Infinite number of planets with life

[Hovik]

I was of course just picking on you in good fun, Stimbo.

Anyway, just as with flammable and inflammable, the 'in' doesn't negate the word it prefixes. So

But, aaaaactually...
The word inflammable is actually inflame + -able and comes from Latin inflammare. Any appearance of relationship to the prefix in- is coincidental.
The prefix in- is a prefix of negation which also has variations in form such as im-, il-, and ir-, found in such words as inadequate, impossible, illegible and irresponsible. These are all the same prefix, the form of which changes depending on the initial sound of the word to which it attaches, the underlying or base form of which is in-. This is a concept known as allomorphy in linguistics.

Minor nit: We know how big the observable universe is.

You're right; that was my error. Nonetheless, the universe is not infinite in any practical sense (at least so far as modern physics can tell us).