Author: Joseph Lazio <jlazio@patriot.net>

[A portion of this entry is based on a lecture by Alain Leger (IAS) at
the SPIE Astronomical Telescopes and Instrumentation 2000 Conference.]

As far as SETI, the search for extraterrestrial intelligence, is
concerned, we do not assume that other being must be based on carbon.
In fact, SETI is a bit of a misnomer.  We are searching for
extraterrestrial *technological* intelligences, technological
intelligences capable of broadcasting their existence over
interstellar distances.  Whether the technological civilizations is
based on carbon or some other substance is largely irrelevant.  (Of
course, one might worry that intelligences based on some substance
other than carbon might have such different perspectives on the
Universe that, even if they broadcast electromagnetic radiation, they
would do so in a fashion that we would never consider.)

However, when one moves to finding life on other bodies in the solar
system or traces of life on extrasolar planets, there is a definite
carbon chauvinism in our thinking.  The most commonly mentioned
alternate to carbon (C) is silicon (Si).  It has similar chemical
properties as C, lying just below C in the periodic table of the
elements.

Carbon chauvinism has arisen because C is able to form quite
complicated molecules, in part because its atomic structure is such
that C can bond with up to four other elements.  Not only can it bond
with up to four other elements, but C can form multiple bonds with
other elements, particularly itself.  (Atoms bond by sharing
electrons, when two atoms share more than one electron they have a
multiple bond.  For instance, water is formed by an oxygen atom
sharing the two electrons from two hydrogen atoms.  In contrast, there
are many C compounds in which a single C atom shares multiple
electrons with other atom.)

A clear indication of the versatility of C is found in interstellar
chemistry.  Interstellar chemistry typically occurs on the surface of
microscopic dust grains contained with large clouds of gas between the
stars.  The physical conditions are much different than anything on
the surface of a habitable planet.  Nonetheless, of the molecules
identified in interstellar space as of 1998, 84 are based on C and 8
are based on Si.  Moreover of the eight Si-based compounds, 4 also
include C.

Thus, while there is definitely a C bias in our thinking, there is at
least some evidence from Nature supporting this bias.