A planet (from the Greek for "wandering star") is a celestial body orbiting a star or stellar remnant that is massive enough to be rounded by its own gravity, is not massive enough to cause thermonuclear fusion at it core, and has cleared its neighbouring region of planetismals.
Planets are generally divided into two main types: large, low-density gas giant, and smaller, rocky terrestrials. Under IAU definitions, there are eight planets in the Solar System. In order of increasing distance from the Sun, they are the four terrestrials, Mercury, Venus, Earth and Mars, then the four gas giants, Jupiter, Saturn, Uranus and Neptune. Six of the planets are orbited by one or more natural satellites, or "moons" (only Mercury and Venus aren't).