Hard SF : Aliens : Alien Anatomy
If there are extraterrestrials, what will they be like? This requires a lot of imagination. First, it may be pointless to try to picture a lifeform we’d find difficult to identify as living and difficult to distinguish as acting intelligently. It might be possible to exist, but by definition it’s outside our science, so we’d just be making wild guesses.
It’s easy to suppose they will look significantly like us. How likely that is depends on what “significantly like us” means. There is a branch of mathematics called topology which has to do with the shapes of objects. In topology, shapes can be defined in very generalized or abstract ways. Suppose a donut and a coffee cup were made of a material that allows them to be stretched and manipulated as much as we wanted without puncturing, tearing or breaking them. If the donut could be manipulated to be like a coffee mug, they would be “topologically equivalent”. Using criteria at this level of abstraction, the chances of extraterrestrials “like us” might be good.
If dolphins can have their level of intelligence without being tool users, having appendages capable of the kinds of activities arms with hands can do, etc., it should be possible to have beings on another world with a comparable intelligence and lack of tools and appendages. There is a logic which suggests that a higher level of intelligence would be associated with tool use and corresponding appendages. To successfully use substantial tools and have appendages that can really do the work, it is unlikely something like a blob, jellyfish or other sort of soft-bodied being could meet the requirements. (To some degree, an aquatic being such as an squid might be soft-bodied and be able to use tools in a liquid environment.) Manipulative appendages and something like a skeleton or exo-skeleton are probably needed for non-aquatic beings.
The purpose of intelligence is essentially to choose actions in the world that benefit you. In order to make intelligent choices, you need to know about the world around you. So, in order to develop intelligence, it is probably necessary to have a substantial amount of sensory information about one’s surroundings. There are practical reasons why vision is one of the more important senses for most animals. There are practical reasons why mostly-lightless astronomical bodies where vision would be less useful would not be promising places for advanced lifeforms to evolve. If pre-sapient species evolved in caves or other places where vision was not helpful, but animals elsewhere on the planet had evolved vision, the species probably could not safely move out into the rest of the world and would remain an isolated niche lifeform. So vision seems highly likely. For similar reasons hearing also seems likely.
Opportunities for self-protection, tool use, gaining varied experiences for learning about the world, etc. would seem limited for a species without locomotion. Even if an intelligent species could evolve from beings without locomotion, it would seem limited technological development, especially industrialization, would be possible. For reasons like these, extraterrestrials are likely to have body parts designed to help them move around.
From this train of thought, we conclude it’s likely aliens will not be soft-bodied, have manipulative appendages, have locomotive body parts, have vision and hearing. The laws of mechanics and other practical factors will influence the relative placement of locomotive parts, manipulative appendages, sense organs, nourishment intake, waste output, etc.
We could deduce other constraints. Temperature limits familiar to us would tend to apply to water-based lifeforms. Perhaps we could infer some facts about lifeforms with too little water to have those temperature limits.
We can ask what kinds of astronomical bodies and locations would tend to have elements heavier than what are gases at Earth temperatures. From this we could estimate likely factors in life with heavier elements. We can ask what kinds of bodies could have only lighter elements; have colder, hotter or similar temperatures to Earth. What kinds of celestial bodies are mostly gas or liquid or solid. What kinds of celestial bodies are likely to be large enough to evolve a complex enough ecosystem to develop intelligent life. At what low temperature would chemical processes be too restrained for consumption of material to provide enough energy for activity? And so on.
What are the major types of stars? Of those which tend to go nova too soon to allow complex life to evolve? Which kinds tend to be in multiple star systems that could interfere with successful evolution of complex life? What types are believed to be less likely to have planets in general or particular kinds of planets? What kinds of stars does that leave us with?
I suppose there are too many potential substances out there to list all the possible combinations to try to make a lifeform. However, we should be able to establish some unworkable combinations….