Astrobiology is a rapidly evolving, interdisciplinary field of research that concerns the origin, frequency, and evolution of life in the universe. Given, however, that so far we only know of one example of life -- the one on Earth -- astrobiology generally proceeds with the assumption that, in terms of its basic requirements, extraterrestrial life should resemble the terrestrial template.
Ingredients that appear to have been crucial for life on Earth to emerge were: a certain level of environmental stability (e.g., not too many impacts by asteroids); the presence of liquid water; temperatures and levels of radiation that are not too extreme; a reliable energy source (the Sun); and the availability of certain elements such as oxygen, carbon, and phosphorus. It is not unreasonable to assume as a first guess that many or maybe even all of these ingredients are essential for life anywhere (after all, carbon, for instance, is quite unique in its ability to form complex molecules), but until we find alien life we wouldn't know for sure which of these are absolutely necessary.
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