Our Sun and planets formed at about the same time from a collection of dust and gases called a nebular cloud. A. The early solar system was made mostly of heavier materials, such as rock-forming minerals, with a small percentage of light elements such as hydrogen, helium, oxygen, carbon, and nitrogen. B. As the outer parts of the nebular cloud cooled, they became home to a region of rocky and metallic debris known as meteoroids. C. The inner planets formed when certain minerals collided and coalesced into larger bodies with high temperatures and weak gravitational fields that were not able to retain the lightest elements. D. Perhaps as the result of an explosive event, the nebular cloud collapsed and began contracting as gravity caused to particles to interact. E. A supernova sent a shock wave through the nebular cloud, causing it to expand until its heavier elements were forced to the outer solar system. F. Being farther from the Sun, the outer planets were cooler than the inner plants, giving them a higher percentage of elements in the form of ice and a large quantity of the lightest gases: hydrogen and helium.