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
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
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
Like Uranus and Jupiter, Neptune's rings are very dark but their composition
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
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 such as Starry
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
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)
Bill Arnett; last updated: 2003 Nov 2