As the Solar System formed billion of years ago, the Sun's heat was not as strong farther out in the nebula cloud. Cooler temperatures allowed chunks of ice, floating among the rocks in the hydrogen and helium cloud, to grow.
The four planets formed in these cooler outer regions -- Jupiter, Saturn, Uranus and Neptune -- were larger worlds of mostly ice and gas. Referred to as Jovian planets after Jupiter, they have very little rock content.
Unlike the inner planets, how did the Jovian planets form? And where did the outermost planet, Pluto, come from? It's hard to say today, but future explorers are likely to find out.