I’m bored so let’s imagine an example. Enter a truly exotic organism: nuclear power life form. We don’t have anything like this on earth but we can imagine one just the same.

First we know we need a way to draw radioactive elements from whatever soil or rock we’re on. We want to maximize surface area for the transfer of ions so we’ll build tube like structures to absorb nuclear elements and transport them into our reactor organ. We’ll want to minimize the radiation leaks otherwise we’ll die so we’ll need a working fluid system and heat transfer chamber and a system for dissipating that heat and in the process creating chemical energy.

For the chamber we can build special structures to hold the “rods” with structures using fluid pressure to move the control rods (or surfaces) between them. The most optimal solution for waste disposal is to grow the rods from their base and then have specialized cells that travers the rods and wear them down at the ends, collecting material that has spent the most time in the reactor, and then have those cells leaving our body through a specialized opening. A similar process can ensure the walls of the chamber never become unstable due to neutron damage.

The solid portions of these structures will need to be strong but light and be easily removed by chemical reaction. We know this kind of structure is possible because it’s literally how bones work. Maybe there’s a more efficient chemical reaction to use like the production of silicate surfaces but I doubt it.

These rods will be relatively heavy and we want them to orient naturally otherwise we’ll be doing extra work so if we assume gravity exists, we’ll build the reactor vertically. We can then build separate rocky structures to support the chamber that don’t need changing as often.

Lastly we need a method for heat transfer, assuming there is an atmosphere, we could just use fleshy flexible membranes to do this. Assuming we are in a more viscous fluid that allows good heat transfer, we could pump the fluid through us to exchange the heat. Or in the absence of atmosphere we could build a specialized large surface area sheet that can radiate heat into space effectively. Again calcium carbonate works for this purpose but so would some metals or a wide variety of materials if we don’t need to worry about electromagnetic radiation from a star.

Water as the working fluid would be optimal as its incompressibility would give us better options for raising the control rods. Furthermore it’s one of the most common fluids in the entire universe.

We could deal with high pressure easily but low pressure would require a more rigid structure probably with a near spherical shape if we really want to maximize efficiency like life does.

Now the only thing left to deal with is reproduction. This is actually relatively simple if we don’t have an atmosphere or we have one that isn’t dense: build a smaller version of ourself with some starting plutonium, put it in a specialized channel, open the back of the channel to superheated water and let the expansion of steam yeet our child a long distance. That way it won’t compete with us for resources.

Sure likely the egg would need to be built with some odd shape to deal with the impact and to make sure some viable roots made it below the surface but that shouldn’t be too hard.

Anyway this has been a very fun little exercise, but more importantly, I created life that wasn’t at all based around life on earth (the mention of bones was a proof of concept, the idea of solid structures is definitely not just earth specific). It doesn’t need to exist in an earth like environment, and it mostly doesn’t look like life on earth.

There are probably some more organic ways to structure things besides rods, like interlocking spirals, but other than that everything else earth-like is literally just from applied physics. Not just the roots but even like pushing the fluid around would only be efficient if it was done like it is inside us. How will we push our fluid around? Through tubes that undergo peristalsis. That’s not because I think things have to act like humans but because humans are bound by physics and physically, that’s the best way to move large amounts of fluids in a body (assuming you can’t construct an optimized turbine  and compressor of course).

We definitely can never say we know what ALL aliens will look like, but it’s almost guaranteed that if there is life in the universe, some of it will look like the life we have here. And all of it will be designed the way it is because of its environment, an environment whose physics can be understood. We can and possibly already have thought up some life that isn’t on earth but is somewhere else in the universe.