Neptune is the eighth planet from the Sun and last of the known
planets. While it is the third largest planet with respect to mass, it
is only the fourth largest in terms of diameter. Due to its blue
coloration, Neptune was named after the Roman god of the Sea.
When scientific discoveries are made there is often a debate
(sometimes heated) as to who deserves credit. The discovery of Neptune
is one such example. Shortly after the discovery of the planet Uranus in
1781, scientists noticed that its orbit had significant fluctuations
that were not expected. To solve this mystery, they proposed the
existence of another planet whose gravitational field would account for
such orbital variances.
In 1845, the English astronomer John Couch Adams completed his
calculations as to the position of this unknown planet. Although he
submitted his findings to the Royal Society (the leading English
scientific organization), his work was met with little interest.
However, a year later the French astronomer Jean Joseph Leverrier made
known his calculations that were strikingly similar to those of Adams.
As a result of the two men's independent estimates being so close, the
scientific community took notice and began its search for the planet in
the region of the sky Adams and Leverrier had predicted. On September
23, 1846, the German astronomer Johann Gall observed the new planet near
to where Adam's calculations had forecasted and even closer to those of
Leverrier.
Leverrier was initally given credit for the discovery. As a
result, an international dispute arose, with one faction championing
Adams and the other Leverrier. This conflict, however, was not shared
between the two men themselves. Eventually, the campaign for each side
cooled, and both men were given credit.
Until the Voyager 2 spacecraft fly-by in 1989, little was known
about Neptune. This mission provided new information about Neptune's
rings, number of moons, atmosphere and rotation. Additionally, Voyager 2
discovered significant features of the moon Trition. There are no
official planetary missions scheduled to Neptune in the near future.
Atmosphere
Neptune's upper atmosphere is composed of 80% hydrogen (H
2),
19% helium and trace amounts of methane. Similar to Uranus, the blue
coloration of Neptune is due in part to its atmospheric methane, which
absorbs light having a wavelength corresponding to red. Unlike Uranus,
Neptune is a deeper blue, and, therefore, some other atmospheric
component must be present in the Neptunian atmosphere that is not found
in Uranus' atmosphere.
Two significant weather patterns have been observed on Neptune.
The first, seen during the Voyager 2 fly-by mission, are the Dark Spots.
These are storms comparable to the Great Red Spot found on Jupiter.
However, a difference between these storms is there duration. Whereas
the Great Red Spot has lasted for centuries, the Dark Spots are much
more shortly lived as is evident by their disappearnce when Neptune was
viewed by the Hubble Space Telescope just four years after the Voyager 2
fly-by.
The second of the two weather patterns observed by Voyager 2 is
the swiftly moving white storm system, nicknamed Scooter. This type of
storm system, which is much smaller than the Dark Spots, also appears to
be short-lived.
As with the other gas giants, Neptune's atmosphere is divided
into latitudinal bands. The wind speed achieved in some of these bands
is almost 600 m/s, the fastest known in the Solar System.
Interior
The interior of Neptune, similar to that of Uranus, is made of
two layers: a core and mantle. The core is rocky and estimated to be 1.2
times as massive as the Earth. The mantle is an extremely hot and dense
liquid composed of water, ammonia and methane. The mantle is between
ten to fifteen times the mass of the Earth.
Although Neptune and Uranus share similar interiors, they are,
however, quite distinct in one way. Whereas Uranus emits only about the
same amount of heat that it recieves from the Sun, Neptune emits nearly
2.61 times the amount of the sunlight it receives. To place this in
perspective, the two planets' surface temperatures are approximately
equal, yet Neptune receives only 40% of the sunlight that Uranus does.
Additionally, this large internal heat is also what powers the extreme
winds found in the upper atmosphere.
Orbit & Rotation
With the discovery of Neptune, the size of the known Solar System
increased by a factor of two. With an average orbital distance of
4.50 x 109 km,
it takes sunlight almost four hours and forty minutes to reach Neptune.
Moreover, this distance also means that a Neptunian year lasts about
165 Earth years!
Neptune's orbital eccentricity of .0097 is second smallest behind
that of Venus. This small eccentricity means that the orbit of Neptune
is very close to being cicular. Another way of looking at this is to
compare Neptune's perihelion of
4.46 x 109 km and its aphelion of
4.54 x 109 km and notice that this is a difference of less than two percent.
Like Jupiter and Saturn, Neptune rotates very quickly as compared
to the terrestrial planets. With a rotational period of a little over
16 hours, Neptune has the third shortest day in the Solar System.
The axial tilt of Neptune is 28.3°, which is relatively close to
the Earth's 23.5°. What is amazing is that, even at such a far distance
from the Sun, Neptune still experiences seasons (though more subtly)
similar to those on Earth as a result of its axial tilt.
Rings
Currently, Neptune is known to have thirteen moons. Of these
thirteen only one is large and spherical in shape. This moon, Triton, is
believed to have originally been a dwarf planet captured by Neptune's
gravitational field, and, thus, not a natural satellite of the planet.
Evidence for this theory comes from Triton's retrograde orbit of
Neptune; that is, Triton orbits in the opposite direction that Neptune
rotates. With a recorded surface temperature of
-235° C, Triton is the coldest known object in the Solar System.
Neptune has three major rings—Adams, LeVerrier and Galle. This
ring system is much fainter than that of the other gas giants. In fact,
some of the rings are so dim that it was believed at one time that they
were incomplete. However, images from the Voyager 2 fly-bys show
extremely faint rings.
Discovery Date: 1846
Number of Moons: 13
Rings: Yes
Time it takes light to reach Neptune from the Sun:
4 hours 40 minutes
Mass:
5.68 x 1026 kg
Mean Diameter:
49,244 km
Mean Circumference:
154,705 km
Volume:
6.25 x 1013 km3
Density:
1,638 kg/m3
Surface Gravity:
11.6 m/s2
Length of Day:
16 hours 7 minutes
Length of Year:
60,190 Earth Days
Orbital Eccentricity:
0.0086
Mean Orbital Distance from the Sun:
4.50 x 109 km
Perihelion:
4.46 x 109 km
Aphelion:
4.54 x 109 km
Axial Tilt: 28.3°
Recent Planetary Missions:
Voyager 2
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