Over recent years, microbiologists have discovered hardy microorganisms surviving in an astounding range of extreme, hostile environments. Pretty much every moist niche on the planet appears to be alive, from boiling hot pools of acidic water, to alkaline salty lakes and solid icebergs at subzero temperatures. These arch-survivalists, the extremophiles, define the boundaries of the survival envelope of terrestrial life - a volume of physicochemical parameter space with respect to environmental conditions like temperature, pH and salinity. A habitable environment is, by definition, one which can support biology, and so within astrobiology, these extremophiles are often used to determine the habitability of similar environments on other planets and moons, and therefore how special the Earth may be in providing the conditions for life. But several important questions remain: why doesn’t life on Earth bulge even further within this parameter space? Why are certain regions of this physico­chemical parameter space not populated by terrestrial organisms? Why has biology not colonized these apparent gaps? Are the limits deter­mined by genuine biological restrictions (such as the degradation of organic molecules), which may be shared with extraterrestrial organisms, or are they more a consequence of the available range of environments on Earth to which life has had the opportunity to adapt, and so per­haps idiosyncratic to terrestrial life?

Furthermore, while extremo­philes demonstrate the incredible adaptability of life once it has arisen, and its capacity for regulating its own internal state in contrast to its environment, they make no statement about the likelihood of life arising in the first place. It is almost certain that the physicochemi­cal conditions able to nurture emerging self­-organizing networks of prebiotic chemistry in the origins of life are much more tightly constrained than the conditions for survival of cells, protected within a membrane barrier and able to regulate internal conditions. So in this sense, might extremophiles be irrelevant to the search for life beyond Earth?

There are therefore two major challenges that remain in fully defining the biological con­straints on habitability, and thus which extra­ terrestrial locales offer the best hopes for life. First, it is key to characterize the theoretical lim­its for supporting biological processes as distinct from the limits exhibited by terrestrial extremo­philes, which may be idiosyncratic to the Earth and its repertoire of habitats available for life to adapt to. Secondly, researchers in the field of prebiotic chemistry can provide insights into the necessary physical and chemical conditions for the original synthesis of life, from which starting point organisms can adapt to the full survival range of terrestrial extremophiles.