Xenobiology (or exobiology, or astrobiology) is the term for a speculative field within biology which considers the possibility of, and possible nature of, extraterrestrial life.
Although this is a speculative field, the absence of life in the rest of the Universe is a falsifiable hypothesis, making Xenobiology a valid field for scientific enquiry.
For this reason the search for extraterrestrial life is of great relevance to xenobiologists; a famous equation, the Drake equation appears to predict that extraterrestrial life is almost certain to exist. However, there is no current evidence for extraterrestrial life (as of 2001).
Xenobiology also features in much science fiction as the fictional science of the biology of alien organisms.