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09-08-2014, 10:52 AM
Solar system
[attachment=66779]
Solar system
The Solar System or solar system
consists of the Sun and the other
celestial objects gravitationally bound to
it: the eight planets, their 166 known
moons,
[1] three dwarf planets (Ceres,
Pluto, and Eris and their four known
moons), and billions of small bodies. This
last category includes asteroids, Kuiper
belt objects, comets, meteoroids, and
interplanetary dust.
In broad terms, the charted regions of
the Solar System consist of the Sun, four
terrestrial inner planets, an asteroid
belt composed of small rocky bodies, four
gas giant outer planets, and a second
belt, called the Kuiper belt, composed of icy objects. Beyond the Kuiper belt
is the scattered disc, the heliopause, and ultimately the hypothetical Oort
cloud.
In order of their distances from the Sun, the planets are Mercury, Venus,
Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Six of the eight planets
are in turn orbited by natural satellites, usually termed "moons" after
Earth's Moon, and each of the outer planets is encircled by planetary rings
of dust and other particles. All the planets except Earth are named after
gods and goddesses from Greco-Roman mythology. The three dwarf planets
are Pluto, the largest known Kuiper belt object; Ceres, the largest object in
the asteroid belt; and Eris, which lies in the scattered disc.
Terminology
Objects orbiting the Sun are divided into
three classes: planets, dwarf planets, and
small Solar System bodies.
A planet is any body in orbit around the
Sun that a) has enough mass to form
itself into a spherical shape and b) has
cleared its immediate neighbourhood of
all smaller objects. There are eight
known planets: Mercury, Venus, Earth,
Mars, Jupiter, Saturn, Uranus, and
Neptune.
Formation
The Solar System is believed to have
formed according to the nebular
hypothesis, first proposed in 1755 by
Immanuel Kant and independently
formulated by Pierre-Simon Laplace.
[7]
This theory holds that 4.6 billion years
ago the Solar System formed from the
gravitational collapse of a giant
molecular cloud. This initial cloud was
likely several light-years across and
probably birthed several stars.[8] Studies of ancient meteorites reveal traces
of elements only formed in the hearts of very large exploding stars,
indicating that the Sun formed within a star cluster, and in range of a
number of nearby supernovae explosions. The shock wave from these
supernovae may have triggered the formation of the Sun by creating regions
of overdensity in the surrounding nebula, allowing gravitational forces to
overcome internal gas pressures and cause collapse.[9]
The region that would become the Solar System, known as the pre-solar
nebula,
[10] had a diameter of between 7000 and 20,000 AU[11] [12] and a mass
just over that of the Sun (by between 0.1 and 0.001 solar masses).[13] As the
nebula collapsed, conservation of angular momentum made it rotate faster.
As the material within the nebula condensed, the atoms within it began to
tostar; then gathered by
direct contact into clumps between one and ten metres in diameter; then
collided to form larger bodies (planetesimals) of roughly 5 km in size; then
gradually increased by further collisions at roughly 15 cm per year over the
course of the next few million years.[21]
The inner Solar System was too warm for volatile molecules like water and
methane to condense, and so the planetesimals which formed there were
relatively small (comprising only 0.6% the mass of the disc)[22] and composed
largely of compounds with high melting points, such as silicates and metals.
These rocky bodies eventually became the terrestrial planets. Farther out,
the gravitational effects of Jupiter made it impossible for the protoplanetary
objects present to come together, leaving behind the asteroid belt.
[23]
Farther out still, beyond the frost line, where more volatile icy compounds
could remain solid, Jupiter and Saturn became the gas giants. Uranus and
Neptune captured much less material and are known as ice giants because
their cores are believed to be made mostly of ices (hydrogen compounds).[24]
[25]
Once the young Sun began producing energy, the solar wind (see below)
blew the gas and dust in the protoplanetary disk into interstellar space and
ended the growth of the planets. T Tauri stars have far stronger stellar
winds than more stable, older stars
Mercury
Mercury (0.4 AU) is the closest planet to the Sun and the smallest
planet (0.055 Earth masses). Mercury has no natural satellites, and its
only known geological features besides impact craters are
"wrinkle-ridges", probably produced by a period of contraction early in
its history.[41] Mercury's almost negligible atmosphere consists of atoms
blasted off its surface by the solar wind.[42] Its relatively large iron core
and thin mantle have not yet been adequately explained. Hypotheses
include that its outer layers were stripped off by a giant impact, and
that it was prevented from fully accreting by the young Sun's energy.[43]
[44]
Venus
Venus (0.7 AU) is close in size to Earth (0.815 Earth masses) and, like
Earth, has a thick silicate mantle around an iron core, a substantial
atmosphere and evidence of internal geological activity. However, it is
much drier than Earth and its atmosphere is ninety times as dense.
Venus has no natural satellites. It is the hottest planet, with surface
temperatures over 400 °C, most likely due to the amount of greenhouse
gases in the atmosphere.[45] No definitive evidence of current geological
activity has been detected on Venus, but it has no magnetic field that
would prevent depletion of its substantial atmosphere, which suggests
that its atmosphere is regularly replenished by volcanic eruptions.[46]
Notes
1. Capitalization of the name varies. The IAU, the authoritative body
regarding astronomical nomenclature, specifies capitalizing the names
of all individual astronomical objects (Solar System). However, the
name is commonly rendered in lower case (solar system) including in
the Oxford English Dictionary, Merriam-Webster's 11th Collegiate
Dictionary, and Encyclopædia Britannica.
2. The mass of the Solar System excluding the Sun, Jupiter and Saturn can
be determined by adding together all the calculated masses for its
largest objects and using rough calculations for the masses of the Oort
cloud (estimated at roughly 3 Earth masses),[110] the Kuiper Belt
(estimated at roughly 0.1 Earth mass)[111] and the asteroid belt
(estimated to be 0.0005 Earth mass)[112] for a total, rounded upwards, of
~37 Earth masses, or 8.9 percent the combined mass of Jupiter and
Saturn.
[attachment=66779]
Solar system
The Solar System or solar system
consists of the Sun and the other
celestial objects gravitationally bound to
it: the eight planets, their 166 known
moons,
[1] three dwarf planets (Ceres,
Pluto, and Eris and their four known
moons), and billions of small bodies. This
last category includes asteroids, Kuiper
belt objects, comets, meteoroids, and
interplanetary dust.
In broad terms, the charted regions of
the Solar System consist of the Sun, four
terrestrial inner planets, an asteroid
belt composed of small rocky bodies, four
gas giant outer planets, and a second
belt, called the Kuiper belt, composed of icy objects. Beyond the Kuiper belt
is the scattered disc, the heliopause, and ultimately the hypothetical Oort
cloud.
In order of their distances from the Sun, the planets are Mercury, Venus,
Earth, Mars, Jupiter, Saturn, Uranus, and Neptune. Six of the eight planets
are in turn orbited by natural satellites, usually termed "moons" after
Earth's Moon, and each of the outer planets is encircled by planetary rings
of dust and other particles. All the planets except Earth are named after
gods and goddesses from Greco-Roman mythology. The three dwarf planets
are Pluto, the largest known Kuiper belt object; Ceres, the largest object in
the asteroid belt; and Eris, which lies in the scattered disc.
Terminology
Objects orbiting the Sun are divided into
three classes: planets, dwarf planets, and
small Solar System bodies.
A planet is any body in orbit around the
Sun that a) has enough mass to form
itself into a spherical shape and b) has
cleared its immediate neighbourhood of
all smaller objects. There are eight
known planets: Mercury, Venus, Earth,
Mars, Jupiter, Saturn, Uranus, and
Neptune.
Formation
The Solar System is believed to have
formed according to the nebular
hypothesis, first proposed in 1755 by
Immanuel Kant and independently
formulated by Pierre-Simon Laplace.
[7]
This theory holds that 4.6 billion years
ago the Solar System formed from the
gravitational collapse of a giant
molecular cloud. This initial cloud was
likely several light-years across and
probably birthed several stars.[8] Studies of ancient meteorites reveal traces
of elements only formed in the hearts of very large exploding stars,
indicating that the Sun formed within a star cluster, and in range of a
number of nearby supernovae explosions. The shock wave from these
supernovae may have triggered the formation of the Sun by creating regions
of overdensity in the surrounding nebula, allowing gravitational forces to
overcome internal gas pressures and cause collapse.[9]
The region that would become the Solar System, known as the pre-solar
nebula,
[10] had a diameter of between 7000 and 20,000 AU[11] [12] and a mass
just over that of the Sun (by between 0.1 and 0.001 solar masses).[13] As the
nebula collapsed, conservation of angular momentum made it rotate faster.
As the material within the nebula condensed, the atoms within it began to
tostar; then gathered by
direct contact into clumps between one and ten metres in diameter; then
collided to form larger bodies (planetesimals) of roughly 5 km in size; then
gradually increased by further collisions at roughly 15 cm per year over the
course of the next few million years.[21]
The inner Solar System was too warm for volatile molecules like water and
methane to condense, and so the planetesimals which formed there were
relatively small (comprising only 0.6% the mass of the disc)[22] and composed
largely of compounds with high melting points, such as silicates and metals.
These rocky bodies eventually became the terrestrial planets. Farther out,
the gravitational effects of Jupiter made it impossible for the protoplanetary
objects present to come together, leaving behind the asteroid belt.
[23]
Farther out still, beyond the frost line, where more volatile icy compounds
could remain solid, Jupiter and Saturn became the gas giants. Uranus and
Neptune captured much less material and are known as ice giants because
their cores are believed to be made mostly of ices (hydrogen compounds).[24]
[25]
Once the young Sun began producing energy, the solar wind (see below)
blew the gas and dust in the protoplanetary disk into interstellar space and
ended the growth of the planets. T Tauri stars have far stronger stellar
winds than more stable, older stars
Mercury
Mercury (0.4 AU) is the closest planet to the Sun and the smallest
planet (0.055 Earth masses). Mercury has no natural satellites, and its
only known geological features besides impact craters are
"wrinkle-ridges", probably produced by a period of contraction early in
its history.[41] Mercury's almost negligible atmosphere consists of atoms
blasted off its surface by the solar wind.[42] Its relatively large iron core
and thin mantle have not yet been adequately explained. Hypotheses
include that its outer layers were stripped off by a giant impact, and
that it was prevented from fully accreting by the young Sun's energy.[43]
[44]
Venus
Venus (0.7 AU) is close in size to Earth (0.815 Earth masses) and, like
Earth, has a thick silicate mantle around an iron core, a substantial
atmosphere and evidence of internal geological activity. However, it is
much drier than Earth and its atmosphere is ninety times as dense.
Venus has no natural satellites. It is the hottest planet, with surface
temperatures over 400 °C, most likely due to the amount of greenhouse
gases in the atmosphere.[45] No definitive evidence of current geological
activity has been detected on Venus, but it has no magnetic field that
would prevent depletion of its substantial atmosphere, which suggests
that its atmosphere is regularly replenished by volcanic eruptions.[46]
Notes
1. Capitalization of the name varies. The IAU, the authoritative body
regarding astronomical nomenclature, specifies capitalizing the names
of all individual astronomical objects (Solar System). However, the
name is commonly rendered in lower case (solar system) including in
the Oxford English Dictionary, Merriam-Webster's 11th Collegiate
Dictionary, and Encyclopædia Britannica.
2. The mass of the Solar System excluding the Sun, Jupiter and Saturn can
be determined by adding together all the calculated masses for its
largest objects and using rough calculations for the masses of the Oort
cloud (estimated at roughly 3 Earth masses),[110] the Kuiper Belt
(estimated at roughly 0.1 Earth mass)[111] and the asteroid belt
(estimated to be 0.0005 Earth mass)[112] for a total, rounded upwards, of
~37 Earth masses, or 8.9 percent the combined mass of Jupiter and
Saturn.