ELI5: why are we so certain that water and oxygen are required for alien life?

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> We’re assuming that alien life requires the same things we do We aren’t. There might well be alien life that doesn’t require any of that. It might be strange and completely unrecognizable. And there is the problem; if the alien life is unrecognizable then what do we look for? We only have the one example of life to work from, the life on our planet, so it makes sense to look for the kind of life that we know the signs of rather than something we don’t know anything about.


I don’t think we *are* that certain. But we have no idea what other conditions could lead to life, because the only example we know, in the universe, is us. So we’re assuming that any place that looks kinda like Earth is the best place to find life that looks kinda like life as we understand it. Water plays a critical roles in our biology, and in our evolution. Water allowed unicellular structures to evolve, since they only needed to develop a membrane and not a skeleton or other support system as they would on land. Oxygen is very useful, but isn’t a strict requirement; there are life forms on earth that don’t use oxygen as much as most organisms do.


In addition to the other good answers, I’d also like to add that the reasons also have to do with chemistry, particularly with carbon and it’s interaction with oxygen and chemical reactions occurring in water based environments. Based on the modern understanding of chemistry, only certain molecules can react with others in complex ways so this restricts the possibilities of what other life forms are likely to have. This conclusion is not necessarily dependent on studying living organisms we see on earth, it is just a fact about how chemicals interact. It doesn’t mean this is correct of course, only that our understanding of chemistry makes it seem unlikely that other types of molecules would lead to life. There are some they disagree and have created alternative hypothetical biochemistries with different molecules. To date none of these have been found in any life form on earth.


It’s not just a blind assumption based on our own situation on Earth. Water is a pretty common compound. It has a reasonably wide range of temperatures that it remains liquid at. It is a good solvent, and works really well for storing and mixing chemicals for organisms. It has a high specific heat, and in the proper configuration with fat and skin, will maintain a regular temperature pretty well, and keep homeostasis in your organism. There are other neat features, like the fact the solid version (ice) is less dense, and will float on the surface. Most compounds don’t do this. This means that a lake will freeze on the top, and that ice acts an insulating barrier, meaning all the fish below will stay protected and have nice liquid water to live in. Also, H2O does a lot of magic stuff on a molecular level, in the ways it interacts with proteins, interacts with buffers for H+ and OH-, etc. **Basically, H2O does a lot a special shit that is very handy for the concept of a organism that is made of chemicals that interact with each other. It’s not that scientists forgot to consider aliens might use other compounds.**


If I showed you a red box, and told you to find something like it, you’d look for things that are red and boxy because that’s what you know to look for. You’re not assuming that my *only* interests are red boxes, but you’re only able to look for my interests *as you know them*. We’re not assuming all life requires the same things we do, but we’re only able to identify life as we know it.