When trying to find a solution to hard questions the best way is to start by closely look at the question. The question really implies other questions that need to be addressed first in order to answer “What do aliens look like?”. The implied questions are firstly, “what is life?” and the secondly, “what environment is that life living in?”

Assuming you mean intelligent life like human beings we would have to realise that we are a product of our environment. In our environment we perform a number of tasks. We reproduce, we efficiently find and distribute energy, we fight entropy, we evolve. Really if you boil it down to the core element we are a really long number. Almost a real number. This number is finely tuned to our environment. The more we evolve the longer and more tuned that number becomes. Each of us is a weighted constantly changing very precise number.

Lower life forms may also have a number that represents them. The number does not have to be as precise for lower life forms. The number representing a single ant would not need to be as long or complex as the number representing a human being.

The number is closely matched to the environment. In fact the tuned number forms part of a feedback loop. A person in a coma for 10 years will have a simpler number than a person active in the community. 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.

Are most organisms symmetrical? Most organisms are bacterial, and I think a lot of them are symmetrical, except for, of course, spirochaetes. Since he is a human studying extremophile bacteria, this declaration makes sense.

However, most eukaryotes are not symmetrical at all, because most eukaryotes are things like plants, fungi, algae and other protists that are definitely not symmetrical. One only has to think about the beautiful symmetries of flowers. Eye-catching, right? Why are they eye-catching? Because their symmetry stands out in the chaotic mass of fractal plants that covers so much of the landscape.

Do I expect alien intelligent life to be assymetrical? Not particularly. However, that’s because this primitive human can only define intelligence by how it interacts with me, and that requires things like moving symbolic communication. As a counter-example, plants have some amazingly complex interactions with their local environment–symbioses with fungi, bacteria, pollinators, chemical communications with other plants, competition with plants, anti-predator devices, and so forth–yet even if there is a controlling intelligence behind all this, I can’t communicate with it, and therefore, by default, I treat it as unintelligent.

Problem here is, intelligence can only be determined through interaction, and a truly alien intelligence would probably not even be recognizable by us, even if we lived together.

Dr. Mancinelli made a good contribution here, but inadvertently, he also showed how our biases shape what we’re looking for. It’s an important lesson.

Much of this comes from practical decisions, of course. It’s easier to put a costume on an actor than it is to create something truly novel with which the audience might not be able to identify.

An alien life form, therefore, would most likely be symmetrical. The type of symmetry would be influenced on the environment in which it lived. From our basic knowledge of survival of macroscopic organisms whether they be aquatic or … What Are Alien …