An object can be broken up by a planet's gravity once it passes the the jovian planets are composed primarily of and helium. hydrogen and helium don't exist in earth's because the terrestrial planets of mercury, venus, earth, and mars couldn't exert a strong gravitational pull on hydrogen and helium gas within the nebula. is the planet closest to the sun, has almost no atmosphere, and what little atmosphere exists is constantly getting blown away by solar wind. the atmosphere of is very hot and dense, comprised of approximately 95 percent carbon dioxide, and the surface is composed of molten bedrock.