We’re assuming that alien life requires the same things we do, why?
In: Biology
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.
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