Hard SF : Aliens : Introduction
Extraterrestrials are a common enough element is SF. They certainly provide a means to explore alternative possibilities and analyze human characteristics with the subtly allowed when you picture the characteristic in somebody else.
Of course, there is the question of whether there are extraterrestrials out there, and if so why we don't detect more evidence of them. "Absence of evidence is not evidence of absence." So finding extraterrestrials can prove they exist, but not finding them will never prove they don't exist. There is one theory that the reason we don't detect aliens is because technological societies tend to destroy themselves before they can contact beings in other solar systems. I would say humans have yet to prove or disprove this is true of Earth, but there are reasons to question the future of our species and planet.
Aliens in fiction often fall into two categories: monstrous looking bad aliens and humanoid aliens who can be either good guys or bad guys. It's rather understandable that a TV show like Star Trek found it cheaper and easier to have almost every extraterrestrial be very humanoid. In movies where they can afford more special effects, monsters and more extensively modified humanoids are more common. Even in books, where "special effects" cost nothing, the central characters are generally humanoid. The aliens are often interesting, but the non-humanoid ones also often have bodies that are unlikely to have been evolutionary successes or have doubtful body mechanics.
There is a wild card to keep in mind. A number of rules apply to what would be a plausible result of evolution on another world. However, once a species reaches a level of technology, it becomes possible to engineer different bodies for themselves or newly designed species. As long as these engineered bodies do not have to survive natural selection in the wild and have technology to make their environment more convenient to their bodies, fewer restraints apply to what kind of body is plausible. The laws of physics still apply, but not the law of the jungle.
See also: In Science Issues section: Who Is Human, What Can Think