The nebula from which our Solar system is supposed to have been formed, started its collapse and core formation some time 5-5.6 billion years ago and the planets were formed about 4.6 billion years ago.
Our solar system consists of the sun (the star), 8 planets, 63 moons, millions of smaller bodies like asteroids and comets and huge quantity of dust-grains and gases.
Out of the eight planets, mercury, venus, earth and mars are called as the inner planets as they lie between the sun and the belt of asteroids the other four planets are called the outer planets.
Alternatively, the first four are called Terrestrial, meaning earth-like as they are made up of rock and metals, and have relatively high densities.
The rest four are called Jovian or Gas Giant planets. Jovian means jupiter-like.
Most of them are much larger than the terrestrial planets and have thick atmosphere, mostly of helium and hydrogen.
The difference between terrestrial and jovian planets can be attributed to the following conditions:
The terrestrial planets were formed in the close vicinity of the parent star where it was too warm for gases to condense to solid particles. Jovian planets were formed at quite a distant location.
The solar wind was most intense nearer the sun; so, it blew off lots of gas and dust from the terrestrial planets. The solar winds were not all that intense to cause similar removal of gases from the Jovian planets.
The terrestrial planets are smaller and their lower gravity could not hold the escaping gases.
All the planets were formed in the same period sometime about 4.6 billion years ago.
Till recently (August 2006), Pluto was also considered a planet. However, in a meeting of the International Astronomical Union, a decision was taken that Pluto like other celestial objects (2003 UB313)discovered in recent past may be called ‘dwarf planet’
Let us briefly review the conditions on the other seven planets of the solar system.
Mercury is closest to the sun. It has a temperature range of 427ºC on its side facing the Sun and – 270ºC, on its dark side. It has no atmosphere.
Venus is the closest neighbour of the earth. It is about 40 mk away. It is an extremely hot planet with a temperature of 480ºC. Its atmosphere has 96% carbon dioxide and poisonous gases like sulphur dioxide and carbon monoxide.
Earth is the only planet known to sustain life.
Mars is also close to earth. It is called the red planet. It has 95% carbon monoxide and reddish dust. It is relatively a very cold planet and as of now presence of life on it has not been conclusively established.
Jupiter is the largest planet of the solar system. It is mainly a rapidly spinning ball of gas specially clouds of ammonia, and has no solid surface.
Saturn consists mainly of hydrogen and helium. Its atmosphere has 90% nitrogen and a temperature of (-184ºC). It is also made up of hydrogen cyanide which is a highly poisonous gas. It is characterized by a ring that surrounds it.
Uranus is also a very cold planet. Uranus is a distant planet of solar system and 7th in order from the sun. Uranus and Neptune are the outermost planets of the solar system. Uranus has a highly tilted rotational axis.
Neptune is cold and dark with its surface coated with frozen methane.