We’re assuming that alien life requires the same things we do, why?
In: Biology
We think that life has to be really complicated stuff all mixed together. So that means that whatever a new system of life is like we think it will have to be stuff that can be in a bunch of different shapes and still be able to mix together.
So far the stuff on earth that makes up life is really good for this. All of our stuff is like LEGO bricks and it’s shapes can do a lot of cool things really easily and all the water we have is like the lego base that helps all the bricks come together and make really cool stuff.
It’s possible that there is really cool stuff that doesn’t work like lego but can still do complicated stuff, but right now it’s much easier to search for lego stuff in space so we’re trying to find out what planets might have cool new lego bricks on it.
We have lifeforms on earth that don’t require either of those things, and aren’t based on carbon. That allows us to assume that life can develop pretty much everywhere. With that assumption we can’t predict anything, and we don’t have any parameters to search for. That’s why we look for planets that could support our life instead of searching Venus for bacteria in the crust 2km down.
I could be wrong, but this makes sense.
https://science.nasa.gov/science-news/science-at-nasa/2010/02dec_monolake
https://www.google.com/amp/s/www.independent.co.uk/news/science/deep-cave-microbe-kidd-mine-university-toronto-aliens-evolution-a9097341.html%3famp
Oxygen is not a necessary condition and is really a “poison”. See what it does to cars!
But water is a different beast.
If water isn’t necessary then you would think after 4 billion years of evolution life on Earth would have evolved to not need it, or at least some life. The evolutionary advantage of not needing water would have easily created a huge advantage over life that did.
So we can conclude that water is (probably) necessary for life (that we can recognize).
Have you ever heard the phrase “life, as we know it” in a sci-fi show?
We only know of life on one planet. All the life *that we know of* needs liquid water in some form. We do already know of [life that doesn’t need oxygen.](https://en.wikipedia.org/wiki/Anaerobic_organism)
Scientists are very interested in finding a planet with water and oxygen for two reasons. First, water means that life *like Earth life* could exist there. Second, oxygen likes to combine with other things, and that means that if a planet has a lot of free oxygen, there’s likely something actively separating the oxygen and releasing it back to the air. The best candidate *that we know of* is life. This means that if you’re looking for life similar to Earth life, a planet with water and oxygen is a good place to start looking.
There *could* be all kinds of other life out there. We don’t know. But, that also means that we don’t know what to look for or where to look.
It’s not absolutely required, it’s just that when searching for alien life, we’re playing the odds. There are three things that life absolutely needs.
1. A building block to create complex molecules. On Earth that’s carbon.
2. A means of generating energy. Most earth life burns fuel for energy and that reaction requires oxygen.
3. All life requires chemical reactions to exist and chemical reactions need a medium to occur in. For Earth life, that medium is water.
It is possible to think up replacements for those three things but the replacements often require rarer elements, more unlikely circumstances or are simply more limited in what they can achieve. And thus make it far less likely to find *complex* alien life using those alternatives.
Take carbon for instance. Carbon makes a good building block because it eagerly combines with other atoms to create complex molecules. Carbon can enter into four covalent bonds with other atoms and actually prefers bonding to create molecules over reacting with other atoms. Carbon is also the fourth most common atom in the universe.
Silicon is often mentioned as an alternative to carbon. But silicon has many more limitations. It’s not nearly as eager as carbon to form stable covalent bonds for instance. And in the presence of oxygen, silicon immediately reacts to create silica. In other words, in the presence of oxygen, silicon just turns to stone. That’s not a very useful quality in a building block.
Especially because one of the easiest ways to generate energy requires oxygen. Burning fuel for energy by reacting with oxygen is a very easy way to generate a lot of energy. And once again, oxygen is a very common element. If you want an alternative way to generate energy that doesn’t require oxygen, you’re looking at methods that require rarer elements while generating energy more slowly and in more limited amounts.
And then there’s water. You need a reaction medium for chemical reactions to occur in. Hydrogen is the most common element in the universe, oxygen is the third most common element. H2O is pretty common.
So if you’re looking for alien life, especially complex alien life, it’s not a bad start to go looking for carbon, water and oxygen. Those are very common elements that form the foundation of extremely useful processes for the formation of complex life. You could look for alternatives but you’d essentially be looking for rarer elements that overcomplicate the formation of life while simultaneously limiting its potential for complexity.
We only have a very limited capacity for searching the universe for life. So we might as well search for the conditions that provide the most likely opportunities for life to arrise.
Because it is necessary for life *as we know it*.
We know how to find life *as we know it* because we know that if we detect liquid water and molecular oxygen in a planet’s atmosphere, it *has to* mean photosynthesis, for example. We don’t know any similar markers for life based on other chemistries.
There might be, for example, methane and fluoride based life out there, but we world be able to detect it because we don’t know what to look for. If you don’t know what to look for, there’s no point in looking for it.
Water also has some rare properties that make it uniquely suitable for life. For example, solid water (ice) is lighter than liquid water, meaning oceans don’t freeze from the bottom up, which would be bad for life.
Also, Oxygen is not required as it is a “toxic” byproduct of sunlight-based cellular respiration. However, free oxygen is quite reactive and readily absorbed by rocks and in the water. What does that mean? It means finding free oxygen in an atmosphere is a very good sign that something is *creating* more oxygen on a constant basis and that has a fairly good chance of being some sort of plant life.
Secondly, Oxygen cellular reactions are much higher energy than CO2 based. If the planet evolved in a similar manner to ours we might find higher order creatures like animals.
There’s another point, in that we are made of carbon and water *because* they are the most successful compounds. We may find life on other planets based on silicon and methane, but that planet is subject to same laws of physics as us, so there must be a good reason why silicon and methane are used there instead of carbon and water.
Scientist here. As many have already said: we aren’t certain in the least. One of them isn’t even a requirement here on Earth.
There are microbes that cannot exist in oxygen. It’ll kill them. There are microbes that exist in harsh environments. Look up Lake Bonney in Antarctica. That is one weird biome.
And, there is life that subsists on chemicals vented from the Earth instead of sunlight. Areas around hydrothermal vents, for example.
All life on Earth needs water, though. Water has some incredible properties that few other materials have. That’s why we tend to think of life needing water to exist.
We don’t, but we are confident that they will be controlled by the same laws of physics that we are
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