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Diameter: 49,244 km
Distance from Sun: 4,503,000,000 km
Surface area: 7,618,272,763 km²
Length of day: 0d 16h 6m
Gravity: 11.15 m/s²
Circumference: 155,600 km
Mass: 102.4E24 kg (17.15 Earth mass)
Length of year: 165 years
Surface temp: -214 C

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.

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.

Neptune's Great Dark Spot

At the time of the Voyager encounter,  Neptune’s most  prominent feature was the Great Dark Spot (above) 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).

Neptune's Great Dark Spot2

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” (above). It may be a plume rising from lower in the atmosphere  but its true nature remains a mystery.

Neptune's spot's gone in HST photo

However,  HST observations of  Neptune (above) 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's rings 2

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's moon

Neptune has 13 known moons; 7 small named ones and Triton plus four discovered in  2002 and one  discovered in 2003.




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