Astrobiology (in Greek astron = star, bios = life and logos = word/science), also known as exobiology (Greek: exo = out) or xenobiology (Greek: xenos = foreign) is a speculative field within biology which considers the possible variety of extraterrestrial life. It also 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.
The term is also used in science fiction, with the difference that is not speculative, but an established science. A xenobiologist is usually a human doctor or biologist who is expert on the physiology of alien organisms and life forms.
Although this is currently a speculative field, the presence of life in the rest of the Universe is a verifiable hypothesis (though it has yet to be verified), 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.
Research primarily involves studying data from telescopes and planetary space probes, comparing the conditions on other planets in the solar system with extreme environments on Earth. Microbes, classed as extremophiles, have been found in apparently inhospitable environments such as frozen lakes, within rocks, at extreme depths in the oceans, at deep-sea hydrothermal vents and in geysers, among others. Several planets in the Solar System have similar conditions and could well harbour similar life forms. Such research is conducted at many universities around the world.
Missions that have specifically searched for life include the Viking and Beagle 2 probes, both directed to Mars. The Viking results were inconclusive and Beagle 2 failed to reach the surface of Mars. A future mission with a strong exobiology role will be JIMO, designed to study the frozen moons of Jupiter, some of which may have liquid water.
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.
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.
As of 2005, there is no definite evidence of extraterrestrial life. 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. In 2004, the spectral signature of methane was detected in the Martian atmosphere by both Earth-based telescopes as well as by the Mars Express probe. Methane has a relatively short half-life in the Martian atmosphere, so there must be a recent source of it. Since one possible source, active volcanism, has thus far not been detected on Mars, this has led scientists to speculate that the source could be (microbial) life.
Missions to other planets (such as Beagle 2: Evolution to Mars, Cassini to Saturn's moon Titan, and the future JIMO mission to Jupiter's icy moons) hope to further explore the possibilities of life on other planets in our solar system.