Inner solar system planets. Computer artwork and diagram of the orbits of the innermost four planets. This overhead angle looks down on the planets, which are: Mars (far right), Earth (lower left), Venus (upper right), and Mercury (left of Sun). The asteroids are seen beyond the orbit of Mars. These four planets are known as the rocky, terrestrial planets, as opposed to the outer gas giants. The inner solar system (out to Mars) has a radius of around 250 million kilometres.