The planets have certain common characteristics. They are all roughly spherical (round), and they all follow orbital paths which take them around the Sun.

We can divide them into two broad categories: Terrestrial planets, those like Earth, and Jovian planets, which are like Jupiter. The members of the first group tend to be low mass, high density, and orbit near to the Sun, while the second group contains planets which have both high mass and low density, and are found further away from the Sun (they all lie more than five times as far away from the Sun than the Earth does).

You may wonder why the Jovian planets weigh more than the Terrestrial planets, if they are lower density objects. After all, don't pillows weight less than bricks? The answer lies in their large volumes. Which would you rather have dropped on your foot – one hammer, or two thousand pounds of light, fluffy pancakes (piled up into a giant, tall stack)?