The search for Life in the Universe generally assumes three basic life's
needs: I) building block elements (i.e., CHNOPS), II) a solvent to life's
reactions (generally, liquid water) and III) a thermodynamic disequilibrium. It
is assumed that similar requirements might be universal in the Cosmos. On our
planet, life is able to harvest energy from a wide array of thermodynamic
disequilibria, generally in the form of redox disequilibrium. The amount of
different redox couples used by living systems has been estimated to be in the
range of several thousands of reactions. Each of these reactions has a specific
midpoint redox potential accessible thanks to specialised proteins called
oxidoreductases, that constitute life's engines. These proteins have one or
more metal cofactors acting as catalytic centres to exchange electrons. These
metals are de facto the key component of the engines that life uses to tap into
the thermodynamic disequilibria needed to fuel metabolism. The availability of
these transition metals is not uniform in the Universe, and it is a function of
the distribution (in time and space) of the supernovae explosions and complex
galaxy dynamics. Despite this, Life's need for specific metals to access
thermodynamic disequilibria has been so far completely overlooked in
identifying astrobiological targets. We argue that the availability of at least
some transition elements appears to be an essential feature of habitability,
and should be considered a primary requisite in selecting exoplanetary targets
in the search for life.
Preprint
Subjects: Astrophysics - Earth and Planetary Astrophysics; Astrophysics - Astrophysics of Galaxies; Astrophysics - Solar and Stellar Astrophysics