The formation and composition of planets depend on various factors, including the protoplanetary disk from which they form, the distance from their parent star, and the availability of different elements and compounds. While it's true that solid elements are denser and can create a stronger gravitational pull, the formation of gas giants like Jupiter and Saturn is not solely determined by gravitational forces.
Gas giants typically form farther from their parent star, where the protoplanetary disk contains a larger amount of volatile substances such as hydrogen and helium. These substances are abundant in the outer regions of the disk, where temperatures are lower, allowing them to condense into gas rather than remain in a solid state.
In the early stages of planetary formation, solid planetesimals and protoplanets can begin to accumulate these volatile gases through a process called accretion. As the protoplanet grows in mass, its gravitational pull becomes stronger, enabling it to attract and retain larger amounts of gas. This process continues until the protoplanet becomes massive enough to accumulate a substantial gas envelope, transforming it into a gas giant.
In contrast, rocky planets, like Earth, tend to form closer to their parent star, in regions of the protoplanetary disk where it is hotter. The higher temperatures prevent volatile gases from condensing into a gas state, resulting in a lower abundance of gas available for accretion. Consequently, rocky planets primarily accrete solid materials, such as rock and metal, which eventually form their solid cores.
It's important to note that the distinction between gas giants and rocky planets is not solely based on gravitational pull. It is a complex interplay of various factors, including the availability of different materials, temperature, and distance from the star, which ultimately determine the composition and characteristics of a planet.