People often answer this very question on a smaller scale. They perceive the question in a way that only concerns them. They'll say something like: To better yourself, to meet your companion, to live comfortably etc. But then there are others like you and I. Those who think outside the box, the minority. When asked this question we instantly perceive it as pertaining to life and humanity as a whole. I think that's interesting.
As to the answer of your question, there is none and I don't know of a possible way that we can ever come to. It's all speculation. It is interesting, though, to think about life on other planets. Different species, races, cultures, ideas, inventions, beliefs, advancements, overall knowledge. It's fascinating and unlimited. Also a little overwhelming and dissapointing too, because I want to know it all.
Nice thread.
How ignorant of an answer this is. So sure of yourself, as if you were speaking fact. Wow.