Ask HN: Why do we assume aliens would depend on water and oxygen like us? - cvaidya1986
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thedevindevops
I'm a carbon chauvinist. I freely admit it. Carbon is tremendously abundant in
the cosmos and it makes marvelously complex organic molecules that are
terrifically good for life. I'm also a water chauvinist. It's an ideal solvent
for organic molecules and it stays liquid over a very wide range of
temperatures. -Carl Sagan, Cosmos s01e05

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db48x
On TV it's usually just narrative convenience.

But there are real reasons to suspect that it's likely. Some atoms are more
common than others. In general, light atoms are more common than heavy atoms.
Life is going to use the tools which are commonly available on its planet, so
life that has uranium as a basic constituent of their biochemistry would have
to come from somewhere quite strange.

And you can also rank the atoms by how many different types of bonds they can
form with other atoms. Noble gasses form almost no bonds at all, with
anything, so it seems pretty unlikely that any form of life would use them.
Carbon and oxygen, and the other atoms in those columns of the periodic table,
form lots of types of bonds and can do so with several other atoms at the same
time. This gives you lots of complex molecules.

Carbon and oxygen are in the sweet-spot, and therefore it seems likely that
alien biochemistries will use them. Of course, the set of chemicals that you
can make with with them is quite vast; it's also likely that an alien
biochemistry would have only the simplest and most basic molecules with ours,
even if it's based on carbon and oxygen. Luckily water is quite a simple
molecule.

