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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.
Neptune has been visited by only one spacecraft, Voyager 2 on Aug 25 1989. Almost everything we know about Neptune comes from this encounter.
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 the result of absorption of red light by methane in the atmosphere.
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 faded away or is currently being hidden 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, 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 has 8 known moons; 7 small ones and Triton.
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
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)
... Sun ... Uranus
... S/1997U1 ... Neptune ... Naiad
... Pluto ... 
