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. It also necessarily includes the concept of artificial life, since any life form that might naturally evolve elsewhere could conceivably be created in a laboratory using a future technology. It might be difficult to tell whether a truly strange life form had in fact arisen in space, or was designed much nearer to home.
Although this is currently a speculative field, the absence of life in the rest of the Universe is a falsifiable hypothesis (though it is yet to be proven false), making Xenobiology a valid field for scientific enquiry. Likewise, computer simulations of basic life processes have made it possible to do exploratory engineering of alternate life forms (like left-handed DNA) to determine their characteristics.
For these reasons the search for extraterrestrial life is of great relevance to xenobiologists. Some contend that the number of planets with intelligent extraterrestrial life can be estimated from the Drake equation if and when we ascertain the values of its variables. However uncertainties in the term of the equation make it impossible to predict whether life is rare or common. Another associated topic in xenobiology is the Fermi paradox, which suggests that if intelligent life is common in the universe then there should be obvious signs of it.
There is no current evidence for intelligent extraterrestrial life (as of 2003). However examination of meteors from Antarctica which are presumed to have originated from the planet Mars have provided what some scientists believe to be microfossils of extraterrestrial life, although that interpretation of the evidence is still controversial.
Xenobiology also figures in much science fiction as the fictional science of the biology of alien organisms. This use of the term demonstrates the speculative generation of possible models of such life, e.g. silicon-based.