Now, what always bothered me, is that scientists try to look for planets with similar conditions like earth. But it's very subjective to estimate that for life to be possible, life conditions have to be exact or similar to earth.
Well I think scientists look for a certain amount of similarity simply because of the universality of the laws of physics.
The greatest example of this would of course be gravity, the concept of "up" and "down" in creature (or alien) designs must be taken into consideration.
An animal like a dog, lizard, or human has appendages on the bottom of his body for movement of the body against gravity. But they have no such appendages on the tops of their bodies because of the gravity it would be ridiculous and useless to have appendages on top of the body. Also birds have landing/take off gear on the bottoms of their bodies.
Of course, because their is no distinction between right and left on a planet with gravity then you would also see symmetry in most aliens probably, just like us they would have two eyes, two ears, two arms, or maybe not two arms but the same amount on each side.
Another universal sort of science that would limit alien conditions would be the amount of elements available for chemical compounds. There are more only around 115 elements and they all have very strict parameters in terms of whether they are metals or non-metals or what they can form bonds with, and their various properties.
But I think your question is an interesting one, and there are many ways alien life and conditions could be completely different then us. For example many scientists wonder why our dna is composed of 4 base pairs, what would life have been different if they were anything other than c,g,a,t?
Basically without writing a thousand page response I would also have to add the fact about the chemical composition of water and its relationship to life and the environment. My mind starts to tilt when I try to think of how much h2o has of an effect on everything. I simply have a very difficult time figuring out how life could exist as we know it without H2O.
Anyway I have given my amatuer two cents now, and I doubt it was very insightful, but those are my intial thoughts to your question, Im sure someone else knows more.
One more thing....A lot of scientists feel that ET would probably be microscopic in nature, like a bacterium, and I would agree that it would be more likely, but I have difficulty in thinking that any
intelligent life forms would be that small simply because of the chemical reactions needed for a synapse and the space that is needed. But since we dont really understand how the brain works at its cognitive level it would be difficult to say much more.
