I'm trying to get a basic understanding of Earth's origins in order to teach an advanced oceanography course to high school students this summer. The course starts with one lecture on the origins of the universe, solar system, the earth, and the ocean. I'm trying to understand, why did terrestrial planets form in the inner solar system while gas giants formed in the outer solar system? I understand that planets formed as the disk of the solar nebula accreted into clumps, and that only dense metals and rocky minerals could coalesce into the terrestrial planets in the inner solar system where it is hot. I understand that farther from the protostar where it was cooler, less dense material, methane and ammonia, was able to accrete to form the gas giants. It is also my understanding that in general there were less heavy elements available in the nebula than light ones, so the inner planets had less material to accumulate and are therefore much smaller than the gas giants. My question is, why do we not see terrestrial type planets in the outer solar system? Is it because that denser material was pulled closer to the center of the nebula due to gravity and so none was available in the outer reaches of the solar system? If so, how can we explain the Kuiper Belt? Thanks in advance for your help!