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| The Sun and planets
(distances not to scale). |
Solar System
The Solar System is the Sun and all the objects that
orbit around it. The Sun is orbited by planets,
asteroids, comets and other things.
The Solar System is about 4.6 billion years old. It
formed by gravity in a large molecular cloud. Most of
this matter gathered in the center, and the rest
flattened into an orbiting disk that became the Solar
System. It is thought that almost all stars form by this
process.
The Sun is a star. It contains 99.9% of the Solar
System's mass. This means that it has strong gravity.
The other objects are pulled into orbit around the Sun.
The Sun is mostly made out of hydrogen, and some helium.
There are eight planets in the Solar System. From
closest to farthest from the Sun, they are: Mercury,
Venus, Earth, Mars, Jupiter, Saturn, Uranus and Neptune.
The first four planets are called terrestrial planets.
They are mostly made of rock and metal, and they are
mostly solid. The last four planets are called gas
giants. This is because they are much larger than other
planets and are mostly made of gas.
The Solar System also contains other things. There are
asteroid belts, mostly between Mars and Jupiter. Further
out than Neptune, there is the Kuiper belt and the
scattered disc. These areas have dwarf planets,
including Pluto, MakeMake,Haumea,Ceres and Eris. There
are thousands of very small objects in these areas.
There are also comets, centaurs, and there is
interplanetary dust.
Six of the planets and three of the dwarf planets are
orbited by moons. Furthermore, planetary dust orbits the
gas giants. Many other systems like the Solar System
have been found. Each of the billions of stars in the
Milky Way galaxy might have a planetary system. |
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| Artist's conception
of a protoplanetary disk. |
Evolution of the Solar
System
The formation and evolution of the Solar System began
4.6 billion years ago with the gravitational collapse of
a small part of a giant molecular cloud.
Most of the collapsing mass collected in the centre,
forming the Sun, while the rest flattened into a
protoplanetary disk of loose dust, out of which the
planets, moons, asteroids, and other Solar System bodies
formed.
This model, known as the nebular hypothesis, was
developed in the 18th (1700s) century by Emanuel
Swedenborg, Immanuel Kant, and Pierre-Simon Laplace. It
has been adjusted by scientific disciplines such as
astronomy, physics, geology, and planetary science. As
our knowledge of space has grown, the models have been
changed to account for the new observations.
The Solar System has evolved considerably since its
initial formation. Many moons have formed from circling
discs of gas and dust around their parent planets, while
other moons are believed to have formed and were later
captured by their planets. Still others, as the Earth's
Moon, may be the result of giant collisions.
Many collisions between bodies have occurred, and have
been important to the evolution of the Solar System. In
the early stages, the positions of the planets sometimes
shifted, and planets have switched places. This
planetary migration is thought to have been responsible
for much of the Solar System's early evolution.
Earth's orbit
The Earth's orbit around the Sun is nearly a perfect
circle, but when mapped it is found that the Earth moves
around the Sun in a very slightly oval shaped orbit,
called an elliptical orbit. The other planets in the
Solar System also orbits the Sun in slightly elliptical
orbits. Mercury has a more elliptical orbit than the
others, and some of the smaller objects orbit the Sun in
very eccentric orbits. |
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| Andreas Cellarius's
illustration of the Copernican system, from the
Harmonia Macrocosmica (1660). |
Discovery and exploration
For thousands of years, people had no need for a name
for the "Solar System". They thought the Earth stayed
still at the center of everything (geocentrism).
Although the Greek philosopher Aristarchus of Samos
suggested that there was a special order in the sky,
Nicolaus Copernicus was the first to develop a
mathematical system that described what we now call the
"solar system". This was called a new "system of the
world".
In the 17th century, Galileo Galilei, Johannes Kepler
and Isaac Newton began helping people understand physics
more clearly. People began to accept the idea that the
Earth is a planet and moves around the Sun, and that the
planets are worlds with the same physical laws that
control Earth. More recently, telescopes and space
probes have led to discoveries of mountains and craters,
and seasonal meteorological phenomena such as clouds,
dust storms and ice caps on the other planets. |
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| Size comparison of
the Sun and the planets. |
Structure and composition
The principal component of the Solar System is the Sun,
a star that contains 99.86% of the system's known mass
and dominates its gravitation. The Sun's four largest
orbiting bodies, the giant planets, account for 99% of
the remaining mass, with Jupiter and Saturn together
comprising more than 90%. The remaining objects of the
Solar System including the four terrestrial planets, the
dwarf planets, moons, asteroids, and comets) together
comprise less than 0.002% of the Solar System's total
mass.
All the planets, and most other objects, orbit the Sun
in the same direction that the Sun is rotating
(counter-clockwise, as viewed from above Earth's north
pole). There are exceptions, such as Halley's Comet.
Most of the planets in the Solar System have secondary
systems of their own, being orbited by planetary objects
called natural satellites, or moons, two of which, Titan
and Ganymede, are larger than the planet Mercury and, in
the case of the four giant planets, by planetary rings,
thin bands of tiny particles that orbit them in unison. |
|
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| The sun and the
inner planets. |
The Sun
At the center of the Solar System is the Sun.
It is a star, like the billions of other stars in the
sky.
The Sun is important to us because it gives us heat and
energy that allows life. None of the life on Earth could
exist without the Sun.
The rest of the objects in the Solar System orbit
(travel around) the Sun. The planets are the largest of
these. The planets closest to the Sun are called the
inner planets. These are Mercury, Venus, Earth, and
Mars. Then comes a big ring of asteroids, chunks of rock
much smaller than planets. This ring is called the
asteroid belt. Within the asteroid belt, there is a
dwarf planet (smaller than a normal planet) named Ceres.
Then come the outer planets: Jupiter, Saturn, Uranus,
and Neptune. Farther out there are two dwarf planets,
Pluto and Eris.
Activity on the Sun's surface, such as solar flares and
coronal mass ejections, disturbs the heliosphere,
creating space weather and causing geomagnetic storms. |
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| The inner planets.
From top to bottom rightwards: Earth, Mars,
Venus, and Mercury (sizes to scale). |
Inner planets
The first four planets closest to the Sun are called the
inner planets. They are small and dense terrestrial
planets, with solid surfaces.
Mercury
Mercury (0.4 AU from the Sun) is the closest planet to
the Sun and the smallest planet in the Solar System
(0.055 M⊕). (An Astronomical unit (AU) is a unit of
length derived from the Earth's orbit. It is the average
distance the Earth gets from the Sun on the long axis of
the ellipse.)
Mercury has no natural satellites; besides impact
craters, its only known geological features are lobed
ridges or rupes that were probably produced by a period
of contraction early in its history.
Venus
Venus (0.7 AU from the Sun) is close in size to Earth
(0.815 M⊕) and, like Earth, has a thick silicate mantle
around an iron core, a substantial atmosphere, and
evidence of internal geological activity.
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 (752 °F), most likely due to the amount of
greenhouse gases in the atmosphere.
Earth
Earth (1 AU from the Sun) is the largest and densest of
the inner planets, the only one known to have current
geological activity, and the only place where life is
known to exist.
Its liquid hydrosphere is unique among the terrestrial
planets, and it is 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 oxygen.
It has one natural satellite, the Moon, the only large
satellite of a terrestrial planet in the Solar System.
Mars
Mars (1.5 AU from the Sun) is smaller than Earth and
Venus (0.107 M⊕). It has an atmosphere of mostly carbon
dioxide with a surface pressure of 6.1 millibars
(roughly 0.6% of that of Earth).
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
as recently as 2 million years ago.
Its red colour comes from iron oxide (rust) in its soil.
Mars has two tiny natural satellites (Deimos and Phobos)
thought to be either captured asteroids, or ejected
debris from a massive impact early in Mars's history.
Asteroid belt
Asteroids except for the largest, Ceres, are classified
as small Solar System bodies and are composed mainly of
rocky and metallic minerals, with some ice. They range
from a few metres to hundreds of kilometres in size.
Asteroids smaller than one meter are usually called
meteoroids and micrometeoroids (grain-sized).
The asteroid belt occupies the orbit between Mars and
Jupiter. It is thought to be remnants from the Solar
System's formation that failed to form because of the
gravitational interference of Jupiter.
The asteroid belt is very sparsely populated; spacecraft
routinely pass through without incident.
Ceres
Ceres (2.77 AU) is the largest asteroid, a protoplanet,
and a dwarf planet. Its mass is large enough for its own
gravity to pull it into a spherical shape.
Ceres was considered a planet when it was discovered in
1801, and was reclassified to asteroid in the 1850s as
further observations revealed additional asteroids. It
was classified as a dwarf planet in 2006 when the
definition of a planet was created. |
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| The outer planets
(in the background) Jupiter, Saturn, Uranus and
Neptune, compared to the inner planets Earth,
Venus, Mars and Mercury (in the foreground). |
Outer planets
Jupiter
Jupiter (5.2 AU), at 318 M⊕, is 2.5 times the mass of
all the other planets put together. It is composed
largely of hydrogen and helium.
Jupiter's strong internal heat creates semi-permanent
features in its atmosphere, such as cloud bands and the
Great Red Spot.
Jupiter has 79 known satellites. The four largest,
Ganymede, Callisto, Io, and Europa, show similarities to
the terrestrial planets, such as volcanic activity and
internal heating. Ganymede, the largest satellite in the
Solar System, is larger than Mercury.
Saturn
Saturn (9.5 AU), with its extensive ring system, has
several similarities to Jupiter. Although Saturn has 60%
of Jupiter's volume, it is less than a third as massive,
at 95 M⊕.
Saturn is the only planet of the Solar System that is
less dense than water.
The rings of Saturn are made up of small ice and rock
particles.
Saturn has 62 confirmed satellites composed largely of
ice. Two of these, Titan and Enceladus, show signs of
geological activity. Titan, the second-largest moon in
the Solar System, is larger than Mercury and the only
satellite in the Solar System with a substantial
atmosphere.
Uranus
Uranus (19.2 AU), at 14 M⊕, is the lightest of the outer
planets. Unlike the planets, it orbits the Sun on its
side.
It has a much colder core than the other giant planets
and radiates very little heat into space.
Uranus has 27 known satellites, the largest ones being
Titania, Oberon, Umbriel, Ariel, and Miranda.
Neptune
Neptune (30.1 AU), though slightly smaller than Uranus,
is more massive (17 M⊕) and hence more dense.
It radiates more internal heat, but not as much as
Jupiter or Saturn.
Neptune has 14 known satellites. The largest, Triton, is
geologically active, with geysers of liquid nitrogen.
Triton is the only large satellite with a retrograde
orbit.
Neptune is accompanied in its orbit by several minor
planets, termed Neptune trojans, that are in 1:1
resonance with it.
Dwarf planets
Pluto had been called a planet since it was discovered
in 1930, but in 2006 astronomers meeting at the
International Astronomical Union decided Pluto did not
fit into the definition of a planet. Instead they called
it a dwarf planet along with some others. These small
planets are sometimes called plutinos. |
- Pluto
- Ceres
- Haumea
- Eris
- Makemake
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Trans-Neptunian region
Beyond the orbit of Neptune lies the area of the
"trans-Neptunian region", with the doughnut-shaped Kuiper
belt, home of Pluto and several other dwarf planets, and an
overlapping disc of scattered objects, which is tilted
toward the plane of the Solar System and reaches much
further out than the Kuiper belt.
The entire region is still largely unexplored.
It appears to consist overwhelmingly of many thousands of
small worlds—the largest having a diameter only a fifth that
of Earth and a mass far smaller than that of the
Moon—composed mainly of rock and ice.
This region is sometimes described as the "third zone of the
Solar System", enclosing the inner and the outer Solar
System.
Ultima Thule is a trans-Neptunian object located in the
Kuiper belt.
With the New Horizons space probe's flyby on 1 January 2019
(UTC time), it became the farthest and most primitive object
in the Solar System visited by a spacecraft.
It was discovered on 26 June 2014 by astronomer Marc Buie
using the Hubble Space Telescope.
Farthest regions
The point at which the Solar System ends and interstellar
space begins is not precisely defined because its outer
boundaries are shaped by two separate forces: the solar wind
and the Sun's gravity. It is thought to extend up to a
thousand times farther and contain the Oort cloud. |
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| Hale–Bopp seen in
1997. |
Oort cloud
The Oort cloud is a hypothetical spherical cloud of up to a
trillion icy objects. It is thought to be composed of comets
that were ejected from the inner Solar System by
gravitational interactions with the outer planets. The Oort
cloud objects move very slowly, and can be interrupted by
infrequent events, such as collisions, the gravitational
effects of a passing star, or the galactic tide, the tidal
force exerted by the Milky Way.
Boundaries
Much of the 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). Objects may yet be discovered in
the Solar System's uncharted regions.
Galactic context
The Solar System is located in the Milky Way, a barred
spiral galaxy with a diameter of about 100,000 light-years
containing about 100 billion stars. The Sun resides in one
of the Milky Way's outer spiral arms, known as the
Orion–Cygnus Arm or Local Spur.
The Solar System's location in the Milky Way is a factor in
the evolutionary history of life on Earth. Its orbit is
close to circular, and orbits near the Sun are at roughly
the same speed as that of the spiral arms. |
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