Flexi Says: Both the outer and inner planets of our solar system share some similarities. They all orbit the Sun, rotate on their own axes, have a core, are spherical in shape, and have cleared their orbital paths of most other debris. Additionally, all of the planets have metal in their cores. The cores of the inner planets are made almost entirely of iron and nickel, while the outer gas giants have cores that are combinations of rock, metal, and hydrogen compounds. Scientists think that the gas giants are likely to have some iron-nickel alloys in their cores.