the International Astronomical Union's current definitions,
there are Eight planets in the Solar System.
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
In increasing distance from the Sun, they are:
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.
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.