One of the critical things in astrobiology is predicting how an alien organism’s biochemistry might work. Indeed there are some proposed biochemistries utterly different from what we find here on earth including hypothetical silicon-based life, various alternatives to water as a solvent for biochemistry, and even last biochemistries that might be possible at shallow temperatures, such as on the surface of Titan. But, in the end, there are some probable constraints to those chemistries that may prevent intelligent forms of that kind of life from developing. For example, intelligent alien life would probably be dependant on oxygen and its chemistry to run a large brain because there aren’t a lot of suitable chemical alternatives that can do it. Simple anaerobic life can handle the absence of oxygen just fine. Indeed the first life on earth had no use for it, and it was poisonous. But that same reactivity that caused that extinction can also be used to run trillions of neurons in an intelligent being. For complicated carbon-based life, over 25 elements are used chemically, but there are six major ones that complicated life on earth can’t survive without. Oxygen and carbon being two, but also hydrogen, nitrogen, sulfur, and phosphorus.
For Earth-like intelligent life to occur, the area in the galaxy where it happens is going to need to be rich in these elements. For five of them, availability isn’t going to be too much of a problem, primarily hydrogen, it’s everywhere, locked up here mostly as water. But for one of those elements, phosphorus, scarcity may be a severe enough issue that entire sections of galaxies may not have enough of it for complicated life to arise. This all comes from preliminary work done by astronomers at Cardiff University led by Jane Greaves and Phil Cigan. They were looking at two supernova remnants to see how much phosphorus was created in the explosions. Models had predicted a certain amount, but the observations didn’t match, opening up the possibility that the amount of phosphorus built and blown out in a supernova varies and some exoplanets may end up with very little in the way of reactive phosphorus compounds being delivered by meteorite impacts.
In short, as at least a partial solution to the Fermi Paradox, it may be that complicated life in the universe is rare only because the distribution of elements such as phosphorus may not be uniform, leaving areas destitute in that element without complicated life. However, some areas would be rich in those elements, we arose in one of those, but it’s anyone’s guess right now how common areas like that are.
Add that with the other hurdles that seemingly face life arising on exoplanets and that it seems increasingly clear that earth’s conditions that led to our development had to be just so and subject to multiple factors and a little luck to get this far, and it seems that intelligent life may be rare. But the sheer astronomical numbers of galaxies, stars, and planets in this universe still are firmly in favor of intelligent life not being unique; it’s still very likely out there.
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