While I agree about this being pretty on par for earth, you are wrong about us having no idea what aliens would look like.

I get that maybe you were using hyperbole but seriously if physics is consistent across the universe, we can make pretty good guesses. Also science fiction is definitely not just based on life on earth and I can provide an example.

First, life according to nasa is any self sustaining chemical system that can undergo Darwinian evolution.

Basically if a thing can make more of itself and those “children” have the chance to be at least slightly different from their parents, it is life.

Well guess what, the universe tends to disorder. There is only one way to be the exact self replicating thing you are. Ergo, given time, you will stop working unless you are able to fight entropy which requires the production of entropy (see the second law of thermodynamics).

Basically all living things MUST take in some form of energy and output it in a more disordered form. Every living thing must eat and every living thing must produce waste.

Now this doesn’t have to be in the form of chemical energy. It should be possible to create an organism that can sustain itself by taking in quanta of high frequency light and emitting more quanta of lower frequency light.

However, that is strictly to stay alive which is only part of the definition of life and not even the real important one. The important aspect of life is that it can reproduce itself and equally important: reproduce itself not always exactly the same.

Building a copy of yourself requires more elements and moving any amount of mass requires applying a force (newtons laws). Now you could simply sit around and let diffusion bring nutrients to you. In which case you either need to be a machine that simply slowly build itself by chance, or you could be a cell with a semipermeable membrane that uses ion channnels to create an electrical potential across said membrane to facillitate your acquisition of those building blocks and outcompeting the former kind of life. Which one is more likely? So which one will become more complex and possibly large enough to be seen as an alien life form and not alien bacteria?

Anyway if you do work via diffusion, you’ll want surface area but you don’t want volume because force over distance is energy, so bigger than necessary means loss of energy, means getting out competed. This forces life which relies on diffusion alone to become more round shaped though it’s not a big loss since that’s how most simple membrane materials want to be anyway.

Now if you eat something, you need a way to turn that food into work you need done. This means you need to have (or parasitize) some chemical machinery that takes food and does something useful. If you need to replicate yourself then you also need a machine or machines that create more of each part of you. In most cases specialization of machines reduces waste, so a living thing will produce little units that each do specific tasks rather than a single protein that does everything because that’d would require more order and thus more energy. Instead the cell becomes a little factory that does the same stuff in a way that doesn’t require perfect rigid order because that’d be a waste.

Ta da, we have earth like life. If it’s beneficial for these units to work together, they will. Maybe they’ll merge into a single cell like thing like slime mold. Maybe one will use the other like a mitochondria. Maybe they’ll stay separate but signal each other as a colony. Maybe multiple colonies will combine to form something like a man o war jelly fish. Maybe a cell will be able to differentiate itself later allowing it to form a more complex multicellular organism with different systems specializing for specific tasks.

This is where structure becomes a diverse thing, but see we already know what a living thing needs and these structures will be built to facilitate those. You need a system for acquiring food and possibly a separate one for removing waste. And you need a system for reproduction.

All life needs this and we’re familiar with it because that’s how life on earth works.

Now depending on what energy you eat, things get a lot more diverse but they follow from physics so we can predict them.

You eat light from a directional source? Then you want broad structures that face that source.

You need nutrients from diffusion? Then you want a network of tubular shapes to maximize surface area and minimize volume.

Need to trap prey? Build a net, build a harpoon gun, grow prehensile limbs and claws to grab them. Trap them with slippery walls or sticky substances. Immobilize them with venom or vapor and then enclose them for digestion. Grow legs or other methods of propulsion and get after them. Grow fangs to stop them from getting way.

All of those things are things life on earth uses. Because they’re the options that work and guess what: they will work anywhere else in the universe.

Are your oceans made of ammonia? Maybe kerosene? Who cares. If there are life forms in it that are small, filter feeding will be the optimal strategy for life. If there are big ones then direct predation with teeth will be efficient.

Need to move through a fluid? Fins will be the best option. Need to move through a really viscous fluid? Spiral propulsion systems like flagellum with be the way to go. Have to move along the ground and can’t propel yourself by the means above? Well you’ll develop a foot of some kind to use friction to move.

If physics is at all consistent across the universe, there will be similarities between life across its entirety because that’s what life is. Life is optimizing physics, optimizing energy/resource use to reproduce more life. Sure maybe there are weird situations we don’t have here on earth so we don’t have life adapted to that but that’s the entire idea behind science fiction.

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.