Neptune - The 8th Planet from our Sun
Composition,
structure and atmosphere
Magnetosphere
Planetary
rings
Orbit
and rotation
Moons
Neptune is the
eighth and farthest planet from the Sun in the Solar System. It is the fourth-largest planet
by diameter and the third-largest by mass. Among the giant planets in the Solar System, Neptune is the most
dense. Neptune is 17 times the mass of Earth
and is slightly more massive than its near-twin Uranus, which is 15 times the mass of Earth
and slightly larger Neptune.[c] Neptune orbits the Sun at an average
distance of 30.1 astronomical units (4.50×109 km).
Named after the Roman god of the sea,
its astronomical symbol is ♆, a stylised version of the god Neptune's trident.
Neptune
is not visible to the unaided eye and is the only planet found by mathematical
prediction rather than by empirical observation.
Unexpected changes in the orbit of Uranus led Alexis Bouvard to deduce that its orbit was subject to gravitational perturbation by an unknown planet. Neptune was
subsequently observed with a telescope on 23 September 1846 by Johann Galle within a degree of the position predicted by Urbain Le Verrier. Its largest moon, Triton, was discovered shortly thereafter,
though none of the planet's remaining 14 moons were
located telescopically until the 20th century. The planet's distance from Earth
gives it a very small apparent size, making it challenging to study with
Earth-based telescopes. Neptune was visited by Voyager 2, when it flew by the
planet on 25 August 1989. The
advent of Hubble Space
Telescope and
large ground-based
telescopes with adaptive optics has allowed for more-detailed observations.
Neptune
is similar in composition to Uranus, and both have compositions that differ
from those of the larger gas giants, Jupiter and Saturn. Neptune's atmosphere, like Jupiter's
and Saturn's, is composed primarily of hydrogen and helium, along with traces of hydrocarbons and possibly nitrogen; it contains a higher proportion of
"ices" such as water, ammonia, and methane. Scientists sometimes categorise
Uranus and Neptune as "ice giants" to
emphasise this distinction. The
interior of Neptune, like that of Uranus, is primarily composed of ices and
rock. Traces
of methane in the outermost regions in part account for the planet's blue
appearance.
In
contrast to the hazy, relatively featureless atmosphere of Uranus, Neptune's
atmosphere has active and visible weather patterns. For example, at the time of
the 1989 Voyager
2 flyby, the planet's southern hemisphere had a Great Dark Spot comparable to the Great Red Spot on Jupiter. These weather patterns are
driven by the strongest sustained winds of any planet in the Solar System, with
recorded wind speeds as high as 2,100 kilometres per hour (580 m/s;
1,300 mph). Because
of its great distance from the Sun, Neptune's outer atmosphere is one of the
coldest places in the Solar System, with temperatures at its cloud tops
approaching 55 K (−218 °C). Temperatures at the planet's centre are
approximately 5,400 K (5,100 °C). Neptune has a faint and fragmented ring system (labelled "arcs"), which may have
been detected during the 1960s but was indisputably confirmed only in 1989 by Voyager 2.
Composition,
structure and atmosphere
Neptune's mass of 1.0243×1026 kg, is
intermediate between Earth and the
larger gas giants: it is 17
times that of Earth but just 1/19th that of Jupiter. Its surface gravity is surpassed only by Jupiter. Neptune's equatorial radius of 24,764 km is
nearly four times that of Earth. Neptune, like Uranus, is an ice giant, a subclass of giant planet, due to their
smaller size and higher concentrations of volatiles relative to
Jupiter and Saturn. In the
search for extrasolar planets, Neptune has
been used as a metonym: discovered bodies of
similar mass are often referred to as "Neptunes", just as
scientists refer to various extrasolar bodies as "Jupiters".
Neptune's internal structure resembles that of Uranus. Its atmosphere forms
about 5% to 10% of its mass and extends perhaps 10% to 20% of the way towards
the core, where it reaches pressures of about 10 GPa, or about 100,000 times that of Earth's
atmosphere. Increasing concentrations of methane, ammonia and water are found in the
lower regions of the atmosphere.
The
mantle is equivalent to 10 to 15 Earth masses and is rich in water, ammonia and
methane. As is customary in planetary science, this mixture is referred
to as icy even
though it is a hot, dense fluid. This fluid, which has a high electrical
conductivity, is sometimes called a water–ammonia ocean. The mantle may consist of a layer of ionic
water in which the water molecules break down into a soup of hydrogen and oxygen ions, and deeper down superionic water in which the oxygen crystallises but the hydrogen ions float around freely within the oxygen
lattice. At a
depth of 7000 km, the conditions may be such that methane decomposes into
diamond crystals that rain downwards like hailstones. Very-high-pressure experiments at the Lawrence
Livermore National Laboratory suggest that the base of the mantle may
comprise an ocean of liquid diamond, with floating solid 'diamond-bergs'.
The core of
Neptune is composed of iron, nickel and silicates, with an interior model giving a
mass about 1.2 times that of Earth. The
pressure at the centre is 7 Mbar (700
GPa), about twice as high as that at the centre of Earth, and the temperature
may be 5,400 K.
At high
altitudes, Neptune's atmosphere is 80% hydrogen and 19% helium. A trace amount of methane is also present.
Prominent absorption bands of methane exist at wavelengths above 600 nm,
in the red and infrared portion of the spectrum. As with Uranus, this
absorption of red light by the atmospheric methane is part of what gives Neptune its blue hue, although Neptune's vivid azure differs from
Uranus's milder cyan.
Because Neptune's atmospheric methane content is similar to that of Uranus,
some unknown atmospheric constituent is thought to contribute to Neptune's
colour.
Neptune's
atmosphere is subdivided into two main regions: the lower troposphere, where temperature decreases with
altitude, and the stratosphere, where
temperature increases with altitude. The boundary between the two, the tropopause, lies at a pressure of 0.1 bars
(10 kPa). The
stratosphere then gives way to the thermosphere at a pressure lower than 10−5 to 10−4 microbars (1 to 10 Pa). The thermosphere gradually transitions to
the exosphere.
Models
suggest that Neptune's troposphere is banded by clouds of varying compositions
depending on altitude. The upper-level clouds lie at pressures below one bar,
where the temperature is suitable for methane to condense. For pressures
between one and five bars (100 and 500 kPa), clouds of ammonia and hydrogen sulfide are thought to form. Above a pressure of
five bars, the clouds may consist of ammonia, ammonium sulfide, hydrogen sulfide and water.
Deeper clouds of water ice should be found at pressures of about 50 bars
(5.0 MPa), where the temperature reaches 273 K (0 °C).
Underneath, clouds of ammonia and hydrogen sulfide may be found.[51]
High-altitude
clouds on Neptune have been observed casting shadows on the opaque cloud deck
below. There are also high-altitude cloud bands that wrap around the planet at
constant latitude. These circumferential bands have widths of 50–150 km
and lie about 50–110 km above the cloud deck. These altitudes are in the layer where weather
occurs, the troposphere. Weather does not occur in the higher stratosphere or
thermosphere. Unlike Uranus, Neptune's composition has a higher volume of
ocean, whereas Uranus has a smaller mantle.
Neptune's spectra suggest that its lower stratosphere is hazy
due to condensation of products of ultraviolet photolysis of methane, such as ethane and ethyne. The stratosphere is also home to trace
amounts of carbon monoxide and hydrogen cyanide. The stratosphere of Neptune is warmer than
that of Uranus due to the elevated concentration of hydrocarbons.
For
reasons that remain obscure, the planet's thermosphere is at an anomalously
high temperature of about 750 K. The
planet is too far from the Sun for this heat to be generated by ultraviolet radiation. One candidate for a heating
mechanism is atmospheric interaction with ions in the planet's magnetic field. Other candidates are gravity waves from the interior that dissipate in the
atmosphere. The thermosphere contains traces of carbon dioxide and water, which may have been deposited
from external sources such as meteorites and dust.
Magnetosphere
Neptune
also resembles Uranus in its magnetosphere, with a magnetic field strongly tilted relative to its rotational axis at 47° and offset at least
0.55 radii, or about 13500 km from the planet's physical centre.
Before Voyager
2's arrival at Neptune, it was hypothesised that Uranus's tilted
magnetosphere was the result of its sideways rotation. In comparing the
magnetic fields of the two planets, scientists now think the extreme
orientation may be characteristic of flows in the planets' interiors. This
field may be generated by convective fluid motions in a thin spherical shell of electrically
conducting liquids
(probably a combination of ammonia, methane and water)[51] resulting in a dynamo action.
The
dipole component of the magnetic field at the magnetic equator of Neptune is
about 14 microteslas (0.14 G). The
dipole magnetic moment of Neptune is about 2.2 × 1017 T•m3 (14 μT•RN3, where RN is the radius of Neptune). Neptune's
magnetic field has a complex geometry that includes relatively large
contributions from non-dipolar components, including a strong quadrupole moment that may exceed the dipole moment in strength. By contrast, Earth, Jupiter
and Saturn have only relatively small quadrupole moments, and their fields are
less tilted from the polar axis. The large quadrupole moment of Neptune may be
the result of offset from the planet's centre and geometrical constraints of
the field's dynamo generator.
Neptune's bow shock, where the magnetosphere begins to
slow the solar wind, occurs at a distance of 34.9 times
the radius of the planet. The magnetopause, where the pressure of the
magnetosphere counterbalances the solar wind, lies at a distance of 23–26.5
times the radius of Neptune. The tail of the magnetosphere extends out to at
least 72 times the radius of Neptune, and likely much farther.
Planetary
rings
Neptune
has a planetary ring system, though one much less substantial
than that of Saturn. The rings may consist of ice particles
coated with silicates or carbon-based material, which most likely gives them a
reddish hue. The
three main rings are the narrow Adams Ring, 63,000 km from the centre of
Neptune, the Le Verrier Ring, at 53,000 km, and the broader, fainter Galle
Ring, at 42,000 km. A faint outward extension to the Le Verrier Ring has
been named Lassell; it is bounded at its outer edge by the Arago Ring at 57,000 km.
The
first of these planetary rings was detected in 1968 by a team led by Edward Guinan. In the early 1980s, analysis of this data
along with newer observations led to the hypothesis that this ring might be
incomplete. Evidence
that the rings might have gaps first arose during a stellar occultation in 1984 when the rings obscured a star on
immersion but not on emersion. Images
by Voyager
2 in 1989
settled the issue by showing several faint rings. These rings have a clumpy
structure, the
cause of which is not understood but which may be due to the gravitational
interaction with small moons in orbit near them.
The
outermost ring, Adams, contains five prominent arcs now named Courage, Liberté, Egalité 1, Egalité 2 and Fraternité (Courage, Liberty, Equality and
Fraternity). The
existence of arcs was difficult to explain because the laws of motion would
predict that arcs would spread out into a uniform ring over short timescales.
Astronomers now estimate that the arcs are corralled into their current form by
the gravitational effects of Galatea, a moon just inward from the ring.
Earth-based
observations announced in 2005 appeared to show that Neptune's rings are much
more unstable than previously thought. Images taken from the W. M. Keck
Observatory in 2002
and 2003 show considerable decay in the rings when compared to images by Voyager 2. In particular, it
seems that the Liberté arc might disappear in as little as one
century.
Orbit
and rotation
The
average distance between Neptune and the Sun is 4.50 billion km (about 30.1 astronomical units (AU)), and it completes an orbit on
average every 164.79 years, subject to a variability of around
±0.1 years. The perihelion distance is 29.81 AU; the aphelion
distance is 30.33 AU.
On 11
July 2011, Neptune completed its first full barycentric orbit since its discovery in 1846, although it did not appear at its
exact discovery position in the sky, because Earth was in a different location
in its 365.26-day orbit. Because of the motion of the Sun in relation to the barycentre of the Solar System, on 11 July
Neptune was also not at its exact discovery position in relation to the Sun; if
the more common heliocentric coordinate
system is used, the discovery longitude was reached on 12 July 2011.
The
elliptical orbit of Neptune is inclined 1.77° compared to that of Earth.
The axial
tilt of Neptune is 28.32°, which
is similar to the tilts of Earth (23°) and Mars (25°). As a result, this planet
experiences similar seasonal changes. The long orbital period of Neptune means
that the seasons last for forty Earth years. Its
sidereal rotation period (day) is roughly 16.11 hours. Because its axial tilt is comparable
to Earth's, the variation in the length of its day over the course of its long
year is not any more extreme.
Because
Neptune is not a solid body, its atmosphere undergoes differential rotation.
The wide equatorial zone rotates with a period of about 18 hours, which is
slower than the 16.1-hour rotation of the planet's magnetic field. By contrast,
the reverse is true for the polar regions where the rotation period is
12 hours. This differential rotation is the most pronounced of any planet
in the Solar System, and it
results in strong latitudinal wind shear.
Neptune's
orbit has a profound impact on the region directly beyond it, known as the Kuiper belt. The Kuiper belt is a ring of
small icy worlds, similar to the asteroid belt but far larger, extending from
Neptune's orbit at 30 AU out to about 55 AU from the Sun. Much in the same way that Jupiter's
gravity dominates the asteroid belt, shaping its structure, so
Neptune's gravity dominates the Kuiper belt. Over the age of the Solar System,
certain regions of the Kuiper belt became destabilised by Neptune's gravity,
creating gaps in the Kuiper belt's structure. The region between 40 and
42 AU is an example.
There
do exist orbits within these empty regions where objects can survive for the
age of the Solar System. These resonances occur
when Neptune's orbital period is a precise fraction of that of the object, such
as 1:2, or 3:4. If, say, an object orbits the Sun once for every two Neptune
orbits, it will only complete half an orbit by the time Neptune returns to its
original position. The most heavily populated resonance in the Kuiper belt,
with over 200 known objects, is
the 2:3 resonance. Objects in this resonance complete 2 orbits for every 3 of
Neptune, and are known as plutinos because
the largest of the known Kuiper belt objects, Pluto,
is among them. Although Pluto
crosses Neptune's orbit regularly, the 2:3 resonance ensures they can never collide. The 3:4, 3:5, 4:7 and 2:5 resonances
are less populated.
Neptune
has a number of known trojan objects occupying both the Sun–Neptune L4 and L5 Lagrangian points—gravitationally stable
regions leading and trailing Neptune in its orbit, respectively. Neptune trojans can be viewed as being in a 1:1
resonance with Neptune. Some Neptune trojans are remarkably stable in their
orbits, and are likely to have formed alongside Neptune rather than being
captured. The first and so far only object identified as associated with
Neptune's trailing L5 Lagrangian point is 2008 LC18. Neptune also has a temporary quasi-satellite, (309239) 2007 RW10. The object has been a quasi-satellite
of Neptune for about 12,500 years and it will remain in that dynamical state
for another 12,500 years.
Moons
Neptune
has 14 known moons. Triton is
the largest Neptunian moon, comprising more than 99.5% of the mass in orbit
around Neptune,[e]and it is the only one massive
enough to be spheroidal. Triton was
discovered by William Lassell just 17 days after the discovery
of Neptune itself. Unlike all other large planetary moons in the Solar System,
Triton has a retrograde orbit, indicating that it was
captured rather than forming in place; it was probably once a dwarf planet in
the Kuiper belt. It is close
enough to Neptune to be locked into a synchronous rotation,
and it is slowly spiralling inward because of tidal acceleration.
It will eventually be torn apart, in about 3.6 billion years, when it
reaches the Roche limit. In
1989, Triton was the coldest object that had yet been measured in the Solar
System, with estimated
temperatures of 38 K (−235 °C).
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