The Solar System

The Solar System comprises of the Sun, 8 planets, satellites of the planets and the asteroid belt. The Kuiper belt and the Oort Cloud are composed of rocks and ice and are situated beyond the Solar System. The solar System is situated in the Milkyway Galaxy on its Orion Spiral arm at a distance of about 30000 light years away from the Galactic centre. It has been found by meteorite radiometric measurements that the Solar System is about 4.6 billion years old

The planets

The planets of the Solar System are divided into two categories,

1. The Inner or Terrestrial Planets

2. The Outer or Jovian Planets

The Terrestrial Planets

The terrestrial planets are, Mercury, Venus, Earth and Mars. These planets have hard rocky surfaces and are composed of elements such as Fe Ni Al and rocky silicates.

During the formation of the Solar System the newborn sun was so hot that only materials with a high melting point could survive to form planets near the sun. The air and water seen on earth and the gases on Venus are believed to have arrived by means of asteroids bombarding into the terrestrial planets.

The "rocky" terrestrial planets have got no rings like unlike the Jovian planets, and have relatively low speeds of rotation They also have very few satellites.

The Jovian Planets

The Jovian Planets are Jupiter, Saturn, Uranus and Neptune. During the early stages of the Solar System, at a considerable distance from the sun the gases could condense to form planets. Thus the terrestrial planets were born. It is known that Jupiter formed before Saturn and therefore accumulated more gas than Saturn could. Therefore Jupiter is Larger than Saturn. These planets actually have no Surface. the atmosphere gradually gets dense as depth increases and at some time liquefy. Then gradually it turns into solid. However these transitions are not sharply defined.

The Jovian Planets have a lot of satellites and have relatively high speeds of rotation. Because of the size and the speed of Uranus and Neptune, it is believed that they were formed between Jupiter and Saturn and later on swept away to where they are now.

*Jupiter and Saturn underwent orbital changes to achieve 2:1 orbital resonance, and this caused Neptune which was formed between Jupiter and Saturn to move further away from the Sun.

Pluto was sometime back considered a planet. But now is not taken as a planet, but as a member of the Kuiper belt that resides beyond the Solar System.

Formation of the Solar System

The current theory which explains the formation of the Solar System best is the Nebulae Theory, which originated in the 18th century and has been refined since then.

According to this theory, a nebula (i.e an interstellar cloud of dust and gases) spun fast causing it to collapse to form a disk like structure. It turned into a "Protoplanetary disk" in which the centre became a hot glowing cloud of gas-the Proto-Sun. Within 100 million years the temperature and pressure in the centre of the sun became so great that it started producing energy by hydrogen fusion. Thus this fusion energy countered the gravitational contraction of the sun and brought it to a state of Hydrostatic Equilibrium in which it attained stability.

Then the small particles in the nebula cloud underwent accretion(sticking together) to form the planets.

The nebulae theory has been strengthened by the discovery of extra solar planets.