Final answer:
The inner planets are made mostly of rock and metal because the high temperatures near the Sun evaporated lighter materials like ice and gas. This left metals and rocks, which could withstand the heat, to form the inner planets. Outer solar system objects are ice-dominated due to cooler temperatures, confirming this distinction.
Step-by-step explanation:
The reason why the inner planets are made mostly of rock and metal is largely related to the conditions present in the early solar system. The inner region, close to the Sun, was too hot for volatile compounds such as ices and gases to condense. These lighter materials would evaporate under the intense heat. Hence, the planets that formed in this region, which includes Earth, Mars, Venus, and Mercury, ended up being composed of the materials that could resist those high temperatures, which are metals and rocks.
Furthermore, in the outer solar system, where temperatures were lower, it was possible for ices and gases to remain stable, leading to the formation of gas giants with very different compositions. These planets, like Jupiter and Saturn, have heavy elements in their cores due to the gravitational separation of materials during their formation, with lighter materials forming their extensive atmospheres.
Analogously, spectroscopic analysis shows that the composition of the planets gives us hints about their origins. Objects in the outer solar system are largely ice-dominated, whereas terrestrial planets are rich in heavier elements such as iron and silicon. This points to a solar system where the lighter materials were more abundant further out, leaving a heavier-element-rich residue in the inner solar system where the terrestrial planets formed.