Planet 8. Neptune

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


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

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

Few big Satellites of Neptune 1. Triton

Triton ("TRY ton") is the seventh and by far the largest of Neptune's satellites:

orbit: 354,760 km from Neptune
diameter: 2700 km
mass: 2.14e22 kg


Discovered by Lassell in 1846 only a few weeks after the discovery of Neptune itself.

In Greek mythology, Triton is a god of the sea, the son of Poseidon (Neptune); usually portrayed as having the head and trunk of a man and the tail of a fish.

Triton has been visited by only one spacecraft, Voyager 2 on Aug 25 1989. Almost everything we know about it comes from this encounter.

Triton's orbit is retrograde. It is the only large moon to orbit "backwards", the only other moons with retrograde orbits are Jupiter's moons Ananke, Carme, Pasiphae and Sinope and Saturn's Phoebe all of which are less than 1/10 the diameter of Triton. Triton could not have condensed from the primordial Solar Nebula in this configuration; it must have formed elsewhere (perhaps in the Kuiper Belt?) and later been captured by Neptune (perhaps involving a collision with another now shattered Neptunian moon). A capture scenario could account not only for Triton's orbit but also for the unusual orbit of Nereid and provide the energy needed to melt and differentiate Triton's interior.

Because of its retrograde orbit, tidal interactions between Neptune and Triton remove energy from Triton thus lowering its orbit. At some very distant future time it will either break up (perhaps forming a ring) or crash into Neptune.

The unusual nature of Triton's orbit, the similarity of bulk properties between Pluto and Triton, and the highly eccentric, Neptune-crossing nature of Pluto's orbit suggest some historical connection between them. Exactly what this might be is purely conjecture at this time however.

Triton's axis of rotation is also unusual, tilted 157 degrees with respect to Neptune's axis (which is in turn inclined 30 degrees from the plane of Neptune's orbit). This adds up to an orientation with respect to the Sun somewhat like Uranus's with polar and equatorial regions alternately pointing toward the Sun. This probably results in radical seasonal changes as one pole then the other moves into the sunlight. During the Voyager 2 encounter, Triton's south pole was facing the Sun.

Triton's density (2.0) is slightly greater than that of Saturn's icy moons (e.g. Rhea). Triton is probably only about 25% water ice with remainder rocky material.

Voyager found that Triton has an atmosphere, albeit a very tenuous one (about 0.01 millibar), composed mostly of nitrogen with a small amount of methane. A thin haze extends up 5-10 km.

The temperature at the surface of Triton is only 34.5 K (-235 C, -391 F), as cold as Pluto. This is due in part to its high albedo (.7 - . which means that little of the Sun's meager light is absorbed. At this temperature methane, nitrogen and carbon dioxide all freeze solid.

There are very few craters visible; the surface is relatively young. Almost the entire southern hemisphere is covered with an "ice cap" of frozen nitrogen and methane (right).

There are extensive ridges and valleys in complex patterns all over Triton's surface. These are probably the result of freezing/thawing cycles.

The most interesting (and totally unexpected) features of this unusually interesting world are the ice volcanoes. The eruptive material is probably liquid nitrogen, dust, or methane compounds from beneath the surface. One of Voyager's images shows an actual plume rising 8 km above the surface and extending 140 km "downwind" (left).

Triton, Io and Venus are the only bodies in the solar system besides Earth that are known to be volcanically active at the present time (though Mars clearly was in the past). It's also interesting to note that very different volcanic processes occur in the outer solar system. Earth's and Venus' (and Mars') eruptions are of rocky material and are driven by internal heat. Io's eruptions are probably sulfur or sulfur compounds driven by tidal interactions with Jupiter. Triton's eruptions are of very volatile compounds like nitrogen or methane driven by seasonal heating from the Sun

Few big Satellites of Neptune 2. Proteus

Proteus ("PROH tee us") is the sixth of Neptune's known satellites and the second largest:

orbit: 117,600 km from Neptune
diameter: 418 km (436 x 416 x 402)
mass: ?


Proteus was a sea god who could change his shape at will.

Discovered in 1989 by Voyager 2. Though it is larger than Nereid, it was not discovered earlier because it is very dark and so close to Neptune that it is difficult to see in the glare of the much brighter planet.

Proteus is irregular (non-spherical) in shape. Proteus is probably about as big as an irregular body can be before its gravity pulls it into a more spherical shape.

The heavily cratered surface shows no signs of geologic activity