The study of life is called biology: people who study life are called biologists. A lifespan is the average length of life in a species. Most life on Earth is powered by solar energy: the only known exceptions are the chemosynthetic bacteria living around the hydrothermal vents on the ocean floor. All life on Earth is based on the chemistry of carbon compounds, specifically involving long-chain molecules such as proteins and nucleic acid. With water, which all life needs, the long molecules are wrapped inside membranes as cells. This may or may not be true of all possible forms of life in the Universe: it is true of all life on Earth today.