A planet is a body that directly orbits a star, is large enough to be round because of self gravity, and is not so large that it triggers nuclear fusion in its interior. All the planets revolve around the sun. There are nine planets altogether- Mercury, Venus, Earth, Mars, Jupiter, Saturn, Uranus, Neptune and Pluto. Among these nine planets Mercury, Venus, Earth and Mars are called "rocky" or "terrestial" planets and the planets like Jupiter, Saturn, Uranus and Neptune are called the "gaseous" planets. A popular mnemonic used to remember the planets in order is: My Very Educated Mother Just Showed Us Nine Planets.
It is not known with certainty
how planets are formed. The prevailing theory is that, when
a protostar forms from a condensing nebula, the remnant of
the nebula becomes a thin disc of gas and dust that revolves
around the protostar.
Localised mass concentrations within this disc form increasingly dense pockets of matter, which then collapse inward under gravity to form planets. When the star's core ignites its solar wind blows away the remaining material, leaving asolar systemlike our own.
This theory is being greatly challenged with the discovery of extrasolar systems that vary greatly from our own.
View the details of the following planets