Difference between revisions 3758650 and 3758652 on simplewiki

[[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.

== 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]]" (Latin: ''trans'' prep + 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. Thise 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 up of debris [[wikt:similar|similar]] to the asteroid belt. However, 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 reclassified as dwarf planets. There are over 100,000 Kuiper belt objects with a diameter grater than 50 km. The total mass of the Kuiper belt is thought to be only a tenth or even a hundredth the mass of the Earth. Many Kuiper belt objects have multiple [[satellite]]s, and most have orbits that take them outside of the [[ecliptic]].

The Kuiper belt can be roughly divided into the "[[Classical Kuiper belt object|classical]]" belt and the [[Resonant trans-Neptunian object|resonances]]. Resonances are orbits linked to that 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 classified as cubewanos, after the first of their kind to be discovered, (15760) 1992 QB1, and are still in near primordial, low-eccentricity orbits.

=== Plutinos, Makemake and Haumea ===
The largest object in the Kuiper belt is [[Pluto]], which is 39 AU. It was discovered in 1930 and was considered to be the ninth planet. As a result of the 2006 definition of a planet, Pluto was described as a dwarf planet. Pluto's largest moon, [[Charon]], sometimes orbit each other. Beyond Charon, four smaller moons [[P5]], [[Nix]], [[P4]] and [[Hydra]] orbit around Pluto. Pluto orbits the Sun twice for every three Neptunian orbit. Kuiper belt objects who orbits 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 much more [[wikt:erratic|erratic]]. SDOs can often move almost as much in a [[wikt:vertical|vertical]] direction as they do in a [[wikt:horizontal|horizontal]] 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% more larger and about the same diameter of Pluto. It is the largest known dwarf planet in the Solar System. It has one moon, [[Dysnomia]]. Eris and Pluto have a similar orbit.