The protoplanet hypothesis states that solar systems have their origins in rotating disks of dust coated in ice from frozen gases, which slowly grow into planets. The first bodies of dust and gas brought together by gravity encounter other, smaller bodies and add them to their mass.
According to the protoplanet hypothesis, the planets that form farther from the forming star tend to be larger because the heat from the sun evaporates the gases closer to it, reducing the mass of the forming bodies. Greater mass allows the larger planets to accumulate mass faster due to their greater gravitational pull. The rocky planets toward the interior of solar systems have much less hydrogen and other gases in their compositions.