I believe in aliens simply because of probability.
Also, I agree with BK in that they'd be completely different, but not in that they're unimaginable. There are no real boundaries to imagination, it's just that the likelihood of predicting their appearance through guesswork is... Not likely.
After all, unless they're completely incomprehendable (which seems to me to be unlikely), we'd have to be able to interpret them ourselves. If we can see and interpret their designs it must be possible to imagine them, even if in memory.
For example: If you saw a creature that you had never imagined anything even vaguely similar to, would you be able to remember it? Yes, unless you have a very bad memory. Your imagination would piece together bits of your memory to form an image of what you saw, and would likely fill in spaces.
Memory is influenced by imagination, and that's why it is frequently inaccurate. It doesn't snap photos; it fills in details in even the most photographic memories.
Now, if imagination fills in the details of something "unimaginable," is it really unimaginable? No, we simply don't have a place to start guessing about aliens. It is therefore possible to predict what aliens look like, but the chances of doing so are astronomical. (Pun not intended.)
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I don't think that made sense at all. I need sleep...