The solar system can be divided into two areas: the inner and outer planets. The inner planets (Mercury, Venus, Earth and Mars) are made up of solid, rocky material. The planets tend to be smaller (in comparison to outer planets). They have warmer temperatures, which is vital for our planet to sustain life. While the exact reasons the inner planets formed this way is unknown, scientists have a theory.

The solar system started out as a cloud of gas (the nebula disk). Matter closer to the Sun in the nebula disk was warm enough to boil away most of the gases and ices, leaving only rocky material behind. Gravity brought the rocky material together over time and that formed the inner planets.