O comentário de um leitor a What Are Alien Species Like? Symmetrical, Solid, and Seeing (Probably) no Science Not Fiction:
Consequently if we want to understand what an alien looks like we must understand the environment they come from.
I could point here to the Drake equation. The drake equation does not have enough variables and needs extra inputs. The implied question is would life occur under different environments than ours? The answer is most likely no. There are a number of very unique circumstances that conspired to produce us.
The key is chemistry. Water is a very good solvent. Hydrocarbons cannot dissolve materials as well and this impedes development of complex life. Also carbon has four places to form bonds allowing complex chains necessary for carbon based life. Silicon also has four bonds and is not as easy to use with water. Extremophiles can survive outside the basic chemistry and environment but are restricted in the optimal growth necessary for intelligent life.
Additionally earths liquid metal core maintained by the low orbit of the moon is a fairly unique planetary event. The liquid core creates a magnetic field protecting us from the suns radiation. The tectonic plate action helps to cool the outer layer enough to support life preventing Venus like greenhouse effects. Additionally black smokers, underwater volcanoes allow life to get started. Tides and seasons allow a repeatable cycle helping evolution.
We also have very circular orbits creating stable environments in order for life to develop. We have Jupiter as a big protector from collisions giving life an average window of 700 million years to develop between impacts. The earth is large enough to produce life without risking mars like atmosphere evaporation or Venus greenhouse. The land mass is large enough for creatures to evolve out of the oceans and develop tools and fire. All of these conditions would have to be replicated in order to produce intelligent life.
Certain characteristics are necessary and have evolved several times taking the same form. Eyes for example have evolved several times and always look the same. It would be safe to assume hearing, taste and smell share this characteristic. Theses sensors are usually located close to the brain and usually some place high. Hands with opposable thumbs would be evident if the aliens evolve from trees which they most likely do.
Assume all these factors could be replicated the question becomes what factors could be different and still have intelligent life. The asteroid that destroyed the dinosaurs could be different. Comparing the Ichthyosaurus and Dolphins is a good example. Both creatures look and perform the same. One is a dinosaur and the other is a mammal. Assuming the underlying architectures roughly parallel our evolution our fellow aliens may be more dinosaur like if they missed an asteroid.
If they have had a few extra asteroids they maybe different again.
If the aliens are more evolved they may be more silicon machines than biological.
The surprise would be how similar to us they really are and probably could even be mistaken for human.
The real difference would be language and culture.
If you want to understand the form of an alien species you have to examine its environment.