Obviously, no one can know what life on another planet would look like until we discover such life. However, scientists have speculated that (if I'm not mistaken) all living things must be carbon-based. I think it's also assumed that all life requires water.
Are you aware of a published list of theoretical "universal life guidelines"? For example, I would speculate that locomotory organs (e.g. legs) tend to evolve in pairs (one-legged animals unlikely), and that microscopic creatures have no legs at all (replaced by cilia, tentacles, etc.).
Again, this isn't hard science; I'm just interested in knowing about the speculations of various scientists.
Let me add a few details to make my question more clear. On Earth, most life can be divided into a handful of kingdoms (e.g. plants and animals), and animals are commonly divided between invertebrates and vertebrates. Vertebrates are generally bigger and are bilaterally symmetrical (as are some invertebrates).
I would speculate that the laws of physics dictate that really tiny organisms have locomotory organs like cilia, and some may have just one cilia.
However, larger, more complex animals possess bilateral symmetry. This includes not just vertebrates but arthropods and even some mollusks (e.g cephalopods). There are some obvious advantages to bilateral symmetry among larger life forms.
So, if we discovered a planet with a similar size to Earth, we probably would not expect to find fifty-feet amoebas crawling on land. We might speculate that most large terrestrial creatures would have paired leg-like organs - and probably one or two pairs, not 100. Similarly, paired eyes help with navigation and depth perception.
But those are just very general personal guesses. Lots of alien life forms have been created for science fiction movies, but I wondered if the scientific community has created some more rigorous theories about what alien life forms would really be like.