Does extraterrestrial life exist?
The question of whether or not extraterrestrial life exists is fascinating. We have only one example of a planet where this has happened–our Earth. To address the question scientifically, we must base our reasoning on experiences here. From your questions, I will assume that you are interested in intelligent life that has developed a civilization. This is what we know based on studies of our own civilization and life here on Earth. A. Life on Earth involves complex chemistry (respiration, reproduction, photosynthesis in plants, and metabolism) that centers around the carbon atom. The large molecules of life (DNA, RNA, proteins, etc) use carbon as the backbone with liquid water to bring in materials and expel waste products. Because of the special chemical properties of carbon and water, this combination makes sense and if life does exist elsewhere, odds are that it will have similar chemistry. B. Planets that have rocky surfaces and that are located near enough to their stars so that
