What is a Planet?
A planet is a large, round heavenly body that orbits a star and shines with light reflected from the star. Eight planets orbit the sun in our solar system. In order of increasing distance from the sun, they are: (1) Mercury, (2) Venus, (3) Earth, (4) Mars, (5) Jupiter, (6) Saturn, (7) Uranus, and (8) Neptune. Many nearly planet-sized objects, called dwarf planets, also orbit the sun. Dwarf planets include Pluto and Ceres. Since 1992, astronomers have discovered many planets orbiting other stars.
This site describes what a planet is and has a link to an educator's guide.