what are the Secrets hidden in our solar system?

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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, three dwarf planets, and billions of small bodies.
1. Solar system 1
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
Major features of the Solar System;
interplanetary dust. sizes and distances not to scale. From
In broad terms, the charted regions of left to right): Pluto, Neptune, Uranus,
the Solar System consist of the Sun, four Saturn, Jupiter, the asteroid belt, the
Sun, Mercury, Venus, Earth and its
terrestrial inner planets, an asteroid
Moon, and Mars. A comet is also seen
belt composed of small rocky bodies, four on the left.
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
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.
See also: Definition of planet
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
Planets and dwarf planets of the Solar
known planets: Mercury, Venus, Earth, System; while the sizes are to scale,
Mars, Jupiter, Saturn, Uranus, and the relative distances from the Sun
Neptune. are not.
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2. Solar system 2
On August 24 2006 the International Astronomical Union defined the term
"planet" for the first time, excluding Pluto and reclassifying it under the new
category of dwarf planet along with Eris and Ceres.[2]
A dwarf planet is not required to clear its neighbourhood of other celestial
bodies. Other objects that may become classified as dwarf planets are
Sedna, Orcus, and Quaoar.
From the time of its discovery in 1930 until 2006, Pluto was considered the
Solar System's ninth planet. But in the late 20th and early 21st centuries,
many objects similar to Pluto were discovered in the outer Solar System,
most notably Eris, which is slightly larger than Pluto.
The remainder of the objects in orbit around the Sun are small Solar System
bodies (SSSBs).[3]
Natural satellites, or moons, are those objects in orbit around planets, dwarf
planets and SSSBs, rather than the Sun itself.
A planet's distance from the Sun varies in the course of its year. Its closest
approach to the Sun is called its perihelion, while its farthest distance from
the Sun is called its aphelion.
Astronomers usually measure distances within the Solar System in
astronomical units (AU). One AU is the approximate distance between the
Earth and the Sun, or roughly 149,598,000 km (93,000,000 mi). Pluto is
roughly 38 AU from the Sun while Jupiter lies at roughly 5.2 AU. One light
year, the best known unit of interstellar distance, is roughly 63,240 AU.
Informally, the Solar System is sometimes divided into separate zones. The
inner Solar System includes the four terrestrial planets and the main
asteroid belt. Some define the outer Solar System as comprising
everything beyond the asteroids.[4] Others define it as the region beyond
Neptune, with the four gas giants considered a separate "middle zone".[5]
Layout and structure
The principal component of the Solar
System is the Sun, a main sequence G2
star that contains 99.86% of the system's
known mass and dominates it
gravitationally.[6] Jupiter and Saturn, the
Sun's two largest orbiting bodies, account
for more than 90% of the system's
remaining mass.
Most large objects in orbit around the Sun
The ecliptic viewed in sunlight from
behind the Moon in this Clementinelie near the plane of Earth's orbit, known
as the ecliptic. The planets are very close
image. From left to right: Mercury,
Mars, Saturn. to the ecliptic while comets and Kuiper
belt objects are usually at significantly greater angles to it.
All of the planets and most other objects also orbit with the Sun's rotation in
a counter-clockwise direction as viewed from a point above the Sun's north
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3. Solar system 3
pole. There are exceptions, such as Halley's Comet.
Objects travel around the Sun following
Kepler's laws of planetary motion. Each
object orbits along an approximate ellipse
with the Sun at one focus of the ellipse.
The closer an object is to the Sun, the
faster it moves. The orbits of the planets
are nearly circular, but many comets,
asteroids and objects of the Kuiper belt
follow highly-elliptical orbits.
To cope with the vast distances involved,
many representations of the Solar System
show orbits the same distance apart. In
reality, with a few exceptions, the farther a
The orbits of the bodies in the Solar
planet or belt is from the Sun, the larger System to scale (clockwise from top
the distance between it and the previous left)
orbit. For example, Venus is approximately
0.33 AU farther out than Mercury, while Saturn is 4.3 AU out from Jupiter,
and Neptune lies 10.5 AU out from Uranus. Attempts have been made to
determine a correlation between these orbital distances (see Titius-Bode
law), but no such theory has been accepted.
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
Artist's conception of a protoplanetary
disk molecular cloud. This initial cloud was
likely several light-years across and
probably birthed several stars. 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
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4. Solar system 4
collide with increasing frequency. The centre, where most of the mass
collected, became increasingly hotter than the surrounding disc.[14] As
gravity, gas pressure, magnetic fields, and rotation acted on the contracting
nebula, it began to flatten into a spinning protoplanetary disk with a
diameter of roughly 200 AU[15] and a hot, dense protostar at the center.[16]
Studies of T Tauri stars, young, pre-fusing solar mass stars believed to be
similar to the Sun at this point in its evolution, show that they are often
accompanied by discs of pre-planetary matter.[18] These discs extend to
several hundred AU and reach only a thousand kelvins at their hottest.[19]
After 100 million years, the pressure and
density of hydrogen in the centre of the
collapsing nebula became great enough
for the protosun to begin thermonuclear
fusion. This increased until hydrostatic
equilibrium was achieved, with the
thermal energy countering the force of
gravitational contraction. At this point
the Sun became a full-fledged star.[20]
From the remaining cloud of gas and
dust (the "solar nebula"), the various Hubble image of protoplanetary disks
planets formed. They are believed to in the Orion Nebula, a light-years-wide
"stellar nursery" likely very similar to
have formed by accretion: the planets the primordial nebula from which our
began as dust grains in orbit around the Sun formed.
central protostar; 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]
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.[26] [27]
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5. Solar system 5
The Sun is the Solar System's parent star, and
far and away its chief component. Its large
mass gives it an interior density high enough to
sustain nuclear fusion, which releases
enormous amounts of energy, mostly radiated
into space as electromagnetic radiation such as
visible light.
The Sun is classified as a moderately large
The Sun as seen from Earth yellow dwarf, but this name is misleading as,
compared to stars in our galaxy, the Sun is
rather large and bright. Stars are classified by the Hertzsprung-Russell
diagram, a graph which plots the brightness of stars against their surface
temperatures. Generally, hotter stars are brighter. Stars following this
pattern are said to be on the main sequence; the Sun lies right in the middle
of it. However, stars brighter and hotter than the Sun are rare, while stars
dimmer and cooler are common.[28]
It is believed that the Sun's position on
the main sequence puts it in the "prime
of life" for a star, in that it has not yet
exhausted its store of hydrogen for
nuclear fusion. The Sun is growing
brighter; early in its history it was 75
percent as bright as it is today.[29]
Calculations of the ratios of hydrogen
and helium within the Sun suggest it is
halfway through its life cycle. It will
eventually move off the main sequence
and become larger, brighter, cooler and
redder, becoming a red giant in about
five billion years.[30] At that point its
luminosity will be several thousand times
its present value. The Hertzsprung-Russell diagram; the
main sequence is from bottom right to
The Sun is a population I star; it was top left.
born in the later stages of the universe's
evolution. It contains more elements heavier than hydrogen and helium
("metals" in astronomical parlance) than older population II stars.[31]
Elements heavier than hydrogen and helium were formed in the cores of
ancient and exploding stars, so the first generation of stars had to die before
the universe could be enriched with these atoms. The oldest stars contain
few metals, while stars born later have more. This high metallicity is
thought to have been crucial to the Sun's developing a planetary system,
because planets form from accretion of metals.[32]
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6. Solar system 6
Interplanetary medium
Along with light, the Sun radiates a
continuous stream of charged particles (a
plasma) known as the solar wind. This
stream of particles spreads outwards at
roughly 1.5 million kilometres per hour,[33]
creating a tenuous atmosphere (the
heliosphere) that permeates the Solar
System out to at least 100 AU (see
heliopause). This is known as the
interplanetary medium. The Sun's 11-year
The heliospheric current sheet sunspot cycle and frequent solar flares and
coronal mass ejections disturb the
heliosphere, creating space weather.[34] The Sun's rotating magnetic field
acts on the interplanetary medium to create the heliospheric current sheet,
the largest structure in the solar system.[35]
Earth's magnetic field protects its
atmosphere from interacting with the solar
wind. Venus and Mars do not have
magnetic fields, and the solar wind causes
their atmospheres to gradually bleed away
into space.[36] The interaction of the solar
wind with Earth's magnetic field creates
the aurorae seen near the magnetic poles.
Cosmic rays originate outside the Solar
Aurora australis seen from orbit.
System. The heliosphere partially shields
the Solar System, and planetary magnetic fields (for planets which have
them) also provide some protection. The density of cosmic rays in the
interstellar medium and the strength of the Sun's magnetic field change on
very long timescales, so the level of cosmic radiation in the Solar System
varies, though by how much is unknown.[37]
The interplanetary medium is home to at least two disc-like regions of
cosmic dust. The first, the zodiacal dust cloud, lies in the inner Solar System
and causes zodiacal light. It was likely formed by collisions within the
asteroid belt brought on by interactions with the planets.[38] The second
extends from about 10 AU to about 40 AU, and was probably created by
similar collisions within the Kuiper belt.[39] [40]
Inner Solar System
The inner Solar System is the traditional name for the region comprising the
terrestrial planets and asteroids. Composed mainly of silicates and metals,
the objects of the inner Solar System huddle very closely to the Sun; the
radius of this entire region is shorter than the distance between Jupiter and
Saturn. This region was, in old parlance, denoted inner space; the area
outside the asteroid belt was denoted outer space.
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7. Solar system 7
Inner planets
Main article: Terrestrial planet
The four inner or terrestrial planets have
dense, rocky compositions, few or no
moons, and no ring systems. They are
composed largely of minerals with high
melting points, such as the silicates
which form their solid crusts and
semi-liquid mantles, and metals such as The inner planets. From left to right:
iron and nickel, which form their cores. Mercury, Venus, Earth, and Mars
Three of the four inner planets (Venus, (sizes to scale)
Earth and Mars) have substantial atmospheres; all have impact craters and
tectonic surface features such as rift valleys and volcanoes. The term inner
planet should not be confused with inferior planet, which designates those
planets which are closer to the Sun than Earth is (i.e. Mercury and Venus).
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]
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]
Earth (1 AU) is the largest and densest of the inner planets, the only one
known to have current geological activity, and the only planet known to
have life. Its liquid hydrosphere is unique among the terrestrial planets,
and it is also the only planet where plate tectonics has been observed.
Earth's atmosphere is radically different from those of the other planets,
having been altered by the presence of life to contain 21% free
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8. Solar system 8
oxygen.[47] It has one satellite, the Moon, the only large satellite of a
terrestrial planet in the Solar System.
Mars (1.5 AU) is smaller than Earth and Venus (0.107 Earth masses). It
possesses a tenuous atmosphere of mostly carbon dioxide. Its surface,
peppered with vast volcanoes such as Olympus Mons and rift valleys
such as Valles Marineris, shows geological activity that may have
persisted until very recently.[48] Mars has two tiny natural satellites
(Deimos and Phobos) thought to be captured asteroids.[49]
Asteroid belt
Main article: Asteroid belt
Asteroids are mostly small Solar System
bodies composed mainly of rocky and
metallic non-volatile minerals.
The main asteroid belt occupies the orbit
between Mars and Jupiter, between 2.3
and 3.3 AU from the Sun. It is thought to
be remnants from the Solar System's
formation that failed to coalesce because
of the gravitational interference of
Asteroids range in size from hundreds of
kilometres across to microscopic. All
asteroids save the largest, Ceres, are
classified as small Solar System bodies, Image of the main asteroid belt and
but some asteroids such as Vesta and the Trojan asteroids
Hygieia may be reclassed as dwarf
planets if they are shown to have achieved hydrostatic equilibrium.
The asteroid belt contains tens of thousands, possibly millions, of objects
over one kilometre in diameter.[50] Despite this, the total mass of the main
belt is unlikely to be more than a thousandth of that of the Earth.[51] The
main belt is very sparsely populated; spacecraft routinely pass through
without incident. Asteroids with diameters between 10 and 10-4 m are called
Ceres (2.77 AU) is the largest body in the asteroid
belt and its only dwarf planet. It has a diameter of
slightly under 1000 km, large enough for its own
gravity to pull it into a spherical shape. Ceres was
considered a planet when it was discovered in the
19th century, but was reclassified as an asteroid in
Ceres the 1850s as further observation revealed
additional asteroids.[53] It was again reclassified in
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9. Solar system 9
2006 as a dwarf planet.
Asteroid groups
Asteroids in the main belt are divided into asteroid groups and families
based on their orbital characteristics. Asteroid moons are asteroids that
orbit larger asteroids. They are not as clearly distinguished as planetary
moons, sometimes being almost as large as their partners. The asteroid
belt also contains main-belt comets[54] which may have been the source
of Earth's water.
Trojan asteroids are located in either of Jupiter's L4 or L5 points
(gravitationally stable regions leading and trailing a planet in its orbit); the
term "Trojan" is also used for small bodies in any other planetary or satellite
Lagrange point. Hilda asteroids are in a 2:3 resonance with Jupiter; that is,
they go around the Sun three times for every two Jupiter orbits.
The inner Solar System is also dusted with rogue asteroids, many of which
cross the orbits of the inner planets.
Mid Solar System
The middle region of the Solar System is home to the gas giants and their
planet-sized satellites. Many short period comets, including the centaurs,
also lie in this region. It has no traditional name; it is occasionally referred
to as the "outer Solar System", although recently that term has been more
often applied to the region beyond Neptune. The solid objects in this region
are composed of a higher proportion of "ices" (water, ammonia, methane)
than the rocky denizens of the inner Solar System.
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10. Solar system 10
Outer planets
The four outer planets, or gas giants
(sometimes called Jovian planets),
collectively make up 99 percent of the
mass known to orbit the Sun. Jupiter and
Saturn's atmospheres are largely
hydrogen and helium. Uranus and
Neptune's atmospheres have a higher
percentage of “ices”, such as water,
ammonia and methane. Some astronomers
suggest they belong in their own category,
“ice giants.”[55] All four gas giants have
rings, although only Saturn's ring system
is easily observed from Earth. The term
outer planet should not be confused with
superior planet, which designates planets
outside Earth's orbit (the outer planets
and Mars). From top to bottom: Neptune,
Uranus, Saturn, and Jupiter (not to
Jupiter scale)
Jupiter (5.2 AU), at 318 Earth masses,
masses 2.5 times all the other planets put together. It is composed
largely of hydrogen and helium. Jupiter's strong internal heat creates a
number of semi-permanent features in its atmosphere, such as cloud
bands and the Great Red Spot. Jupiter has sixty-three known satellites.
The four largest, Ganymede, Callisto, Io, and Europa, show similarities
to the terrestrial planets, such as volcanism and internal heating.[56]
Ganymede, the largest satellite in the Solar System, is larger than
Saturn (9.5 AU), famous for its extensive ring system, has similarities to
Jupiter, such as its atmospheric composition. Saturn is far less massive,
being only 95 Earth masses. Saturn has sixty known satellites (and 3
unconfirmed); two of which, Titan and Enceladus, show signs of
geological activity, though they are largely made of ice.[57] Titan is larger
than Mercury and the only satellite in the Solar System with a
substantial atmosphere.
Uranus (19.6 AU), at 14 Earth masses, is the lightest of the outer
planets. Uniquely among the planets, it orbits the Sun on its side; its
axial tilt is over ninety degrees to the ecliptic. It has a much colder core
than the other gas giants, and radiates very little heat into space.[58]
Uranus has twenty-seven known satellites, the largest ones being
Titania, Oberon, Umbriel, Ariel and Miranda.
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11. Solar system 11
Neptune (30 AU), though slightly smaller than Uranus, is more massive
(equivalent to 17 Earths) and therefore denser. It radiates more internal
heat, but not as much as Jupiter or Saturn.[59] Neptune has thirteen
known satellites. The largest, Triton, is geologically active, with geysers
of liquid nitrogen.[60] Triton is the only large satellite with a retrograde
orbit. Neptune is accompanied in its orbit by a number of minor planets
in a 1:1 resonance with it, termed Neptune Trojans.
Main article: Comet
Comets are small Solar System bodies, usually only a
few kilometres across, composed largely of volatile ices.
They have highly eccentric orbits, generally a perihelion
within the orbits of the inner planets and an aphelion far
beyond Pluto. When a comet enters the inner Solar
System, its proximity to the Sun causes its icy surface to
sublimate and ionise, creating a coma: a long tail of gas
and dust often visible to the naked eye.
Short-period comets have orbits lasting less than two
hundred years. Long-period comets have orbits lasting
thousands of years. Short-period comets are believed to Comet Hale-Bopp
originate in the Kuiper belt, while long-period comets, such as Hale-Bopp,
are believed to originate in the Oort cloud. Many comet groups, such as the
Kreutz Sungrazers, formed from the breakup of a single parent.[61] Some
comets with hyperbolic orbits may originate outside the Solar System, but
determining their precise orbits is difficult.[62] Old comets that have had
most of their volatiles driven out by solar warming are often categorised as
The centaurs, which extend from 9 to 30 AU, are icy comet-like bodies
that orbit in the region between Jupiter and Neptune. The largest known
centaur, 10199 Chariklo, has a diameter of between 200 and 250 km.[64]
The first centaur discovered, 2060 Chiron, has been called a comet
since it develops a coma just as comets do when they approach the
Sun.[65] Some astronomers classify centaurs as inward-scattered Kuiper
belt objects along with the outward-scattered residents of the scattered
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12. Solar system 12
Trans-Neptunian region
The area beyond Neptune, often called the outer Solar System or the
"trans-Neptunian region", is still largely unexplored. It appears to consist
overwhelmingly of small worlds (the largest having a diameter only a fifth
that of the Earth and a mass far smaller than that of the Moon) composed
mainly of rock and ice.
Kuiper belt
Main article: Kuiper belt
The Kuiper belt, the region's first
formation, is a great ring of debris
similar to the asteroid belt, but
composed mainly of ice. It extends
between 30 and 50 AU from the Sun.
This region is thought to be the source of
short-period comets. It is composed
mainly of small Solar System bodies, but
many of the largest Kuiper belt objects,
such as Quaoar, Varuna, , and Orcus,
may be reclassified as dwarf planets.
There are estimated to be over 100,000
Kuiper belt objects with a diameter
greater than 50 km, but the total mass of Plot of all known Kuiper belt objects,
the Kuiper belt is thought to be only a set against the four outer planets
tenth or even a hundredth the mass of
the Earth.[67] Many Kuiper belt objects have multiple satellites, and most
have orbits that take them outside the plane of the ecliptic.
The Kuiper belt can be roughly divided into the
"resonant" belt and the "classical" belt. The
resonant belt consists of objects with orbits linked
to that of Neptune (e.g. orbiting twice for every
three Neptune orbits, or once for every two). The
resonant belt actually begins within the orbit of
Neptune itself. The classical belt consists of
objects having no resonance with Neptune, and
extends from roughly 39.4 AU to 47.7 AU.[68]
Members of the classical Kuiper belt are classified
Diagram showing the as cubewanos, after the first of their kind to be
resonant and classical
Kuiper belt discovered, .[69]
Pluto and Charon
Pluto (39 AU average), a dwarf planet, is the largest known object in the
Kuiper belt. When discovered in 1930 it was considered to be the ninth
planet; this changed in 2006 with the adoption of a formal definition of
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13. Solar system 13
planet. Pluto has a relatively eccentric orbit inclined 17 degrees to the
ecliptic plane and ranging from 29.7 AU from the Sun at perihelion
(within the orbit of Neptune) to 49.5 AU at aphelion.
It is unclear whether Charon, Pluto's largest
moon, will continue to be classified as such or as a
dwarf planet itself. Both Pluto and Charon orbit a
barycenter of gravity above their surfaces, making
Pluto-Charon a binary system. Two much smaller
moons, Nix and Hydra, orbit Pluto and Charon.
Pluto lies in the resonant belt, having a 3:2
resonance with Neptune (it orbits twice round the
Sun for every three Neptunian orbits). Kuiper belt
objects whose orbits share this resonance are Pluto and its three known
called plutinos.[70]
Scattered disc
Main article: Scattered disc
The scattered disc overlaps the Kuiper belt but
extends much further outwards. Scattered disc
objects are believed to come from the Kuiper belt,
having been ejected into erratic orbits by the
gravitational influence of Neptune's early outward
migration. Most scattered disc objects (SDOs)
have perihelia within the Kuiper belt but aphelia
as far as 150 AU from the Sun. SDOs' orbits are
also highly inclined to the ecliptic plane, and are
often almost perpendicular to it. Some
astronomers consider the scattered disc to be Black: scattered; blue:
classical; green: resonant
merely another region of the Kuiper belt, and
describe scattered disc objects as "scattered Kuiper belt objects."[71]
Eris (68 AU average) is the largest known
scattered disc object, and caused a debate
about what constitutes a planet, since it is at
least 5% larger than Pluto with an estimated
diameter of 2400 km (1500 mi). It is the
largest of the known dwarf planets.[72] It has
one moon, Dysnomia. Like Pluto, its orbit is
highly eccentric, with a perihelion of 38.2 AU
Eris and its moon (roughly Pluto's distance from the Sun) and an
Dysnomia aphelion of 97.6 AU, and steeply inclined to
the ecliptic plane.
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14. Solar system 14
Farthest regions
The point at which the Solar System ends and interstellar space begins is
not precisely defined, since its outer boundaries are shaped by two separate
forces: the solar wind and the Sun's gravity. The solar wind is believed to
surrender to the interstellar medium at roughly four times Pluto's distance.
However, the Sun's Roche sphere, the effective range of its gravitational
influence, is believed to extend up to a thousand times farther.
The heliosphere is divided into two
separate regions. The solar wind travels at
its maximum velocity out to about 95 AU,
or three times the orbit of Pluto. The edge
of this region is the termination shock, the
point at which the solar wind collides with
the opposing winds of the interstellar
medium. Here the wind slows, condenses
and becomes more turbulent, forming a
The Voyagers entering the great oval structure known as the
heliosheath that looks and behaves very
much like a comet's tail, extending outward for a further 40 AU at its
stellar-windward side, but tailing many times that distance in the opposite
direction. The outer boundary of the heliosphere, the heliopause, is the
point at which the solar wind finally terminates, and is the beginning of
interstellar space.[73]
The shape and form of the outer edge of the heliosphere is likely affected by
the fluid dynamics of interactions with the interstellar medium,[74] as well as
solar magnetic fields prevailing to the south, e.g. it is bluntly shaped with
the northern hemisphere extending 9 AU (roughly 900 million miles) farther
than the southern hemisphere. Beyond the heliopause, at around 230 AU,
lies the bow shock, a plasma "wake" left by the Sun as it travels through the
Milky Way.[75]
No spacecraft have yet passed beyond the heliopause, so it is impossible to
know for certain the conditions in local interstellar space. How well the
heliosphere shields the Solar System from cosmic rays is poorly understood.
A dedicated mission beyond the heliosphere has been suggested.[76] [77]
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15. Solar system 15
Oort cloud
The hypothetical Oort cloud is a great mass of up
to a trillion icy objects that is believed to be the
source for all long-period comets and to surround
the Solar System at around 50,000 AU, and
possibly to as far as 100,000 AU. It is believed to
be composed of comets which were ejected from
the inner Solar System by gravitational
interactions with the outer planets. Oort cloud
objects move very slowly, and can be perturbed by Artist's rendering of the
infrequent events such as collisions, the Kuiper Belt and
gravitational effects of a passing star, or the hypothetical Oort cloud
galactic tide.[78] [79]
Sedna and the inner Oort cloud
90377 Sedna is a large, reddish Pluto-like
object with a gigantic, highly elliptical orbit
that takes it from about 76 AU at perihelion to
928 AU at aphelion and takes 12,050 years to
complete. Mike Brown, who discovered the
object in 2003, asserts that it cannot be part
of the scattered disc or the Kuiper Belt as its Telescopic image of Sedna
perihelion is too distant to have been affected
by Neptune's migration. He and other astronomers consider it to be the
first in an entirely new population, which also may include the object ,
which has a perihelion of 45 AU, an aphelion of 415 AU, and an orbital
period of 3420 years.[80] Brown terms this population the "Inner Oort
cloud," as it may have formed through a similar process, although it is
far closer to the Sun.[81] Sedna is very likely a dwarf planet, though its
shape has yet to be determined with certainty.
Much of our Solar System is still unknown. The Sun's gravitational field is
estimated to dominate the gravitational forces of surrounding stars out to
about two light years (125,000 AU). The outer extent of the Oort cloud, by
contrast, may not extend farther than 50,000 AU.[82] Despite discoveries
such as Sedna, the region between the Kuiper belt and the Oort cloud, an
area tens of thousands of AU in radius, is still virtually unmapped. There are
also ongoing studies of the region between Mercury and the Sun.[83] Objects
may yet be discovered in the Solar System's uncharted regions.
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16. Solar system 16
Galactic context
The Solar System is located in the Milky
Way galaxy, a barred spiral galaxy with a
diameter of about 100,000 light years
containing about 200 billion stars.[84] Our
Sun resides in one of the Milky Way's
outer spiral arms, known as the Orion Arm
or Local Spur.[85] The Sun lies between
25,000 and 28,000 light years from the
Galactic Centre, and its speed within the
galaxy is about 220 kilometres per second,
so that it completes one revolution every
Location of the Solar System within 225–250 million years. This revolution is
our galaxy known as the Solar System's galactic
The Solar System's location in the galaxy is very likely a factor in the
evolution of life on Earth. Its orbit is close to being circular and is at roughly
the same speed as that of the spiral arms, which means it passes through
them only rarely. Since spiral arms are home to a far larger concentration of
potentially dangerous supernovae, this has given Earth long periods of
interstellar stability for life to evolve.[87] The Solar System also lies well
outside the star-crowded environs of the galactic centre. Near the centre,
gravitational tugs from nearby stars could perturb bodies in the Oort Cloud
and send many comets into the inner Solar System, producing collisions
with potentially catastrophic implications for life on Earth. The intense
radiation of the galactic centre could also interfere with the development of
complex life.[88] Even at the Solar System's current location, some scientists
have hypothesised that recent supernovae may have adversely affected life
in the last 35,000 years by flinging pieces of expelled stellar core towards
the Sun in the form of radioactive dust grains and larger, comet-like
The solar apex, the direction of the Sun's path through interstellar space, is
near the constellation of Hercules in the direction of the current location of
the bright star Vega.[90]
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17. Solar system 17
The immediate galactic neighbourhood of
the Solar System is known as the Local
Interstellar Cloud or Local Fluff, an area
of dense cloud in an otherwise sparse
region known as the Local Bubble, an
hourglass-shaped cavity in the
interstellar medium roughly 300 light
years across. The bubble is suffused with
high-temperature plasma that suggests it
is the product of several recent
There are relatively few stars within ten Artist's conception of the Local Bubble
light years (95 trillion km) of the Sun.
The closest is the triple star system Alpha Centauri, which is about 4.4 light
years away. Alpha Centauri A and B are a closely tied pair of Sun-like stars,
while the small red dwarf Alpha Centauri C (also known as Proxima
Centauri) orbits the pair at a distance of 0.2 light years. The stars next
closest to the Sun are the red dwarfs Barnard's Star (at 6 light years), Wolf
359 (7.8 light years) and Lalande 21185 (8.3 light years). The largest star
within ten light years is Sirius, a bright main sequence star roughly twice
the Sun's mass and orbited by a white dwarf called Sirius B. It lies 8.6 light
years away. The remaining systems within ten light years are the binary red
dwarf system Luyten 726-8 (8.7 light years) and the solitary red dwarf Ross
154 (9.7 light years).[92] Our closest solitary sunlike star is Tau Ceti, which
lies 11.9 light years away. It has roughly 80 percent the Sun's mass, but only
60 percent its luminosity.[93] The closest earth-like planet discovered, Gliese
581c, is 20.40 light years away.
Discovery and exploration
Main articles: Geocentric model and Heliocentrism
For many thousands of years, people, with a few notable exceptions, did not
believe the Solar System existed. The Earth was believed not only to be
stationary at the centre of the universe, but to be categorically different
from the divine or ethereal objects that moved through the sky. While
Nicolaus Copernicus and his predecessors, such as the Indian
mathematician-astronomer Aryabhata and the Greek philosopher
Aristarchus of Samos, had speculated on a heliocentric reordering of the
cosmos, it was the conceptual advances of the 17th century, led by Galileo
Galilei, Johannes Kepler, and Isaac Newton, which led gradually to the
acceptance of the idea not only that Earth moved round the Sun, but that
the planets were governed by the same physical laws that governed the
Earth, and therefore could be material worlds in their own right, with such
earthly phenomena as craters, weather, geology, seasons and ice caps.
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18. Solar system 18
Telescopic observations
Main article: Timeline of solar system astronomy
The first exploration of the Solar System was
conducted by telescope, when astronomers first
began to map those objects too faint to be seen
with the naked eye.
Galileo Galilei was the first to discover physical
details about the individual bodies of the Solar
System. He discovered that the Moon was
cratered, that the Sun was marked with sunspots,
and that Jupiter had four satellites in orbit around A replica of Isaac Newton's
it.[94] Christiaan Huygens followed on from telescope
Galileo's discoveries by discovering Saturn's moon
Titan and the shape of the rings of Saturn.[95] Giovanni Domenico Cassini
later discovered four more moons of Saturn, the Cassini division in Saturn's
rings, and the Great Red Spot of Jupiter.[96]
Edmond Halley realised in 1705 that repeated sightings of a comet were in
fact recording the same object, returning regularly once every 75–76 years.
This was the first evidence that anything other than the planets orbited the
In 1781, William Herschel was looking for binary stars in the constellation of
Taurus when he observed what he thought was a new comet. In fact, its
orbit revealed that it was a new planet, Uranus, the first ever discovered.[98]
Giuseppe Piazzi discovered Ceres in 1801, a small world between Mars and
Jupiter that was initially considered a new planet. However, subsequent
discoveries of thousands of other small worlds in the same region led to
their eventual reclassification as asteroids.[99]
By 1846, discrepancies in the orbit of Uranus led many to suspect a large
planet must be tugging at it from farther out. Urbain Le Verrier's
calculations eventually led to the discovery of Neptune.[100] The excess
perihelion precession of Mercury's orbit led Le Verrier to postulate the
intra-Mercurian planet Vulcan in 1859 – but that would turn out to be a red
Further apparent discrepancies in the orbits of the outer planets led
Percival Lowell to conclude that yet another planet, "Planet X," must still be
out there. After his death, his Lowell Observatory conducted a search which
ultimately led to Clyde Tombaugh's discovery of Pluto in 1930. Pluto was,
however, found to be too small to have disrupted the orbits of the outer
planets, and its discovery was therefore coincidental. Like Ceres, it was
initially considered to be a planet, but after the discovery of many other
similarly sized objects in its vicinity it was reclassified in 2006 as a dwarf
planet by the IAU.[101]
In 1992, astronomers David Jewitt of the University of Hawaii and Jane Luu
of the Massachusetts Institute of Technology discovered . This object proved
to be the first of a new population, which came to be known as the Kuiper
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19. Solar system 19
belt; an icy analogue to the asteroid belt of which such objects as Pluto and
Charon were deemed a part.[102] [103]
Mike Brown, Chad Trujillo and David Rabinowitz announced the discovery of
Eris in 2005, a scattered disc object larger than Pluto and the largest object
discovered in orbit round the Sun since Neptune.[104]
Observations by spacecraft
Since the start of the Space Age, a great
deal of exploration has been performed by
robotic spacecraft missions that have been
organized and executed by various space
All planets in the Solar System have now
been visited to varying degrees by
spacecraft launched from Earth. Through
these unmanned missions, humans have
been able to get close-up photographs of
all of the planets and, in the case of
landers, perform tests of the soils and
atmospheres of some.
The first manmade object sent into space
was the Soviet satellite Sputnik 1,
launched in 1957, which successfully
orbited the Earth for over a year. The Artist's conception of Pioneer 10,
American probe Explorer 6, launched in which passed the orbit of Pluto in
1983. The last transmission was
1959, was the first satellite to image the
received in January 2003, sent from
Earth from space. approximately 82 AU away. The
35-year-old space probe is now
receding at over 43,400 km/h
(27,000 mph) from the Sun.[105]
The first successful probe to fly by another Solar System body was Luna 1,
which sped past the Moon in 1959. Originally meant to impact with the
Moon, it instead missed its target and became the first manmade object to
orbit the Sun. Mariner 2 was the first probe to fly by another planet, Venus,
in 1962. The first successful flyby of Mars was made by Mariner 4 in 1964.
Mercury was first encountered by Mariner 10 in 1974.
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20. Solar system 20
The first probe to explore the outer
planets was Pioneer 10, which flew by
Jupiter in 1973. Pioneer 11 was the first
to visit Saturn, in 1979. The Voyager
probes performed a grand tour of the
outer planets following their launch in
1977, with both probes passing Jupiter in
1979 and Saturn in 1980 – 1981. Voyager
2 then went on to make close approaches
to Uranus in 1986 and Neptune in 1989.
The Voyager probes are now far beyond
Neptune's orbit, and are on course to
find and study the termination shock,
heliosheath, and heliopause. According
to NASA, both Voyager probes have
A photo of Earth (circled) taken by encountered the termination shock at a
Voyager 1, 6 billion km (4 billion distance of approximately 93 AU from
miles) away. The streaks of light are
the Sun.[106] [107]
diffraction spikes radiating from the
Sun (off frame to the left). The first flyby of a comet occurred in
1985, when the International Cometary Explorer (ICE) passed by the comet
Giacobini-Zinner,[108] while the first flybys of asteroids were conducted by
the Galileo spaceprobe, which imaged both 951 Gaspra (in 1991) and 243
Ida (in 1993) on its way to Jupiter.
No Kuiper belt object has yet been visited by a spacecraft. Launched on
January 19 2006, the New Horizons probe is currently en route to becoming
the first man-made spacecraft to explore this area. This unmanned mission
is scheduled to fly by Pluto in July 2015. Should it prove feasible, the
mission will then be extended to observe a number of other Kuiper belt
Orbiters, landers and rovers
In 1966, the Moon became the first Solar System body beyond Earth to be
orbited by an artificial satellite (Luna 10), followed by Mars in 1971
(Mariner 9), Venus in 1975 (Venera 9), Jupiter in 1995 (Galileo), the asteroid
433 Eros in 2000 (NEAR Shoemaker), and Saturn in 2004
(Cassini–Huygens). The MESSENGER probe is currently en route to
commence the first orbit of Mercury in 2011, while the Dawn spacecraft is
currently set to orbit the asteroid Vesta in 2011 and the dwarf planet Ceres
in 2015.
The first probe to land on another Solar System body was the Soviet Luna 2
probe, which impacted the Moon in 1959. Since then, increasingly distant
planets have been reached, with probes landing on or impacting the
surfaces of Venus in 1966 (Venera 3), Mars in 1971 (Mars 3, although a fully
successful landing didn't occur until Viking 1 in 1976), the asteroid 433 Eros
in 2001 (NEAR Shoemaker), and Saturn's moon Titan (Huygens) and the
comet Tempel 1 (Deep Impact) in 2005. The Galileo orbiter also dropped a
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21. Solar system 21
probe into Jupiter's atmosphere in 1995; since Jupiter has no physical
surface, it was destroyed by increasing temperature and pressure as it
To date, only two worlds in the Solar System, the Moon and Mars, have been
visited by mobile rovers. The first rover to visit another celestial body was
the Soviet Lunokhod 1, which landed on the Moon in 1970. The first to visit
another planet was Sojourner, which travelled 500 metres across the
surface of Mars in 1997. The only manned rover to visit another world was
NASA's Lunar rover, which travelled with Apollos 15, 16 and 17 between
1971 and 1972.
Manned exploration
Manned exploration of the Solar System is currently confined to Earth's
immediate environs. The first human being to reach space (defined as an
altitude of over 100 km) and to orbit the Earth was Yuri Gagarin, a Soviet
cosmonaut who was launched in Vostok 1 on April 12, 1961. The first man to
walk on the surface of another Solar System body was Neil Armstrong, who
stepped onto the Moon on July 21, 1969 during the Apollo 11 mission. The
United States' Space Shuttle is the only reusable spacecraft to successfully
make multiple orbital flights. The first orbital space station to host more
than one crew was NASA's Skylab, which successfully held three crews from
1973 to 1974. The first true human settlement in space was the Soviet space
station Mir, which was continuously occupied for close to ten years, from
1989 to 1999. It was decommissioned in 2001, and its successor, the
International Space Station, has maintained a continuous human presence
in space since then. In 2004, SpaceShipOne became the first privately
funded vehicle to reach space on a suborbital flight. That same year,
President George W. Bush announced the Vision for Space Exploration,
which called for a replacement for the aging Shuttle, a return to the Moon
and, ultimately, a manned mission to Mars.
See also
• List of Solar System objects:
• By orbit
• By mass
• By radius
• By name
• By surface gravity
• Attributes of the largest solar system bodies
• Astronomical symbols
• Geological features of the solar system
• Numerical model of solar system
• Table of planetary attributes
• Timeline of discovery of Solar System planets and their natural satellites
All Articles originate from http://wikipedia.org - for usage information see GFDL
22. Solar system 22
• Solar system model
• Space colonization
• Solar System in fiction
• Celestia – Space-simulation on your computer (OpenGL)
• Family Portrait (Voyager)
• The Parable of the Solar System Model
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
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• NASA/JPL Solar System main page
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• SPACE.com: All About the Solar System
• Illustration of the distance between planets
• Illustration comparing the sizes of the planets with each other, the sun,
and other stars
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29. Solar system 29
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