According to the International Astronomical Union's current definitions, there are Eight planets in the Solar System.

In increasing distance from the Sun, they are: 1. Mercury 2. Venus 3. Earth 4. Mars 5. Jupiter 6. Saturn 7. Uranus 8. Neptune Jupiter is the largest, at 318 Earth masses. Mercury is smallest, at 0.055 Earth masses. The gravitational attraction between the Sun and these objects keeps them revolving around it.

The Planets that are similar to Earth (Mercury, Venus and Mars), with bodies largely composed of rock are called Terrestrial Planets. The Planets with a composition largely made up of gaseous material (Jupiter, Saturn, Uranus, Neptune ) and are significantly more massive than terrestrials are called Gas giants.

It is not known with certainty how planets are formed. The prevailing theory is that they are formed during the collapse of a nebula into a thin disk of gas and dust. A protostar forms at the core, surrounded by a rotating protoplanetary disk.