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.
Till recently (August 2006), Pluto was also considered a planet. International Astronomical Union, a
decision was taken that Pluto like other celestial objects (2003 UB313)discovered in recent past may be called ‘dwarf planet’.