Neptune
Neptune is the outermost
gas giant in our
solar system. For most of its 165 year orbit it is the eighth
planet from the
Sun and occasionally the ninth due to
Pluto's
eccentricity. It is the fourth largest planet by diameter and the third largest by mass; Neptune is more massive than its near twin
Uranus. The planet is named after the
Roman god of the sea. Its
astronomical symbol is a stylized version of the god's
trident (♆).
Neptune's atmosphere is primarily composed of
hydrogen and
helium, with traces of
methane that account for the planet's blue appearance. Neptune's blue colour is much more vivid than that of Uranus, which has a similar amount of methane, and an unknown component is presumed to cause the intense colour.
["Neptune overview," Solar System Exploration, NASA.] Neptune also has the strongest winds of any planet in the solar system, with estimates as high as 2,500 km/h. At the time of the 1989
Voyager 2 flyby, it had in its southern hemisphere a
Great Dark Spot comparable to the
Great Red Spot on
Jupiter.
Faint dark colored rings have been detected around the blue planet, but are much less substantial than those of
Saturn. When these rings were discovered by a team lead by
Edward Guinan, it was thought that they might not be complete but this was disproved by
Voyager 2. Neptune possesses nine confirmed moons and four awaiting confirmation. Neptune's largest moon,
Triton, is notable for its
retrograde orbit, extreme cold (38
K), and extremely tenuous (14
microbar) nitrogen/methane atmosphere.
Discovered on
September 23,
1846, Neptune is notable for being the only planet discovered based on mathematical prediction rather than regular observations. Perturbations in the orbit of
Uranus led astronomers to deduce Neptune's existence. It has been visited by only one spacecraft,
Voyager 2, which flew by the planet on
August 25,
1989. In 2003, there was a proposal to
NASA's "Vision Missions Studies" to implement a "
Neptune Orbiter with Probes" mission that does
Cassini-level science without fission-based electric power or propulsion. The work is being done in conjunction with
JPL and the
California Institute of Technology.
[T. R. Spilker and A. P. Ingersoll (November 9, 2004). Outstanding Science in the Neptune System From an Aerocaptured Vision Mission. 36th DPS Meeting, Session 14 Future Missions.]Galileo's astronomical drawings show that he had first observed Neptune on
December 27,
1612, and again on
January 27,
1613; on both occasions Galileo had mistaken Neptune for a
fixed star when it appeared very close (in
conjunction) to Jupiter in the
night sky. Believing it to be a fixed star, he cannot be credited with its discovery. At the time Galileo first observed Neptune on
December 28,
1612, it was stationary in the sky because it had just turned retrograde that very day; because it was stationary in the sky and only beginning the planet's yearly retrograde cycle, its motion was far too slight to be detected with Galileo's small
telescope. Had Neptune been moving at its regular/average speed when Galileo first observed it in 1612 and 1613, he would have most likely realized that it was a planet and not a fixed star due to Neptune's relatively rapid normal motion along the
ecliptic compared to the extremely slow motion of the fixed stars.
|
Size comparison of Neptune and Earth |
In 1821,
Alexis Bouvard published astronomical tables of the
orbit of Uranus. Subsequent observations revealed substantial deviations from the tables, leading Bouvard to hypothesize some perturbing body. In 1843,
John Couch Adams calculated the orbit of an eighth planet that would account for Uranus' motion. He sent his calculations to Sir
George Airy, the
Astronomer Royal, who asked Adams for a clarification; Adams began to draft a reply but never sent it.
In 1846,
Urbain Le Verrier, independently of Adams, produced his own calculations but also experienced difficulties in encouraging any enthusiasm in his compatriots. However, in the same year,
John Herschel started to champion the mathematical approach and persuaded
James Challis to search for the planet.
After much procrastination, Challis began his reluctant search in July 1846. However, in the mean time,
Le Verrier had convinced
Johann Gottfried Galle to search for the planet. Though still a student at the
Berlin Observatory,
Heinrich d'Arrest suggested that a recently drawn chart of the sky, in the region of Le Verrier's predicted location, could be compared with the current sky to seek the displacement characteristic of a
planet, as opposed to a fixed star. Neptune was discovered that very night,
September 23,
1846, within 1° of where Le Verrier had predicted it to be, and about 10° from Adams' prediction. Challis later realized that he had observed the planet twice in August, failing to identify it owing to his casual approach to the work.
In the aftermath of the discovery, there was much nationalistic rivalry between the French and the British over who had priority and deserved credit for the discovery. Eventually an international consensus emerged that both Le Verrier and Adams jointly deserved credit. However, the issue is now being re-evaluated by historians with the rediscovery in 1998 of the "Neptune papers" (historical documents from the
Royal Greenwich Observatory), which had apparently been misappropriated by astronomer
Olin Eggen for nearly three decades and were not rediscovered (in his possession) until immediately after his death. After reviewing the documents, some historians now suggest that Adams did not in fact deserve equal credit with Le Verrier.
[William Sheehan, Nicholas Kollerstrom, Craig B. Waff (December 2004). The Case of the Pilfered Planet - Did the British steal Neptune? Scientific American.]Shortly after its discovery, Neptune was referred to simply as "the planet exterior to Uranus" or as "Le Verrier's planet." The first suggestion for a name came from Galle. He proposed the name
Janus. In England, Challis put forth the name
Oceanus, particularly appropriate for a seafaring people. In France,
Arago suggested that the new planet be called
Leverrier, a suggestion which was met with stiff resistance outside
France. French almanacs promptly reintroduced the name
Herschel for
Uranus and
Leverrier for the new planet.
Meanwhile, on separate and independent occasions, Adams suggested altering the name
Georgian to
Uranus, while Leverrier (through the Board of Longitude) suggested
Neptune for the new planet.
Struve came out in favor of that name on
December 29,
1846, to the
Saint Petersburg Academy of Sciences.
[Second report of proceedings in the Cambridge Observatory relating to the new Planet (Neptune) (1847). Astronomische Nachrichten, volume 25, p.309. Found at articles.adsabs.harvard.edu.] Soon
Neptune became the internationally accepted nomenclature. In
Roman mythology Neptune was the god of the sea, identified with the Greek
Poseidon. The demand for a mythological name seemed to be in keeping with the nomenclature of the other planets all of which, except for Uranus, were named in antiquity.
In the
Chinese,
Korean,
Japanese, and
Vietnamese languages the planet's name is literally translated as the
sea king star (海王星).
[Using Eyepiece & Photographic Nebular Filters, Part 2 (October 1997). Hamilton Amateur Astronomers at amateurastronomy.org.] |
The Great Dark Spot, as seen from Voyager 2. |
Relative size
At 1.0243
kg Neptune is an intermediate body between
Earth and the largest gas giants: it is seventeen Earth masses but just 1/18th the mass of
Jupiter. It and
Uranus are often considered a sub-class of gas giant termed "ice giants", given their smaller size and important differences in composition relative to
Jupiter and
Saturn. In the search for
extra-solar planets Neptune has been used as a
metonym: discovered bodies of similar mass are often referred to as "Neptunes"
["Trio of Neptunes", Astrobiology Magazine, May 21, 2006.] just as astronomers refer to various extra-solar "Jupiters."
Composition
Orbiting so far from the sun, Neptune receives very little heat with the uppermost regions of the atmosphere at −218 °C (55 K). Deeper inside the layers of gas, however, the temperature rises steadily. It is thought that this may be leftover heat generated by
infalling matter during the planet's birth, now slowly radiating away into space.
The internal structure resembles that of Uranus. There is likely to be a
core consisting of molten rock and metal, surrounded by a mixture of rock, water, ammonia, and methane. There is no solid surface and the atmosphere, extending perhaps 10 to 20 percent of the way towards the center, is mostly hydrogen and helium at high altitudes (80% and 19%, respectively). Increasing concentrations of methane, ammonia, and water are found as the atmosphere approaches and finally blends into the liquid interior. The pressure at the center of Neptune is millions of times more than that on the surface of
Earth. Comparing its rotational speed to its degree of
oblateness indicates that it has its mass less concentrated towards the center than does Uranus.
Magnetic field
Neptune also resembles Uranus in its
magnetosphere, with a
magnetic field strongly tilted relative to its
rotational axis at 47° and offset at least 0.55 radii (about 13,500 kilometres) from the planet's physical center. Comparing the magnetic fields of the two planets, scientists think the extreme orientation may be characteristic of flows in the interior of the planet and not the result of Uranus' sideways orientation.
Weather
Neptune's
atmosphere has the highest wind speeds in the solar system, thought to be powered by the flow of internal heat, and its weather is characterized by extremely violent
hurricanes, with winds reaching up to 2000 km/h. In 1989, the
Great Dark Spot (GDS) was discovered by
NASA's
Voyager 2 spacecraft. However, in 1994 the
Hubble Space Telescope did not see the Great Dark Spot on the planet. Instead, a new storm similar to the Great Dark Spot was found in the planet's northern hemisphere. The reason for the Great Dark Spot's disappearance is unknown. Many scientists believe heat transfer from the planet's core disrupted the atmospheric equilibrium and disrupted existing circulation patterns. The
Scooter is another storm described as a white cloud south of the Great Dark Spot. The
Wizard's eye (Great Dark Spot 2) is a southern hurricane, the second most intensive hurricane on the planet.
One difference between Neptune and Uranus is the level of meteorological activity. Uranus is visually quite bland, while Neptune's high winds come with notable weather phenomena. The
Great Dark Spot, a cyclonic storm system the size of
Eurasia, was captured by
Voyager 2 in the 1989 flyby. The storm resembled the
Great Red Spot of Jupiter, but was shown to have disappeared in June 1994. However, a newer image of the planet taken by the
Hubble Space Telescope on
November 2 1994, revealed that a smaller storm similar to its predecessor had formed over Neptune's Northern Hemisphere. Unique among the gas giants is the presence of high clouds casting shadows on the opaque cloud deck below.
Though Neptune's atmosphere is much more dynamic than that of Uranus, both planets are made of the same gases and ices. Uranus and Neptune are not strictly gas giants similar to Jupiter and Saturn, but are rather ice giants.
Neptune has a faint
planetary ring system of unknown composition. The rings have a peculiar "clumpy" structure, the cause of which is not currently understood but which may be due to the gravitational interaction with small moons in orbit near them.
 |
Neptune's rings |
Evidence that the rings are incomplete first arose in the mid-
1980s, when
stellar occultation experiments were found to occasionally show an extra "blink" just before or after the planet occulted the star. Images by
Voyager 2 in 1989 settled the issue, when the ring system was found to contain several faint rings. The outermost ring, Adams, contains three prominent arcs now named
Liberté,
Egalité, and
Fraternité (Liberty, Equality, and Fraternity). The existence of arcs is very difficult to understand because the laws of motion would predict that arcs spread out into a uniform ring over very short timescales. The gravitational effects of
Galatea, a moon just inward from the ring, are now believed to confine the arcs.
Several other rings were detected by the
Voyager cameras. In addition to the narrow
Adams Ring 63,000 km from the centre of Neptune, the
Leverrier Ring is at 53,000 km and the broader, fainter
Galle Ring is at 42,000 km. A faint outward extension to the Leverrier Ring has been named
Lassell; it is bounded at its outer edge by the
Arago Ring at 57,000 km.
[Gazetteer of Planetary Nomenclature Ring and Ring Gap Nomenclature (December 8, 2004). USGS - Astrogeology Research Program.]New Earth-based observations announced in 2005 appeared to show that Neptune's rings are much more unstable than previously thought. In particular, it seems that the
Liberté ring might disappear in as little as one century. The new observations appear to throw our understanding of Neptune's rings into considerable confusion.
[Neptune's rings are fading away (March 26, 2005). New Scientist.]| Name of ring | Radius (km) | Width (km) | Notes | | 1989 N3R ('Galle') | 41,900 | 15 | Named after Johann Galle |
| 1989 N2R ('Leverrier') | 53,200 | 15 | Named after Urbain Le Verrier |
| 1989 N4R ('Lassell') | 55,400 | 6 | Named after William Lassell |
| Arago Ring | 57,600 | - | Named after François Arago |
| Liberté Ring Arc | 62,900 | - | "Leading" arc |
| Égalité Ring Arc | 62,900 | - | "Equidistant" arc |
| Fraternité Ring Arc | 62,900 | - | "Trailing" arc |
| Courage Ring Arc | 62,900 | - |
| 1989 N1R ('Adams') | 62,930 | <50 | Named after John Couch Adams |
Neptune has 13 known
moons. The largest by far, and the only one massive enough to be
spheroidal, is
Triton, discovered by
William Lassell just 17 days after the discovery of Neptune itself. Unlike all other large planetary moons, Triton has a
retrograde orbit, indicating that it was captured, and probably represents the largest known example of a
Kuiper Belt object (although clearly no longer in the Kuiper Belt). It is close enough to Neptune to be locked into a
synchronous orbit, and is slowly spiraling inward. Triton is the coldest object that has been measured in the solar system.
| Triton, compared to Earth's Moon |
|---|
| - style="background:#efefef;" | Name
(Pronunciation key) | Diameter
(km) | Mass
(kg) | Orbital radius (km) | Orbital period (days) |
|---|
| Triton | trye'-tən
| 2700
(80% Luna) | 2.15×1022(30% Luna) | 354,800
(90% Luna) | -5.877
(20% Luna) |
Neptune's second known satellite, the irregular moon
Nereid, has one of the most eccentric orbits of any satellite in the solar system.
From July to September 1989,
Voyager 2 discovered six new Neptunian moons. Of these, the irregularly shaped
Proteus is notable for being as large as a body of its density can be without being pulled into a spherical shape by its own gravity. Although the second most massive Neptunian moon, it is only one quarter of one percent of the mass of Triton. Neptune's innermost four moons,
Naiad,
Thalassa,
Despina, and
Galatea, orbit close enough to be within Neptune's rings. The next farthest out,
Larissa was originally discovered in 1981 when it had blocked a star. This was attributed to ring arcs, but when
Voyager 2 observed Neptune in 1989, it was found to have been caused by the moon. Five new irregular moons discovered between 2002 and 2003 were announced in 2004.
[Holman, Matthew J. et. al. (August 19, 2004). Discovery of five irregular moons of Neptune. Nature, p. 865 - 867.][Five new moons for planet Neptune (August 18, 2004). BBC News.] As Neptune was the Roman god of the sea, the planet's moons have been named after lesser sea gods.
For a timeline of discovery dates, see Timeline of discovery of Solar System planets and their natural satellitesAs of June 2006, there are four
1 known
Trojan asteroids of Neptune which have the same orbital period as the planet
[List Of Neptune Trojans (May 6, 2006) at cfa-www.harvard.edu/iau/.]. They lie in the elongated, curved regions around the L
4 and L
5 Lagrangian points 60° ahead of and behind Neptune. These are , , and .
The discovery of on a high inclination (>25°) orbit is significant as it suggests a ‘thick' cloud of Trojans, resulting from a capture rather than formed in situ.
1, listed earlier as a Neptune Trojan, proved to be a scattered disk object.Neptune is never visible with the
naked eye. The brightness of Neptune is between
magnitudes +7.7 and +8.0, so a telescope or binoculars are required to observe it. With the use of a telescope it appears as a small blue-green disk, similar in appearance to Uranus; the blue-green colour comes from the
methane in its atmosphere. Interestingly, Neptune emits more light than it receives from the sun
["Astronomy picture of the day 18 June 2006," NASA.]With an orbital period of 165 years, Neptune will soon return to the approximate position in the sky where Galle discovered it. This will happen three different times. These are
April 11,
2009, when it will be in
prograde motion;
July 17 2009, when it will be in
retrograde motion; and finally for the last time for the next 165 years, on
February 7 2010. This is explained by the concept of retrogradation. Like all planets in the solar system beyond Earth, Neptune undergoes
retrogradation at certain points during its
synodic period. In addition to the start of retrogradation, other events within the synodic period include
astronomical opposition, the return to
prograde motion, and
conjunction to the Sun.
In its orbit around the Sun, Neptune will return to its original point of discovery in August 2011.
*
Planets in astrology - Neptune*
Neptune in fiction*
Neptune Orbiter - Proposed space probe to Neptune to be launched in 2016
* Adams, J. C., "
Explanation of the observed irregularities in the motion of Uranus, on the hypothesis of disturbance by a more distant planet",
Monthly Notices of the Royal Astronomical Society, Vol. 7, p. 149,
November 13,
1846.
* Airy, G. B., "
Account of some circumstances historically connected with the discovery of the planet exterior to Uranus",
Monthly Notices of the Royal Astronomical Society, Vol. 7, pp. 121-144, November 13, 1846.
* Challis, J., Rev., "
Account of observations at the Cambridge observatory for detecting the planet exterior to Uranus",
Monthly Notices of the Royal Astronomical Society, Vol. 7, pp. 145-149, November 13, 1846.
* Galle, "
Account of the discovery of the planet of Le Verrier at Berlin",
Monthly Notices of the Royal Astronomical Society, Vol. 7, p. 153, November 13, 1846.
*Smith, Bradford A.
"Neptune." World Book Online Reference Center. 2004. World Book, Inc. Accessed at nasa.gov.
* Scott S. Sheppard, Chadwick A. Trujillo
A Thick Cloud of Neptune Trojans and Their Colors, Science, June 2006.
*
NASA's Neptune fact sheet*
MPC's List Of Neptune Trojans*
Planets - Neptune A kid's guide to Neptune.
Future missions to Neptune*
Popular Science article*
Universe Today*
Scientific American Magazine (December 2004 Issue) The Case of the Pilfered Planet
