Neptune
The Mystic
Neptune is the eighth planet from the Sun and the fourth largest (by diameter). Neptune is smaller in diameter but larger in mass than Uranus.
orbit: 4,504,000,000 km (30.06 AU) from Sun
diameter: 49,532 km (equatorial)
mass: 1.0247e26 kg
Hardcopy The New Solar System
Summarizes what we have learned from interplanetary explorations in the last 25 years. My primary reference for The Nine Planets.
Encyclopedia of the Solar System
A more scholarly introduction the planetary science for those who want to dig a little deeper.
The Compact NASA Atlas of the Solar System
This road map of the solar system contains lots of maps and data as well as photos. In Roman mythology Neptune (Greek: Poseidon) was the god of the Sea.
After the discovery of Uranus, it was noticed that its orbit was not as it should be in accordance with Newton's laws. It was therefore predicted that another more distant planet must be perturbing Uranus' orbit. Neptune was first observed by Galle and d'Arrest on 1846 Sept 23 very near to the locations independently predicted by Adams and Le Verrier from calculations based on the observed positions of Jupiter, Saturn and Uranus. An international dispute arose between the English and French (though not, apparently between Adams and Le Verrier personally) over priority and the right to name the new planet; they are now jointly credited with Neptune's discovery. Subsequent observations have shown that the orbits calculated by Adams and Le Verrier diverge from Neptune's actual orbit fairly quickly. Had the search for the planet taken place a few years earlier or later it would not have been found anywhere near the predicted location.
More than two centuries earlier, in 1613, Galileo observed Neptune when it happened to be very near Jupiter, but he thought it was just a star. On two successive nights he actually noticed that it moved slightly with respect to another nearby star. But on the subsequent nights it was out of his field of view. Had he seen it on the previous few nights Neptune's motion would have been obvious to him. But, alas, cloudy skies prevented obsevations on those few critical days.
Neptune has been visited by only one spacecraft, Voyager 2 on Aug 25 1989. Much of we know about Neptune comes from this single encounter. But fortunately, recent ground-based and HST observations have added a great deal, too.
Because Pluto's orbit is so eccentric, it sometimes crosses the orbit of Neptune making Neptune the most distant planet from the Sun for a few years.
Neptune's composition is probably similar to Uranus': various "ices" and rock with about 15% hydrogen and a little helium. Like Uranus, but unlike Jupiter and Saturn, it may not have a distinct internal layering but rather to be more or less uniform in composition. But there is most likely a small core (about the mass of the Earth) of rocky material. Its atmosphere is mostly hydrogen and helium with a small amount of methane.
Neptune's blue color is largely the result of absorption of red light by methane in the atmosphere but there is some additional as-yet-unidentified chromophore which gives the clouds their rich blue tint.
Like a typical gas planet, Neptune has rapid winds confined to bands of latitude and large storms or vortices. Neptune's winds are the fastest in the solar system, reaching 2000 km/hour.
Like Jupiter and Saturn, Neptune has an internal heat source -- it radiates more than twice as much energy as it receives from the Sun.
At the time of the Voyager encounter, Neptune's most prominent feature was the Great Dark Spot (left) in the southern hemisphere. It was about half the size as Jupiter's Great Red Spot (about the same diameter as Earth). Neptune's winds blew the Great Dark Spot westward at 300 meters/second (700 mph). Voyager 2 also saw a smaller dark spot in the southern hemisphere and a small irregular white cloud that zips around Neptune every 16 hours or so now known as "The Scooter" (right). It may be a plume rising from lower in the atmosphere but its true nature remains a mystery.
However, HST observations of Neptune (left) in 1994 show that the Great Dark Spot has disappeared! It has either simply dissipated or is currently being masked by other aspects of the atmosphere. A few months later HST discovered a new dark spot in Neptune's northern hemisphere. This indicates that Neptune's atmosphere changes rapidly, perhaps due to slight changes in the temperature differences between the tops and bottoms of the clouds.
Neptune also has rings. Earth-based observations showed only faint arcs instead of complete rings, but Voyager 2's images showed them to be complete rings with bright clumps. One of the rings appears to have a curious twisted structure (right).
Like Uranus and Jupiter, Neptune's rings are very dark but their composition is unknown.
Neptune's rings have been given names: the outermost is Adams (which contains three prominent arcs now named Liberty, Equality and Fraternity), next is an unnamed ring co-orbital with Galatea, then Leverrier (whose outer extensions are called Lassell and Arago), and finally the faint but broad Galle.
Neptune's magnetic field is, like Uranus', oddly oriented and probably generated by motions of conductive material (probably water) in its middle layers.
Neptune can be seen with binoculars (if you know exactly where to look) but a large telescope is needed to see anything other than a tiny disk. There are several Web sites that show the current position of Neptune (and the other planets) in the sky, but much more detailed charts will be required to actually find it. Such charts can be created with a planetarium program.
Neptune's Satellites
Neptune has 13 known moons; 7 small named ones and Triton plus four discovered in 2002 and one discovered in 2003 which have yet to be named.
Distance Radius Mass
Satellite (000 km) (km) (kg) Discoverer Date
--------- -------- ------ ------- ---------- -----
Naiad 48 29 ? Voyager 2 1989
Thalassa 50 40 ? Voyager 2 1989
Despina 53 74 ? Voyager 2 1989
Galatea 62 79 ? Voyager 2 1989
Larissa 74 96 ? Voyager 2 1989
Proteus 118 209 ? Voyager 2 1989
Triton 355 1350 2.14e22 Lassell 1846
Nereid 5509 170 ? Kuiper 1949
Neptune's Rings
Distance Width
Ring (km) (km) aka
------- -------- ----- -------
Diffuse 41900 15 1989N3R, Galle
Inner 53200 15 1989N2R, LeVerrier
Plateau 53200 5800 1989N4R, Lassell, Arago
Main 62930 < 50 1989N1R, Adams
(distance is from Neptune's center to the ring's inner edge)
More about Neptune and its satellites
Web www.nineplanets.org
more Neptune images
from NSSDC
Changing seasons on Neptune
Neptune's Ring System
Voyager Neptune Science Summary from JPL
Neptunian System Nomenclature Tables
more on the 2002 moons
data on the moons discovered in 2002
Data on the newly discovered moons can be found at JPL and Scott Sheppard's site.
Open Issues
Neptune's magnetic field is off center and at a large angle to its rotation axis. What processes in the interior generate this oddly shaped field?
What accounts for the relative lack of hydrogen and helium in Neptune (and Uranus)?
Why are Neptune's winds so strong in spite of the fact that it is so far from the Sun and has a relatively weak internal heat source?
What happened to the Great Dark Spot?
Can we design a useful Neptune orbiter mission cheap enough to be funded?
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Bill Arnett; last updated: 2004 Sep 2 The most recent version of this page can be found at http://www.nineplanets.org/neptune.html
An Overview of Neptune's Interior
This is a drawing of the interior of Neptune.
Click on image for full size version (48K GIF)
The Giant planets do not have the same kind of layers inside that the Earth-like planets do. Their evolution was quite different than that of the Earth-like planets, and they have much more gas and ice inside.
Neptune's interior make-up is primarily methane ice, just as Uranus' is.
Motions in the interior of Neptune help form the magnetosphere of Neptune. Heat generated within Neptune helps form the unusual winds of the atmosphere.
Neptune's atmosphere shows a striped pattern of clouds. This cloud pattern is very similar to that of Jupiter and Saturn. Neptune even has a Great Dark Spot similar to Jupiter's Great Red Spot.
The history of Neptune's atmosphere is similar to that of the other Giant planets. The composition of Neptune clouds is thought to be methane molecules.
Motions in the cloud patterns give clues about Neptune's weather, which is similar to that of Jupiter and Saturn.
The magnetosphere of Neptune is very much like that of Uranus, medium sized but still much larger than the Earth's. Like that of Uranus, is probably made in the middle and with ice, rather than with iron at the core.
Like Uranus, the magnetosphere of Neptune has an extreme tilt, almost 60 degrees. Because Neptune itself is not tilted however, the magnetosphere of Neptune has a more standard, but still completely unique structure.
Mathematical theory suggests that the rings of Neptune affect the motion of particles in this unique magnetosphere, and also are responsible for the presence of three small plasmaspheres instead of one large version!
Like Saturn, Neptune's magnetosphere produces aurora but very faint ones, as well as radio emissions and other waves, such as whistler waves, chorus and hiss.
Neptune has 13 moons. It also has rings, but its rings are different from Saturn's.
Neptune's largest moon is named Triton. Triton is much larger than any of the planet's other moons. Triton is a very cold place, so the moon is covered with ice. Even though Triton is cold there is a lot going on there. It has geysers like the ones at Yellowstone Park on Earth. The geysers shoot ice 8 km (5 miles) high into Triton's thin atmosphere! There may be water under the ice at Triton. It is even possible that there might be life in that water. The interior of Triton is probably geologically active.
The rest of Neptune's moons are much smaller than Triton. Nereid was discovered by Gerard Kuiper in 1949. Despina, Galatea, Larissa, Naiad, Proteus and Thalassa were found by the Voyager 2 spacecraft in 1989. Five more small moons have been found recently. Three of those were discovered in 2002 and two more were found in 2003. The newest moons don't have official names yet.
Neptune's rings are much darker than Saturn's bright rings. Saturn's rings are made of ice, which reflects lots of light. Neptune's rings are probably made of rocks and dust. Rocks and dust don't reflect as much light.
Planetary Symbol: Name in Roman/Greek Mythology: Neptune/Poseidon
Diameter: 49,493 km (30,760 miles) Rotation Period about Axis: 17.24 hrs
Mass: 102.4x10^24 kilograms (17.2 x Earth's) Revolution Period about the Sun: 165 years
Density: 1,638 kg/m^3 Tilt of Axis: 29o 36"
Minimum Distance from Sun: 4.46 billion km
(2.77 billion miles) Surface Gravity: 11 m/s^2 (1.12 x Earth's)
Maximum Distance from Sun: 4.54 billion km
(2.82 billion miles) Temperature at Cloud Tops: -210o C ( -346o F)
Orbital Semimajor Axis: 30.07 AU (Earth=1 AU) Average Cloud Top Temperature (K): 63K
Minimum Distance from Earth: 4.3 billion km
(2.68 billion miles) Satellites/Rings: 13 known moons, faint rings
If you had a quiz question in school that asked what year Neptune was discovered, you'd probably choose 1846. But Neptune wasn't discovered the way all the other planets in our solar system were. Astronomers didn't scan the sky with their telescopes to find Neptune. They used math instead!
After the discovery of Uranus, scientists were having trouble figuring out the planet's orbit. They realized that there must be another planet farther out than Uranus. They were right! French astronomer Leverrier and English astronomer John Couch Adams made the mathematical calculations of where Neptune should be and German astronomer Johann Galle observed it. Talk about an international effort!
Neptune is the eighth planet in our solar sytem (most of the time, anyway).
All the planets were named after ancient gods. So when it came time to name this one, astronomers chose Neptune. Neptune was the Roman god of the deep seas.
Neptune's largest moon, Triton, was discovered at the same time as the planet. Another satellite, Nereid, wasn't found until 1949. The other six were spotted by Voyager II during its flyby in 1989. A lot of research has been done on Triton, and there is evidence that life may have existed there at one time.