Neptune is the eighth or, occasionally, the ninth planet from
the Sun due to Pluto's eccentric orbit (if Pluto is considered a
planet), and the outermost gas giant in our solar system. Although
the smallest of the gas giants, Neptune is more massive than
Uranus: Its stronger gravitational field has compressed it to a
higher density.
Faint dark rings have been detected around the blue planet, but are
less substantial than those of Saturn. When these rings were
discovered, it was thought that they might not be complete, but
this was disproved by Voyager 2. Neptune also has 2,000 km/h winds
of hydrogen, helium, and methane that gives the planet its blue
appearance. At the time of the 1989 Voyager 2 flyby, it had in its
southern hemisphere a Great Dark Spot comparable to the Great Red
Spot on Jupiter. Neptune possesses nine confirmed moons and four
awaiting confirmation. Neptune's largest moon, Triton, is notable
for its retrograde orbit, extreme cold (38K), and extremely tenuous
(14 microbar) nitrogen/methane atmosphere.
Neptune is named after the Roman god of the sea. It is represented
in Unicode by a stylized version of the god's trident (¦·).
Discovered on September 23, 1846, Neptune has been visited by only
one spacecraft, Voyager 2, which flew by the planet on August 25,
1989. In 2003, there was a proposal to NASA's "Vision Missions
Studies" to implement a "Neptune Orbiter with Probes" mission that
does Cassini-level science without fission-based electric power or
propulsion. The work is being done in conjunction with JPL and the
California Institute of Technology
Galileo's astronomical drawings show that he had first observed
Neptune on December 27, 1612, and again on January 27, 1613; on
both occasions Galileo mistook Neptune for a fixed star when it
appeared very close (in conjunction) to Jupiter in the night sky.
Believing it to be a fixed star, he cannot be credited with its
discovery. At the time Galileo first observed Neptune on December
28, 1612, it was stationary in the sky because it had just turned
retrograde that very day; because it was stationary in the sky and
only beginning the planet's yearly retrograde cycle, its motion was
far too slight to be detected with Galileo's small telescope. Had
Neptune been moving at its regular/average speed when Galileo first
observed it in 1612 and 1613, he would have most likely realized
that it was a planet and not a fixed star due to Neptune's
relatively rapid normal motion along the ecliptic compared to the
extremely slow motion of any random fixed star found in the night
sky.
Orbiting so far from the sun, Neptune receives very little heat ¡ª
in fact the uppermost regions of the atmosphere are −218 ¡ãC (55
K). Because Neptune is a gas giant, there is no solid surface; as
one ventures deeper and deeper inside the layers of gas, however,
the temperature rises steadily. It is thought that this may be
leftover heat generated by infalling matter during the planet's
birth, now slowly radiating away into space. Neptune's atmosphere
has the highest wind speeds in the solar system, up to 2000 km/h,
thought to be powered by this flow of internal heat.
The internal structure resembles that of Uranus. There is likely to
be a core consisting of (molten) rock and metal, surrounded by a
mixture of rock, water, ammonia, and methane. The atmosphere,
extending perhaps 10 to 20 percent of the way towards the center,
is mostly hydrogen and helium at high altitudes, but has increasing
concentrations of methane, ammonia, and water as it approaches and
finally blends into the liquid interior. The pressure at the center
of Neptune is millions of times more than that on the surface of
Earth. Comparing its rotational speed to its degree of oblateness
indicates that it has its mass less concentrated towards the center
than does Uranus.
Neptune also resembles Uranus in its magnetosphere, with a magnetic
field strongly tilted relative to its rotational axis at 47¡ã and
offset at least 0.55 radii (about 13,500 kilometres) from the
planet's physical center. Comparing the magnetic fields of the two
planets, scientists think the extreme orientation may be
characteristic of flows in the interior of the planet and not the
result of Uranus' sideways orientation.
One difference between Neptune and Uranus is the level of
meteorological activity. Uranus is visually quite bland, while
Neptune's high winds come with notable weather phenomena. The Great
Dark Spot, a cyclonic storm system the size of Eurasia, was
captured by Voyager 2 in the 1989 flyby. The storm resembled the
Great Red Spot of Jupiter, but was shown to have disappeared in
June 1994. However, a newer image of the planet taken by the Hubble
Space Telescope on November 2, 1994, revealed that a smaller storm
similar to its predecessor had formed over Neptune¡¯s Northern
Hemisphere. Unique among the gas giants is the presence of high
clouds casting shadows on the opaque cloud deck below.
Neptune has a faint planetary ring system of unknown composition.
The rings have a peculiar "clumpy" structure, the cause of which is
not currently understood but which may be due to the gravitational
interaction with small moons in orbit near them.
Evidence that the rings are incomplete first arose in the
mid-1980s, when stellar occultation experiments were found to
occasionally show an extra "blink" just before or after the planet
occulted the star. Images by Voyager 2 in 1989 settled the issue,
when the ring system was found to contain several faint rings. The
outermost ring, Adams, contains three prominent arcs now named
Libert¨¦, Egalit¨¦, and Fraternit¨¦ (Liberty, Equality, and
Fraternity). The existence of arcs is very difficult to understand
because the laws of motion would predict that arcs spread out into
a uniform ring over very short timescales. The gravitational
effects of Galatea, a moon just inward from the ring, are now
believed to confine the arcs.
Several other rings were detected by the Voyager cameras. In
addition to the narrow Adams Ring 63,000 km from the centre of
Neptune, the Leverrier Ring is at 53,000 km and the broader,
fainter Galle Ring is at 42,000 km. A faint outward extension to
the Leverrier Ring has been named Lassell; it is bounded at its
outer edge by the Arago Ring at 57,000 km.5
New Earth-based observations announced in 2005 appeared to show
that Neptune's rings are much more unstable than previously
thought. In particular, it seems that the Libert¨¦ ring might
disappear in as little as one century. The new observations appear
to throw our understanding of Neptune's rings into considerable
confusion.
