Planets orbit stars that are huge enough to have their own gravity force, but small enough not to have thermonuclear fusion. Furthermore, these stars do not have any planetesimals in around them. People have known about planets for ages and in some cultures they were regarded as emissaries of gods in the sky. Even nowadays many people believe in astrology according to what the movement of the stars affects the lives of people. This notion is deeply rejected by scientists.
Planets are divided into two groups according to type. The first group is made out of large giants that are made out of low- density gas. The second group consists of smaller, rocky terrestrials. Nowadays eight planets are considered to be in our Solar system. The first four (Mercury, Venus, Earth and Mars) belong to the second group of planets and the other four gas giants (Jupiter, Saturn, Uranus and Neptune) belong into the first one mentioned beforehand. Most of these planets are surrounded by one or more moons; in some cases these moons are even larger than small planets. Scientists have recognized also five smaller dwarf planets in our Solar System.
Outside of our Solar system 310 other planets have been indentified by September 2008, these planets range from huge gas giants to small terrestrial ones. So in total there are 318 detected planets by this time. No extrasolar dwarf planets have been identified yet.
