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 4.568 billion years old. The Sun formed by gravity in a large molecular cloud. It is mainly hydrogen, which it converts into helium through nuclear fusion. The planets are in a flattened orbiting disk. This disk was left over from the cloud that formed the Sun. Eventually, the gas and dust of the disk came together into planets. It is thought that almost all stars and their planets form this way.

The Sun is a star. It makes up 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 and higher elements. All heavier elements, called metals in astronomy, account for less than 2% of the Sun's mass. Oxygen is about 1% of the Sun's mass. Iron (0.2%) is the most plentiful of the other elements.

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.

Six of the planets, and the six largest dwarf planets, are orbited by moons. There are more than 200 moons in the Solar System. Mercury and Venus have no moons, and Jupiter and Saturn have the largest number of moons. The largest moon is Ganymede which is a moon of Jupiter. Titan is one of Saturn’s moons. It is the only moon in the Solar System to have an atmosphere, which is mainly composed of nitrogen.

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 interplanetary dust.

In Ancient Greece, Aristarchus of Samos proposed the heliocentric model of the Solar System, where the Sun, is at the center of the known universe. He is sometimes known as the "Greek Copernicus".

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. Others, such 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.

Grand tack hypothesis 
Astronomers now think that the order of the planets was not always as it is today. Knowing what we know today, we can see the Solar System is strange. Most other planetary system we are able to study have their largest planet closer to their star. Also we have noticed other oddities in the Solar System. Mars is smaller than it ought to be, and the asteroid belt has been disturbed.

So, astronomers have put forward the grand tack hypothesis. In it Jupiter was earlier closer to the Sun, and (for some unknown reason) moved out to its present position.

Orbits of the planets 
The Earth's orbit around the Sun is nearly a perfect circle, but in a very slightly oval shaped orbit, an elliptical orbit. The other planets in the Solar System also orbit 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. The planets all orbit the Sun in the same direction.

A full account of the planetary motion needs an account of the n-body problem, which is not treated on this wiki. A page can be found on En wiki.

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). 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 to understand physics more clearly. People began to accept the idea that the Earth is a planet that moves around the Sun, and that the planets are worlds, and that all worlds are governed by the same same physical laws. More recently, telescopes and space probes sometimes let us see details directly. All inner planets have surface features. The gas giants (as the name suggests) have surfaces whose make-up is gradually being discovered.

The eight planets 

In their order from the Sun:
Mercury
Venus
Earth
Mars
Jupiter
Saturn
Uranus
Neptune

The planets are the biggest objects that go around the Sun.  It took people many years of using telescopes to find the objects that were farthest away.  New planets might still be found, and more small objects are found every year.  Most of the planets have moons that orbit around them just as the planets orbit the Sun. There are at least 200 of these moons in the Solar System.

Dwarf planets 

Pluto was discovered by American astronomer Clyde Tombaugh and was declared the 9th planet of the Solar System in 1930. 

This all changed on August 24, 2006, when the International Astronomical Union (IAU) decided on the correct definition for the word "planet" for the first time. By this definition, Pluto was not a planet anymore due to its irregular orbit and size. It became a "dwarf planet" along with Eris and many others. 

Eris was 27% more massive than Pluto. After this, Pluto was put on the list of minor planets and was downgraded in 2006. Instead they defined a new category of dwarf planet, into which Pluto did fit, along with some others. These small planets are sometimes called plutinos.
 Pluto
 Ceres
 Haumea
 Eris
 Makemake

Structure 
There are a few main parts of the Solar System. Here they are in order from the Sun, with the planets numbered, and the dwarf planets marked with the letters a to e.

Inner solar system 

The first four planets closest to the Sun are called the inner planets. They are small and dense terrestrial planets, with solid surfaces. They are made up of mostly rock and metal with a distinct internal structure and a similar size. Three also have an atmosphere. The study of the four planets gives information about geology outside the Earth. 

 Terrestrial planets region contains the four planets closest to the sun, all are rocky planets
 (1) Mercury
 (2) Venus
 (3) Earth
 (4) Mars
 Asteroid belt region contains;
 (a) Ceres (the only dwarf planet in this region)
 Asteroids

Outer solar system 

 Gas giant planets region contains;
 (5) Jupiter
 (6) Saturn
 (7) Uranus
 (8) Neptune

Trans-Neptune region 
 Kuiper belt region contains;
 (b) Pluto
 (c) Haumea
 (d) Makemake
 Kuiper belt objects and possibly other dwarf planets
 short-period comets
 Scattered disc region contains;
 (e) Eris
 Scattered disk objects and possibly other dwarf planets

Oort Cloud  
The Oort cloud is separate from the trans-Neptune region, and much farther out. It contains the long-period comets.

Ecliptic plane 
The plane of the ecliptic is defined by the Earth's orbit around the Sun. All of the planets orbit the Sun roughly around this same orbital plane. The farther away from this plane a planet orbits, the more inclined is its orbit to the ecliptic. If you could look at the Solar System "edge on" then all the planets would be orbiting more or less in the plane of the ecliptic.

References

More reading

Other websites 

 Views of the Solar System
 Your Weight on Other Worlds
 Solar System Citizendium
The Planets in the Solar System