Heidelberg Joint Astronomical Colloquium

Speaker Gerda Horneck
Title From Habitability to the Origin and Evolution of Life

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.