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[[File:Planets2008.jpg|400px|thumb|right|The [[Sun]], [[planet]]s and [[dwarf planet]]s of the Solar System. Compared with each other, the sizes are correct but the distances are not.]]
The '''Solar System''' is the [[Sun]] and all of the objects in [[space]] which [[orbit]] (go around) it. The Sun is orbited by [[planet]]s, [[Satellite (natural)|moons]], [[asteroid]]s, [[comet]]s and other things. The Sun is a [[star]]. It contains nearly all of the Solar System's [[mass]]. This means that it has strong [[gravity]]. The other objects are pulled into orbit around the Sun. The Solar System is in one of the arms of the [[Milky Way]] [[galaxy]].

There are eight [[planet]]s in the Solar System. In order of increasing distance from the Sun, they are: [[Mercury (planet)|Mercury]], [[Venus]], [[Earth]], [[Mars]], [[Jupiter]], [[Saturn (planet)|Saturn]], [[Uranus]] and [[Neptune]]. The first four planets are called [[terrestrial planet]]s. They are mostly made of [[rock]] and [[metal]], and they are mostly [[solid]]. The last four planets are called [[gas giant]]s. This is because they are large planets which are mostly made of [[gas]]. They also have much more mass than the terrestrial planets. [[Satellite (natural)|Moons]] orbit six of the planets. [[Planetary rings]] and [[cosmic dust]] orbit the [[outer planet]]s.

The Solar System also contains other things. There is the [[asteroid belt]] between Mars and Jupiter. Further out than Neptune, there is the [[Kuiper belt]] and [[scattered disc]]. Inside the Kuiper belt, there are [[trans-Neptunian object]]s which are mostly made up of [[ice]]s, [[water]], [[ammonia]] and [[methane]]. Four [[dwarf planet]]s are in the Kuiper belt. These are small planets which are just large enough to be [[sphere]]-shaped. There are five dwarf planets in total: [[Ceres (dwarf planet)|Ceres]], [[Pluto]], [[Haumea (dwarf planet)|Haumea]], [[Makemake (dwarf planet)|Makemake]] and [[Eris (dwarf planet)|Eris]]. Moons orbit three of the dwarf planets. There are also thousands of [[Small Solar System body|small bodies]] in the scattered disc. As well as these, there are comets, [[Centaur (planetoid)|centaurs]] and there is cosmic dust.

== Discovery and exploration ==

For many thousands of years, people did not know about the Solar System. They thought Earth stayed still at the centre of the Universe and other things moved around it. People believed this until Nicolaus Copernicus published a mathematical model in 1543. His model said the Sun was at the centre of the Solar System. Most people did not believe him at first. But, in the 17th century, scientists such as Galileo, Kepler and Newton found out lots of physics. What they found out said that Copernicus was right.

At first, people could only use their eyes to look at the Solar System. After telescopes were invented, scientists discovered much more of the Solar System. They found new moons and planets, and they could look at other things that they could not see before. More recently, scientists have sent spacecraft into the Solar System. Spacecraft have discovered even more moons and features on other planets, such as mountains, craters, clouds, dust storms and ice caps.

== Structure ==

The main part of the Solar System is the Sun. It is a main sequence G2 star which contains 99.86 percent of the Solar System's mass. The largest four planets, the gas giants, make up 99 percent of the mass left.

Most large objects orbiting the Sun lie near the plane of Earth's orbit, called the ecliptic. The planets are very close to the ecliptic. However, comets and Kuiper belt objects are often at greater angles to it. The Sun, Mercury, Earth, Mars, Jupiter, Saturn and Neptune rotate anti-clockwise. Venus and Uranus rotate clockwise. All the planets and most other objects orbit the Sun anti-clockwise. There are some exceptions, such as Halley's comet.

The overall structure of the Solar System is divided into four regions: the Sun, the inner Solar System, the outer Solar System and the trans-Neptunian region. The inner Solar System is made of the four terrestrial planets and the main asteroid belt. The outer Solar System consists of the four gas giants. The trans-Neptunian region is made of all the objects beyond the orbit of Neptune.

Most of the planets in the Solar System are orbited by natural satellites (moons). These are large, ball-shaped bodies mainly made of rock. Most of the large natural satellites are in synchronous rotation. This means that one face of the natural satellite is always turned toward its parent planet. The four gas giants are orbited by planetary rings. These are bands of tiny particles and small rocks.

[[File:Ellipse axis2.png|thumbnail|250px|The black line of this diagram shows the shape of an ellipse. The Sun is always inside the ellipse but it does not have to be in the centre.]]
Kepler's laws of planetary motion explain how things go around the Sun. An object orbits along an ellipse path. The Sun is at one focus. Objects which are closer to the Sun go more quickly. This is because they are more affected by the Sun's gravity. The distance between a body and the Sun changes over the course of its orbital year. When a body is closest to the Sun, the approach is called the perihelion. (''ὁ ἥλιος''; Sun. ''περί'' + acc; near, by.) When a body is most distant from the Sun, the point is called the aphelion. (''ὁ ἥλιος''; Sun. ''ἀπό'' + gen; from, away from.) The orbits of the planets are nearly cirles. But many comets, asteroids and Kuiper belt objects go along highly elliptical paths. We can predict where things are in the Solar System  using models of numbers.

It is difficult for us to imagine the Solar System because of the very large distances between objects. In general, the farther a planet is from the Sun, the larger the distance between it and the previous planet. For example, Venus is approximately 0.33 astronomical units (AU) farther out from the Sun than Mercury while Saturn is 4.3 AU out from Jupiter.

== Sun ==
The Sun is the star of the Solar System. Its large mass (332,900 Earth masses) produces temperatures and densities in its core great enough to keep nuclear fusion going. Nuclear fusion releases enormous amounts of energy. This energy is radiated into space as electromagnetic radiation, mostly in the form of visible light.

The Sun is classified as a type G2 yellow dwarf star. But this name is misleading as, compared to most of stars in our galaxy, the Sun is rather large and bright. The Sun is in the prime of its life at the moment. It has not yet exhausted its store of hydrogen for nuclear fusion. The Sun is gradually growing brighter.

The Sun is a population I star. It was born in the later stages of the Universe's evolution. It contains more elements heavier than hydrogen and helium than older population II stars. Elements heavier than hydrogen and helium (such as metals) were formed in the cores of ancient and exploding stars. So the first generation of stars had to die before the universe could have these atoms. The oldest stars contain few metals, while stars born later have more.

== Material to be used ==
In the Solar System often there is a Solar Wind. This is a flow of [[Plasma (physics)|Plasma]]. The Solar System is located in one of the arms of the Milky Way galaxy. The Milky Way galaxy contains about 200 billion (200,000,000,000) stars.

== Trans-Neptunian region ==
The area beyond [[Neptune]] is called the "[[trans-Neptunian object|trans-Neptunian region]]" (''trans'' + acc; across, over, beyond). We have not explored here very much. The region has a large number of small bodies. These bodies are mostly made up of rock and ice. The trans-Neptunian region is sometimes called the "outer Solar System", though those words are used more often to describe the region beyond the [[asteroid belt]].

=== Kuiper belt ===
The [[Kuiper belt]] is made of [[wikt:scattered|scattered]] pieces of rock just like the asteroid belt. It is mostly made up of ice and can be found between 30 and 50 [[AU]] from the Sun. Inside the Kuiper belt are three [[dwarf planet]]s and [[Small solar system body|small Solar System bodies]]. Many of the largest Kuiper belt objects such as [[Quaoar]], [[Varuna]] and [[Orcus]], may be called dwarf planets at some time. There are over 100,000 Kuiper belt objects which are more than 50 km across. The total mass of the Kuiper belt is thought to be only a tenth or even a hundredth of the mass of the Earth. Many Kuiper belt objects have more than one moon.

The Kuiper belt can be roughly divided into the "[[Classical Kuiper belt object|classical]]" belt and the [[resonant trans-Neptunian object]]s. Resonances are orbits linked to the orbit of Neptune. The first resonance 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. Members of the classical Kuiper belt are called ''cubewanos''. They are called this because the first one of their kind to be discovered, [[(15760) 1992 QB1|(15760) 1992 QB<sub>1</sub>]], was called QB<sub>1</sub> for short. The orbits of cubewanos are still very circular.

=== Plutinos, Makemake and Haumea ===
The largest object in the Kuiper belt is [[Pluto]], which is 39 AU. It was discovered in 1930 and was called the ninth planet. A new definition for the word ''planet'' was made in 2006. Because of this, Pluto is now called a ''dwarf planet''. Pluto has five moons. Pluto's largest moon, [[Charon]], orbits closest to Pluto. Beyond Charon, four smaller moons [[S/2012 P 1|S/2012 (134340) 1]], [[Nix (moon)|Nix]], [[S/2011 P 1|S/2011 (134340) 1]] and [[Hydra (moon)|Hydra]] orbit around Pluto. Pluto orbits the Sun twice for every three orbits of Neptune. Kuiper belt objects who orbit in this area are called "plutinos".

Makemake (45.79 AU) is smaller than Pluto. It is the largest object in the classical Kuiper belt. Makemake is the brightest object in the Kuiper belt after Pluto. In 2008, it was described as a dwarf planet. Haumea (43.13 AU) has a similar orbit of Makemake. However, it is caught in a 7:12 orbital resonance with Neptune. It is about the same size as Makemake and has two satellites. It has a fast 3.9-hour rotation, which makes it seem as though Humea is flat and larger than it really is. As with Makemake, it was described as a dwarf planet in 2008.

=== Scattered disc and Eris ===
The [[scattered disc]] (or scattered disk) is a distant part of the Solar System. The area has small, icy minor planets known as [[scattered disc objects]] (SDOs). The part of the scattered disc closest to the Sun is in an area of space called the Kuiper belt. The first SDO was found in 1995 by [[Spacewatch]].

The scattered disk is still not understood well. Astronomers believe that it was created when objects in the Kuiper belt were "scattered" by the gravity of the outer planets, mainly Neptune. Unlike most objects in the Solar System which move in a round and flat path, the scattered disc is harder to predict. SDOs can often move almost as much in a [[wikt:vertical|up and down]] direction as they do in a [[wikt:horizontal|side to side]] direction.

Eris (68 AU) is the largest known scattered disc object. There has been a debate on weather or not it should be described as a planet. This is because it is 25% larger than Pluto and it is around the same distance across. It is the largest known dwarf planet in the Solar System. It has one moon, [[Dysnomia (moon)|Dysnomia]]. Eris and Pluto have a similar orbit.

===Asteroid belt===
{{Main|Asteroid belt}}
[[File:InnerSolarSystem-en.png|300px|thumb|Image of the [[asteroid belt]] and the [[Trojan asteroids]]]]

[[Asteroid]]s are [[Small Solar System body|small Solar System bodies]] that are made up of mainly [[refractory (astronomy)|refractory]] rocky and [[metallic]] [[mineral]]s, with some ice.<ref>{{cite web|title=Are Kuiper Belt Objects asteroids? Are large Kuiper Belt Objects planets?
|publisher=[[Cornell University]]|url=http://curious.astro.cornell.edu/question.php?number=601|accessdate=2009-03-01}}</ref>

The asteroid belt is located between Mars and Jupiter. It is 2.3 and 3.3&nbsp;AU from the Sun. It is thought to be objects from when the Solar System was formed and failed to form into larger objects because of the gravitational pull of Jupiter.<ref>{{cite journal
 | author=Petit, J.-M.; Morbidelli, A.; Chambers, J.
 | title=The Primordial Excitation and Clearing of the Asteroid Belt
 | journal=Icarus
 | year=2001
 | volume=153
 | issue=2
 | pages=338–347
 | url=http://www.gps.caltech.edu/classes/ge133/reading/asteroids.pdf
 | format=PDF
 | accessdate=2007-03-22 | doi = 10.1006/icar.2001.6702
| bibcode=2001Icar..153..338P
}}</ref>

Asteroids range in size from hundreds of kilometers across to [[micro]]scopic. All asteroids except the largest, Ceres, are classified as small Solar System bodies, but some asteroids such as [[4 Vesta|Vesta]] and [[10 Hygiea|Hygiea]] may be reclassed as [[dwarf planet]]s if they are shown to have [[hydrostatic equilibrium]].<ref>{{cite web|title=IAU Planet Definition Committee|publisher=International Astronomical Union|year=2006|url=http://www.iau.org/public_press/news/release/iau0601/newspaper/|accessdate=2009-03-01}}</ref>

The asteroid belt contains tens of thousands, possibly millions, of objects over one kilometre in diameter.<ref>{{cite web |year=2002 |title=New study reveals twice as many asteroids as previously believed |work=ESA |url=http://www.esa.int/esaCP/ESAASPF18ZC_index_0.html|accessdate=2006-06-23}}</ref> Despite this, the total mass of the asteroid belt is unlikely to be more than a thousandth of that of the Earth.<ref name=Krasinsky2002>{{cite journal |authorlink=Georgij A. Krasinsky |first=G. A. |last=Krasinsky |coauthors=[[Elena V. Pitjeva|Pitjeva, E. V.]]; Vasilyev, M. V.; Yagudina, E. I. |bibcode=2002Icar..158...98K |title=Hidden Mass in the Asteroid Belt |journal=Icarus |volume=158 |issue=1 |pages=98–105 |month=July |year=2002 |doi=10.1006/icar.2002.6837}}</ref> The asteroid belt is scattered, which allows [[spacecraft]]s to pass through without any damage. Asteroids with diameters between 10 and 10<sup>−4</sup>&nbsp;m are called [[meteoroid]]s.<ref>{{cite journal |author=Beech, M. |coauthors=Duncan I. Steel |year=1995 |month=September |title=On the Definition of the Term Meteoroid |journal=[[Quarterly Journal of the Royal Astronomical Society]] |volume=36 |issue=3 |pages=281–284 |bibcode=1995QJRAS..36..281B |last2=Steel}}</ref>

====Ceres====
[[Ceres (dwarf planet)|Ceres]] (2.77&nbsp;AU) is the largest asteroid, a [[protoplanet]], and a dwarf planet. It has a diameter of under 1000&nbsp;km, and a mass large enough for its own gravity to pull it into a [[sphere|spherical shape]]. Ceres was considered a planet when it was discovered in the 19th century, but was reclassified as an asteroid in the 1850s as further observations revealed additional asteroids.<ref>{{cite web |title=History and Discovery of Asteroids |format=DOC |work=NASA |url=http://dawn.jpl.nasa.gov/DawnClassrooms/1_hist_dawn/history_discovery/Development/a_modeling_scale.doc |accessdate=2006-08-29}}</ref> It was classified in 2006 as a dwarf planet.

====Asteroid groups====
Asteroids in the asteroid belt are divided into [[asteroid group]]s and families. These are based on how they orbit. [[Asteroid moon]]s are asteroids that orbit larger asteroids. They are not like planetary moons, but are larger than smaller asteroids that orbits them. The asteroid belt also contains [[main-belt comet]]s, which is thought to have been the reason why Earth has liquid water.<ref>{{cite web |year=2006 |author=Phil Berardelli |title=Main-Belt Comets May Have Been Source Of Earths Water |work=SpaceDaily |url=http://www.spacedaily.com/reports/Main_Belt_Comets_May_Have_Been_Source_Of_Earths_Water.html |accessdate=2006-06-23}}</ref>

[[Trojan asteroid]]s are located in either of Jupiter's [[L5 point|L<sub>4</sub> or L<sub>5</sub> points]]. The term "Trojan" is also used for small bodies in any other planetary or satellite Lagrange point. [[Hilda family|Hilda asteroids]] are in a 2:3 [[Orbital resonance|resonance]] with Jupiter. They go around the Sun three times for every two Jupiter orbits.<ref name=Barucci>{{cite book|last=Barucci|first=M. A.|coauthors=Kruikshank, D.P.; Mottola S.; Lazzarin M.|year=2002 |chapter=Physical Properties of Trojan and Centaur Asteroids|title=Asteroids III|publisher=University of Arizona Press|pages=273–87|location=Tucson, Arizona}}</ref>

[[Near-Earth asteroid]]s are asteroids that are near planet Earth. They sometimes will cross the orbit of other inner planets.<ref name = "MorbidelliAstIII">{{cite journal|url = http://www.boulder.swri.edu/~bottke/Reprints/Morbidelli-etal_2002_AstIII_NEOs.pdf|title = Origin and Evolution of Near-Earth Objects|author = A. Morbidelli, W. F. Bottke Jr., Ch. Froeschlé, P. Michel|journal = Asteroids III|editor = W. F. Bottke Jr., A. Cellino, P. Paolicchi, and R. P. Binzel|pages = 409–422|month = January | year = 2002|publisher = University of Arizona Press|format=PDF|bibcode = 2002aste.conf..409M|last2 = Bottke|last3 = Froeschlé|last4 = Michel}}</ref>

== References ==
{{reflist|2}}