ⓘ 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 ..

Solar System

ⓘ 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 Systems 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. 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.


1. 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 Earths 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 Systems early evolution.


2. Earths orbit

The Earths 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.


3. 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.


4. The eight planets

In their order from the Sun:

  • Uranus
  • Mercury
  • Jupiter
  • Mars
  • Earth
  • Neptune
  • Venus
  • Saturn

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.


5. 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 on the definition of a planet, and Pluto did not fit. 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.

  • Ceres
  • Eris
  • Pluto
  • Haumea
  • Makemake

6. 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.

Outer solar system

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

Oort Cloud

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


6.1. Structure 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
  • 3 Earth
  • 1 Mercury
  • 2 Venus
  • 4 Mars
  • Asteroid belt region contains;
  • Asteroids
  • a Ceres the only dwarf planet in this region

6.2. Structure Outer solar system

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

6.3. Structure Trans-Neptune region

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

6.4. Structure Oort Cloud

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


7. Ecliptic plane

The plane of the ecliptic is defined by the Earths 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.