Final answer:
The inner planets are made of rock and metal because they can survive the heat closer to the Sun, while the outer planets are made of lighter materials like ice and gas. The internal structure of the terrestrial planets also plays a role, with dense metals in the core and lighter silicates near the surface. Option number 1 is correct.
Step-by-step explanation:
The reason why the inner planets are made of rock and metal, while the outer planets are made of lighter materials, is due to a combination of factors. Firstly, the heat of the Sun plays a role in determining the composition of the planets. The inner planets and most asteroids are made of rock and metal because these materials can survive the high temperatures closer to the Sun, while ice and gas evaporate. On the other hand, the outer planets, which are farther from the Sun and have always been cooler, are composed mainly of ice and gas.
Additionally, the internal structure of the terrestrial planets also contributes to their composition. The densest metals are located in the central core, while the lighter silicates are found near the surface. If these planets were liquid, like the giant planets, the sinking of heavier elements due to gravity would explain this arrangement. This suggests that, although the terrestrial planets are solid now, they must have been hot enough to melt in the past.