  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 energy yielding
reactions requires specialised proteins called oxidoreductases, that 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 complex 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.
