Our understanding of life on exoplanets and exomoons must be based on what we know about life on Earth. Water is a common ecological requirement for all life on Earth. Temperature on an exoplanet is the first parameter to consider, both because of its influence on liquid water and because it can be directly estimated from orbital and climate models of exoplanetary systems. Water is required for life, but deserts demonstrate that even a small amount can suffice. Only a small amount of light from the central star is required to support photosynthesis. Some nitrogen is required for life, and
