A planet is in the Solar System.
Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System , after Mercury . Mercury is the smallest and innermost planet in the Solar System . Solar System . The asteroid belt , which lies between the orbits of Mars and Jupiter , mostly contains objects composed , like the terrestrial planets , of rock and metal . Venus is the second planet from the Sun , orbiting it every 224.7 Earth days .