Defining a planet’s habitability: not as simple as all that

Is the life on our planet the fruit of a chemical accident or is it indeed inherent in the laws of nature? Is it a unique feature in the universe or a commonplace ‘cosmic imperative’? The scientists who think that life is not confined to our planet are growing ever more numerous: if favourable conditions exist elsewhere in the universe, why shouldn’t it appear there as well?

Astrobiology is the science which concentrates on the study of the origins and the evolution of life on Earth and its distribution in the universe. By its very nature interdisciplinary, it brings together biologists, biochemists, palaeontologists, astrophysicists, geologists, chemists, etc., whose Holy Grail is the identification of a second ‘biogenesis,’ a second inhabited planet or one which has sheltered life in its past. Astrobiology (also called ‘exobiology’ in France, or ‘bioastronomy’ in English) is the subject of the research and workshops of the FNRS ‘Astrobiology’ contact group, created by the two authors, E. Javaux and V. Dehant.

Before searching for possible extraterrestrial life, several prior definitions are called for. The first is an obvious one only in appearance: what is life? The responses offered are diverse and are to a greater or lesser extent satisfactory. ‘They all take as their starting point the only example of life we have…otherwise all we can offer is intellectual flights of fancy,’ hastens to point out  Emmanuelle Javaux, a micro-palaeontologist at the ULg’s Department of Geology, for whom ‘life is a series of chemical processes, confined within a compartment (a cell), exchanging energy and matter with its environment and transforming it (metabolism), reproducing by the transfer of information (genetic code) through natural selection.’

Even if there does not exist a unanimously accepted definition of life, what is important is to be able to recognize it when it presents itself to the researcher. Clear criteria are thus necessary to distinguish it from a chemistry which imitates it. Every definition of life brings into play an ensemble of criteria of which none, taken separately, is specific to life, apart from maybe the genetic code. These definitions, established on the basis of a study of life such as it exists on Earth, can be directly transposed to astrobiology. It is thus that the palaeontologist intervenes in drawing up a Mars mission to determine, for example, the tools to be taken along, the place to ‘land,’ or the types of rock to take samples of.

Just as crucial as the definition of life is that of habitability. The latter is the subject of an interdisciplinary exposé published this Summer in the Astronomy and Astrophysics Review (1) journal by Emmanuelle Javaux and Véronique Dehant, a mathematician and department head at the Belgian Royal Observatory.

(1) Javaux J.E., Dehant V. 2010. Habitability : from stars to cells. Astron. Astrophys. Rev. 18: 383.

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