Final answer:
The inner planets, called terrestrial planets, are similar to Earth with compositions primarily of rock and metal and solid surfaces. This group includes Mercury, Venus, Earth, and Mars, along with Earth's Moon due to its consistency in composition. They are distinct from the gas giants and are made up of heavier elements like iron and silicon.
Step-by-step explanation:
The inner planets, also known as terrestrial planets, are called so because they are similar in composition to Earth. These include Mercury, Venus, Earth, and Mars. These planets are relatively small, composed primarily of rock and metal, and have solid surfaces that showcase their geological history through features such as craters, mountains, and volcanoes. Additionally, Earth's Moon is often included in this category due to its similar composition, although it is not a planet itself.
The terrestrial planets have a differentiated structure, with denser materials like iron and nickel sinking to their centers during their molten phase, resulting in a core, mantle, and crust composition. Unlike the outer gas giants, the terrestrial planets and the Moon lack significant quantities of light gases and ices, which are more common in the outer Solar System. Instead, they are made mostly of heavier elements, such as iron and silicon, which indicates different processes of planet formation in the inner solar system.