Flexi Says: Several factors influenced the formation of rocky inner planets and gaseous outer planets in our solar system. The planets were originally formed from a spinning cloud of dust and gas that circled a newly formed star, the Sun. Temperatures nearer to the Sun were hotter. Heavier elements that make up rock can condense at high temperatures, but lighter elements like hydrogen and helium don't condense until temperatures are much cooler. As heavier metals condensed, they formed rocks which collided over time to build up the inner planets. Since these planets are relatively small, lighter gases escaped their gravity and were blown outward by solar wind. Far from the Sun in the outer reaches of the solar system, temperatures are cold enough for hydrogen compounds to condense, forming the cores of the outer planets. These much larger planets have enough gravity to hold large volumes of light gases in their atmospheres.