main idea. A. Somewhere between 4 and 25 naturally occurring elements are necessary for life, depending on the complexity of the organism. B. The most common elements used by lifeoxygen, carbon, hydrogen, and nitrogenare also some of the most widely distributed elements in the universe. C. Life is most likely to be found in the oldest star systems, where heavy elements have been continually produced since those systems were formed. D. Planets, moons, asteroids, and comets are all composed of heavy elements, which means they contain the basis for any life form, carbon based or otherwise. E. Life is most likely to exist on those bodies that were not heavily bombarded with asteroids and comets during the formation of the solar system. F. Organic molecules are widely available, but chemical reactions among these molecules probably require either an atmosphere or a liquid medium.