Water is essential for life as we know it, and so the search for life elsewhere often begins with the search for water. Water has been detected in many places in space — in the atmosphere of Saturn, in star-forming clouds, and even (as water vapor) on the surface of the Sun. Water was present in the raw materials that formed the Earth, as well as in comets that rained down on Earth (particularly early in its history).

Since life on Earth requires liquid water to survive, many scientists think that our search for life elsewhere should begin in places where temperatures are neither too hot nor too cold for water to exist in liquid form. By seeking out and studying such places in our own Solar System and beyond, we can learn more about the requirements for life, and also perhaps shed light on the origins of life here on Planet Earth.