The formation of the Solar System led to the creation of the inner planets being primarily composed of gas and ice, while the outer planets are largely rocky and small.

The Solar System formed from a rotating cloud of gas and dust, called a protoplanetary disk, around the young Sun. In the hot inner region of this disk, temperatures were high enough that only metal and rock could condense, so the planets that formed there—Mercury, Venus, Earth, and Mars—became rocky and relatively small. Farther out, the disk cooled, allowing volatile substances like water, methane, and ammonia to freeze into ice, and the cooler temperatures let gas giants grow by capturing large amounts of hydrogen and helium from the surrounding nebula. For example, Jupiter’s core formed first from icy and rocky material and then pulled in a massive envelope of gas, making it a huge, mostly gaseous planet. Thus, the inner planets are rocky because they formed where it was too hot for gases to stay, while the outer planets are gas giants because they formed where ices and gases could accumulate.