none of Triton's original surface had survived. So there must have been considerable geological activity in its early lifetime, possibly caused by gravitational stresses as a result of its hypothesised capture by Neptune. In addition, resurfacing must have continued almost up to the present in geological terms, and some resurfacing may still be occurring today.

In the equatorial regions there were large areas of relatively young, dimpled terrain, quite unlike anything seen elsewhere in the solar system. This so-called cantaloupe terrain was crisscrossed in places by shallow linear ridges up to 30 km wide and 1,000 km long, probably caused when water ice in Triton's mantle froze and expanded. Within this cantaloupe terrain and to its north there were areas of frozen lakes and calderas, showing evidence of liquid flows, possibly as a result of volcanic activity. Some of these frozen liquid flows were relatively old with many craters, but some looked fresh and were almost crater-free. These lakes and calderas cannot be made of nitrogen and/or methane, as the surface temperature is too near their melting points for these ices to be able to support the crater walls, which, in some places, are over 1,000 m high. Instead it appears as if the subsurface here is made of a mixture of water ice, nitrogen and/or methane, with the water ice providing the strength, and the nitrogen and/or methane reducing the freezing point of water to enable it to flow at low temperatures.

Since the Voyager 2 encounter a further 5 small satellites have been discovered, all of which have orbits outside that of Nereid. Three of these are retrograde and two are prograde orbits. Two of the satellites, Psamathe and Neso, have the largest orbits of any planetary satellites discovered to date, with semimajor axes just under 50 million km.


Harold Reitsema and colleagues attempted to find a ring or rings around Neptune in 1981, following the discovery of rings around Uranus and Jupiter in the late 1970s. They observed a stellar occultation by Neptune that year, but it showed an intensity reduction on only one side of the planet, so they could not have discovered a ring. Reitsema concluded instead that they had fortuitously discovered a satellite, at least 180 km in diameter and about 50,000 to 70,000 km from the centre of the planet.

On 22 July 1984 the first unambiguous discovery of a partial ring was found during a stellar occultation. Although the occultation occurred on only one side of Neptune, like that in 1981, this time it was observed by two teams of astronomers at two different observatories about 95 km apart. If this was due to a satellite, it would have been so large as to be easily visible from Earth. So they concluded that the occultation was due to a partial ring or ring arc about 67,000 km from the centre of Neptune. This discovery raised the question as to whether Reitsema and colleagues had also seen a ring arc in 1981. A further occultation in 1985 also showed that there was probably another ring arc 63,000 km from the centre of Neptune.

Voyager 2's intercept of Neptune in 1989 finally resolved the question of rings around Neptune, when it found five continuous rings. The Adams, Arago and LeVerrier rings were narrow and distinct, whereas the Lassell and Galle rings were relatively broad and indistinct. It transpired that the occultations of 1984 and 1985 had discovered parts of the Adams ring, which was remarkably clumpy, with density variations of about a factor of ten along its length. The object discovered in 1981, however, was a satellite, now called Larissa, of about 200 km diameter.

Voyager 2 discovered a satellite, Galatea, orbiting Neptune about 1,000 km inside the clumpy Adams ring, and a satellite, Despina, orbiting about 700 km inside the Le Verrier ring, possibly helping to shepherd the rings. Galatea, which had a 42/43 resonance with Adams, was probably controlling its ring arcs.

Magnetism and magnetosphere

In 1989 Voyager 2 detected Neptune's magnetopause about 24 RN (Neptune radii) from Neptune, and found that all of Neptune's satellites known at that time, except Nereid, were within the planet's largely empty magnetosphere. Neptune's magnetic axis was found to make an angle of about 47° to its spin axis, and its magnetic centre was displaced from its geometric centre by about 0.55 RN. Its dipole moment was about 28 times that of Earth.

The 30° orientation of Neptune's equator to the plane of its orbit around the Sun meant that the angle between Neptune's magnetic axis and the direction of the solar wind can vary enormously. At some times its magnetic axis can be almost pole-on to the solar wind. These radical variations in orientation of magnetic axis to solar wind meant that the configuration of Neptune's magnetosphere and its radiation belt can vary wildly over the course of one Neptune day of 16 hours.

Further information on Neptune's satellites and rings is given in Table 2.5.

See also: Atmospheric constituents of the outer planets; Internal structures of the outer planets

Table 2.5. Neptune's main satellites and rings

(a) Neptune's satellites


Year of discovery

Discovered by

Semimajor axis (103 km)

Orbital inclination

Mean radius (km)




Voyager team






Voyager team






Voyager team






Voyager team






Reitsema et al.






Voyager team




















Jewitt et al.







Holman et al.




(b) Neptune's rings

Ring component

Radial location (km)

Discovered by


(edge of planet)




Voyager 2


Le Verrier


Voyager 2




Voyager 2




Voyager 2




Manfroid and Hubbard


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  • Godwin, Robert; Whitfield, Steve, Deep Space: The NASA Mission Reports, Apogee Books, 2005.
  • Littmann, Mark, Planets Beyond: Discovering the Outer Solar System, John Wiley, 1990.
  • McFadden, L. A.; Weissman, P. R.; Johnson, T. V. (eds.), Encyclopedia of the Solar System, 2nd ed., Academic Press, 2007.
  • Miner, Ellis D.; Wessen, Randii, Neptune: The Planet, Rings and Satellites, Springer-Praxis, 2002.
  • Miner, E. D.; Wessen, R. R.; Cuzzi, J. N., Planetary Ring Systems, Springer-Praxis, 2007.
  • Moore, Patrick, The Planet Neptune: An Historical Survey Before Voyager, 2nd ed., Wiley-Praxis, 1996.
  • Shirley, James H.; Fairbridge, Rhodes W. (eds.), Encyclopedia of Planetary Science, Chapman and Hall, 1997.
  • Standage, Tom, The Neptune File: A Story of Astronomical Rivalry and the Pioneers of Planet Hunting, Walker Publishing, 2000.
  • © Cambridge University Press 2013

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