[ˈnɛptun]) is the eighth
and furthest planet
from the Sun in our
system. It is the fourth largest planet by diameter and the third largest by
mass; Neptune is 17 times the mass of Earth and is slightly more massive than
its near twin
Uranus, which is 14 Earth Masses, but slightly smaller due to its higher
density. The planet is named after the
Roman god of the sea. Its
astronomical symbol (♆,
U+2646) is a stylized version of Poseidon's Trident. A portion of its orbit
(approximately 10 of every 250 years) lies farther from the Sun than the dwarf
Neptune's atmosphere is primarily composed of hydrogen and helium, with
traces of methane that account for the planet's blue appearance. Neptune's blue
color is much more vivid than that of Uranus, which has a similar amount of
methane, so an unknown component is presumed to cause Neptune's intense colour.
Neptune also has the strongest winds of any planet in the solar system, with
estimates as high as 2,500 km/h or 1,500 mph. At the time of the 1989
flyby, it had in its southern hemisphere a
Great Dark Spot comparable to the
Great Red Spot on
Neptune's temperature at its cloud tops is usually close to −210℃ (−346°F), one
of the coldest in the solar system, due to its long distance from the sun.
Neptune's centre is about 7,000℃ (13,000°F), however, hotter than the sun's
surface. This is due to extremely hot gases and rock in the centre.
Faint azure colored rings have been detected around the blue planet, but are
much less substantial than those of
these rings were discovered by a team led by
Edward Guinan, it was thought that they might not be complete. However, this
was disproved by Voyager 2. Neptune possesses thirteen confirmed moons.
Neptune's largest moon,
Triton, is notable for its
retrograde orbit, extreme cold (38K),
and extremely tenuous (14
microbar) nitrogen/methane atmosphere.
September 23, 1846,
Neptune is notable for being the first planet discovered based on mathematical
prediction rather than regular observations. Perturbations in the orbit of
astronomers to deduce Neptune's existence. It 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.
Shortly after its discovery, Neptune was referred to simply as "the planet
exterior to Uranus" or as "Le Verrier's planet". The first suggestion for a name
came from Galle. He proposed the name
England, Challis put forth the name
particularly appropriate for a seafaring people. In France,
Arago suggested that the new planet be called Leverrier, a suggestion
which was met with stiff resistance outside
almanacs promptly reintroduced the name Herschel for Uranus and
Leverrier for the new planet.
Urbain Le Verrier
John Couch Adams
||September 23, 1846
30.327 131 69 AU
29.810 795 27 AU
30.068 963 48
||0.008 585 87
(6.43° to Sun's equator)
Longitude of ascending node:
Argument of perihelion:
g) (At 1
|Sidereal rotation period:
||16.11 h (16 h 6 min 36 s)
|Rotation velocity at equator:
||2.68 km/s = 9660 km/h (at the equator)
Right ascension of North pole:
||299.33° (19 h 57 min 20 s)
Hydrogen - H2
19% ±3.2% Helium
Methane - CH4
Hydrogen Deuteride - HD
1.5 ppm Ethane
Meanwhile, on separate and independent occasions, Adams suggested altering
the name Georgian to Uranus, while Leverrier (through the Board of
Longitude) suggested Neptune for the new planet.
Struve came out in favor of that name on December
29, 1846, to
Saint Petersburg Academy of Sciences.
Soon Neptune became the internationally accepted nomenclature. In
Neptune was the god of the sea, identified with the Greek
The demand for a mythological name seemed to be in keeping with the nomenclature
of the other planets, all of which, except for Uranus, were named in antiquity.
The planet's name is translated literally as the sea king star in the
Vietnamese languages (海王星 in Chinese characters, 해왕성 in Korean).
In India the name given to the planet is Varuna (Devanagari वरुण) the god of
the sea in Vedic / Hindu mythology, the equivalent of Poseidon/Neptune in the
is an intermediate body between
Earth and the
largest gas giants: it is seventeen Earth masses but just 1/18th the mass of
and Uranus are
often considered a sub-class of gas giant termed "ice giants", given their
smaller size and important differences in composition relative to
and Saturn. In
the search for
extra-solar planets Neptune has been used as a
discovered bodies of similar mass are often referred to as "Neptunes"
just as astronomers refer to various extra-solar "Jupiters."
Orbiting so far from the sun, Neptune receives very little heat with the
uppermost regions of the atmosphere at −218 °C (55 K). 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
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
(80% and 19%, respectively). Increasing concentrations of methane, ammonia, and
water are found in the lower regions of the atmosphere. Gradually this darker
and hotter area blends into the superheated liquid interior. The pressure at the
center of Neptune is millions of times more than that on the surface of
its rotational speed to its degree of
indicates that it has its mass less concentrated towards the center than does
Neptune also resembles Uranus in its
magnetosphere, with a
magnetic field strongly tilted relative to its
axis at 47° 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. Neptune's
atmosphere has the highest wind speeds in the solar system, thought to be
powered by the flow of internal heat, and its weather is characterized by
with winds reaching up to 2000 km/h.
In 1989, the
Great Dark Spot, a cyclonic storm system the size of
discovered by NASA's
spacecraft. The storm resembled the
Great Red Spot of Jupiter. However, on
Hubble Space Telescope did not see the Great Dark Spot on the planet.
Instead, a new storm similar to the Great Dark Spot was found in the planet's
northern hemisphere. The reason for the Great Dark Spot's disappearance is
unknown. One possible theory is that heat transfer from the planet's core
disrupted the atmospheric equilibrium and disrupted existing circulation
patterns . The Scooter is another storm described as a
white cloud south of the Great Dark Spot. The
Wizard's eye (Great Dark Spot 2) is a southern hurricane, the second most
intensive hurricane known to exist on the planet.
Unique among the gas giants is the presence of high clouds casting shadows on
the opaque cloud deck below. Though Neptune's atmosphere is much more dynamic
than that of Uranus, both planets are made of the same gases and ices. Uranus
and Neptune are not strictly gas giants similar to Jupiter and Saturn, but are
rather ice giants, meaning they have a larger solid core and are also made of
ices. Neptune is very cold, with temperatures as low as -224°C (-372°F) recorded
at the cloud tops in 1989.
Exploration of Neptune
The closest approach of Voyager 2 to Neptune occurred on
1989. Since this
was the last major planet the spacecraft could visit, it was decided to make a
close flyby of the moon
Triton, regardless of the consequences to the trajectory, similarly to what
was done for
Saturn and its moon
The probe also discovered the
Great Dark Spot, which has since disappeared, according to
Hubble Space Telescope observations. Originally thought to be a large cloud
itself, it was later postulated to be a hole in the visible cloud deck.
Neptune turned out to have the strongest winds of all the solar system's gas
giants. In the outer regions of the solar system, where the Sun shines over 1000
times fainter than on Earth (still very bright with a magnitude of -21), the
last of the four giants defied all expectations of the scientists.
One might expect that the farther one gets from The Sun, the less energy
there would be to drive the winds around. The winds on Jupiter were already
hundreds of kilometres per hour. Rather than seeing slower winds, the scientists
found faster winds (over 1600 km/h) on more distant Neptune.
Scientists now know why this is the case —if enough energy is produced,
is created, which slows the winds down (like those of Jupiter). At Neptune
however, there is so little energy, that once winds are started, they meet very
little resistance, and are able to maintain extremely high velocities.
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,
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 Liberté, Egalité, and Fraternité (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
591-second exposure of the rings of Neptune taken
with the clear filter by the Voyager 2 wide-angle camera. The two main rings are
clearly visible and appear complete over the region imaged.
Also visible in this image is the inner faint ring and the faint
band which extends smoothly from the ring roughly halfway between the two bright
rings. Both of these newly discovered rings are broad and much fainter than the
two narrow rings.
The bright glare is due to overexposure of the crescent on
Neptune. Numerous bright stars are evident in the background. Both bright rings
have material throughout their entire orbit, and are therefore continuous.
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.
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 Liberté ring might disappear in as little as one
century. The new observations appear to throw our understanding of Neptune's
rings into considerable confusion.
|Name of ring
|1989 N3R ('Galle')
|1989 N2R ('Leverrier')
Urbain Le Verrier
|1989 N4R ('Lassell')
|Liberté Ring Arc
|Égalité Ring Arc
|Fraternité Ring Arc
|Courage Ring Arc
|1989 N1R ('Adams')
John Couch Adams
Neptune has 13 known
moons. The largest by far, and the only one massive enough to be
Triton, discovered by
William Lassell just 17 days after the discovery of Neptune itself. Unlike
all other large planetary moons, Triton has a
retrograde orbit, indicating that it was captured, and probably represents a
large example of a
Belt object (although clearly no longer in the Kuiper Belt). It is close
enough to Neptune to be locked into a
synchronous orbit, and is slowly spiraling inward and eventually will be
torn apart when it reaches the
limit. Triton is the coldest object that has been measured in the solar
system, with temperatures of 38.15K (-235°C, -392°F).
|Triton, compared to Earth's
|Orbital radius (km)
||Orbital period (days)
Neptune's second known satellite (by order of distance), the irregular moon
Nereid, has one of the most eccentric orbits of any satellite in the solar
From July to September 1989, Voyager 2 discovered six new Neptunian
moons. Of these, the irregularly shaped
Proteus is notable for being as large as a body of its density can be
without being pulled into a spherical shape by its own gravity. Although the
second most massive Neptunian moon, it is only one quarter of one percent of the
mass of Triton. Neptune's innermost four moons,
Galatea, orbit close enough to be within Neptune's rings. The next farthest
Larissa was originally discovered in 1981 when it had occulted a star. This
was attributed to ring arcs, but when Voyager 2 observed Neptune in 1989,
it was found to have been caused by the moon. Five new irregular moons
discovered between 2002 and 2003 were announced in 2004.
Neptune was the Roman god of the sea, the planet's moons have been named after
lesser sea gods.
Neptune and Triton, by Voyager 2
Appearance and visibility from Earth
Neptune is never visible with the
The brightness of Neptune is between
magnitudes +7.7 and +8.0, so a telescope or binoculars are required to
observe it. With the use of a telescope it appears as a small blue-green disk,
similar in appearance to Uranus; the blue-green color comes from the
its atmosphere. Its small apparent size has made it almost impossible to study
visually; even observatory data was fairly poor until the advent of
With an orbital period of 165 years, Neptune will soon return to the
approximate position in the sky where Galle discovered it. This will happen
three different times. These are
2009, when it will
2009, when it will
retrograde motion; and finally for the last time for the next 165 years, on
2010. This is
explained by the concept of
retrogradation. Like all planets in the solar system beyond Earth, Neptune
retrogradation at certain points during its
synodic period. In addition to the start of retrogradation, other events
within the synodic period include
astronomical opposition, the return to
prograde motion, and
conjunction to the Sun.
In its orbit around the Sun, Neptune will return to its original point of
discovery in August 2011.
Voyager II flew by Neptune and the images relayed back to
Earth became the
basis of a PBS all-night program called
Neptune All Night.
|Rings of Neptune added
|I learn a lot about this
beautiful yet cold and stormy planet. Thank you, Too bad there are
no images of the rings. We need to get back out there.