Perhaps a better question is, what causes the difference between terrestrial and Jovian planets. It has to do with distance from the sun. There are 4 terrestrial and 4 Jovian planets. The terrestrial planets are closer to the sun. As such, they are rocky, dense, and have small masses. In comparison, the Jovian planets, also known as the gas giants have a rocky core but a huge atmosphere that consists of hydrogen, helium, methane and ammonia. Because of this huge atmosphere, the density of these planets are lower than terrestrial. The terrestrial planets are separated from the Jovian planets by the asteroid belt. Because the Jovian planets are so far from the sun, the sun's heat could not vaporize the atmospheres as on some terrestrial planets.
