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 came together 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 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.

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.

Titan, one of Saturn’s moons, is the only moon in the solar system to have an Atmosphere.

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". 

Six of the planets and three of the dwarf planets are orbited by moons. There are about 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.

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.

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.

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.

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 173 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) gave a correct definition to the word "planet" for the first time. By this definition, Pluto was not a planet anymore due to its irregular orbit and size. 

Eris was more massive than Pluto. 

It became a "dwarf planet" along with Eris and many others. After this, Pluto was put on the list of minor planets and was downgraded in 2006 by astronomer Michael E Brown. 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 - 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. Most asteroids are also often counted with the inner planets

 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