Question

is an asteroid a planet? why

Answer 1

No. There is an entire asteroid belt between Mars and Jupiter, comprising of many thousands of asteroids. They are not planets. They are far too small to be classed as such.
http://coolcosmos.ipac.caltech.edu/cosmic_kids/AskKids/images/asteroidbelt.gif

Answer 2

The definition of planet by the International Astronomical Union (IAU) states that:

"A planet is a celestial body that (a) has sufficient mass for its self-gravity to overcome rigid body forces so that it assumes a hydrostatic equilibrium (nearly round) shape, and (b) is in orbit around a star, and is neither a star nor a satellite of a planet."

Source: http://en.wikipedia.org/wiki/IAU_definition_of_planet

Answer 3

Thank uuuu! :) 0-0

Answer 4

Oops....somehow I neglected to mention the third requirement:
(c) has cleared the neighbourhood around its orbit.

Answer 5

o ok