| Abstract |
The history of our biosphere spans over more than 3.5 billion
years. Microorganisms have flourished on Earth during this whole period and
dominated the Earth's biosphere during the first 2 billion years of its
history before the first unicellular eukaryotes (cells with a nucleus and
other organelles) appeared. Microorganisms have invented several strategies to
cope with and adapt to environments of a wide range of physical and chemical
parameters, such as extremes in temperature, salinity, pH, redox potential or
radiation stress. Nearly all sites on Earth are inhabited by microbial
communities, where an energy source is available and which are compatible with
the chemistry of carbon-carbon bonds. However, so far the Earth is the only
planet known to harbour life. Within the on-going and planned ventures of
space exploration, the search for signatures of life beyond the Earth is one
of the major drivers. In order to harbour life, a planet or moon needs to be
habitable. The criteria for habitability are based on the general properties
of terrestrial life, which include the availability of
| + a carbon based chemistry, |
| + an energy source, and |
| + water in its liquid phase. |
In a broader sense, habitability criteria range from the properties of the
host star under consideration (single star, availability of heavy elements,
mass, lifetime, planetary system), the characteristics of the planet or moon
(mass, orbit, atmosphere, discrete liquid sphere, surface), the chemistry
available (solvents, element composition and concentration, energy source,
redox potential, pH range) as well as the biological prerequisites (
replication of informational, molecules, stereospecific catalytic molecules,
information transfer molecules, polymerising molecular assembly, interphasic
molecular assembly). Based on those criteria, four classes of circumstellar
habitability can be distinguished.
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