Abstract
Many people have theorized about what alien life might look like, or where we might find it. Where should we look for it? What do we search for?
In this post, I argue that most alien life life should be carbon-based. I argue that its constituent elements are most likely to be hydrogen, carbon, oxygen, iron and nitrogen. And I propose that the best place to look for it is in the vicinity of liquid water.
At first sight this looks like nothing more than carbon chauvinism. A carbon-based being assuming that all life must be just like himself. But my argument is based on fundamental principles of physics and the nature of life. In spite of the appearance of carbon chauvinism, and lack of imagination, I believe that my argument is solid.
Preliminaries
In this post I will restrict myself to chemical life, out of necessity. I cannot begin to hypothesize what non-chemical life could be like. Second, I won’t argue that all life must be carbon, but rather, that most life should be carbon. Hence, carbon is our best bet in our search for alien life.
What is life? – Complexity
Ok, so what is life? Well, I don’t actually know, but I hope we can agree that whatever it is, it is complex. Not all that is complex is life, but I posit that all chemical life is very complex.
Consider a virus. A virus lies somewhere between “chemical” and “alive”. Too complex for the former, too simple for the latter. A virus is merely a strand of DNA inside a sack of proteins. A virus doesn’t grow, it doesn’t eat and has no internal structure besides a small DNA strand. The simplest unicellular life is immensely more complex. And yet, a virus is an incredibly complex thing. A single DNA strand contains millions of molecules. Far more complex than anything else that you would consider a chemical.
Complexity equals carbon
How do we make very complex chemicals? It turns out that the list of candidates is not very long. Most elements only join to make molecules of 2, 3 atoms. You will struggle to find a 10-atom molecule without carbon or silicon in it. Only carbon and silicon can really combine to make complex molecules. Of these, carbon stands out as being able to make the longest molecules. From nanotubes to DNA strands, carbon readily makes molecules with millions and millions of molecules.
If you want a complex molecule, carbon will be your first choice, and silicon your second. Nothing else would do.
Element Abundance
What are the most common atoms in the universe? All other things being equal, these are our best candidates for life. The most common elements in the universe are:
| Hydrogen | 73.9% |
| Helium | 24.0% |
| Oxygen | 10.7% |
| Carbon | 4.6% |
| Neon | 1.3% |
| Iron | 1.1% |
| Nitrogen | 1.0% |
| Silicon | 0.6% |
Which of these can be used to make complex molecules? Helium and neon are noble gases and don’t combine with other elements. I guess an alien life form could use them for something (e.g. to float) but the life form cannot be made of helium or neon.
Notice that carbon is 7 times more common that silicon. There may be silicon-based life in the universe, but carbon should be the norm. Not only is carbon chemically better, but it is more common. We have a much better chance of finding carbon-based life than silicon-based life.
So what do we have left? Hydrogen, oxygen, carbon, iron and nitrogen. That looks awfully close to our standard organic compounds. Of these, we know that at least hydrogen, oxygen, carbon and nitrogen readily combine with each other to make molecules. Water (H2O), ammonia (NH3), CO2 and methane (CH4) are among the most common molecules in the universe.
I was surprised to see Iron on the list, but I note that iron combines with oxygen, nitrogen and it plays an important role in our chemistry. So I suppose that some alien life forms might include large quantities of iron.
The role of water
Lastly, there is water. First, why do we need a fluid at all? A fluid is helps transport nutrients and excrete waste products. Even a single cell contains fluid to transport nutrients and waste products within the cell.
Water is not the only possible fluid, but it is a very likely one. Water is not only the most common molecule in the universe (see the table above) but it is especially suitable for carbon-based life: It is possible to make carbon based compounds that dissolve readily in water, and compounds that do not dissolve in water. The former are necessary in order to have nutrients, and the latter are necessary in order to contain the fluid.
The only fluids that are more common than water in the universe are gases hydrogen and helium. These gases are very difficult to contain because their atoms are so small – they escape easily. Helium doesn’t react with anything, so it probably can’t transport nutrients. Hydrogen reacts with most things, and is explosive. It is difficult to see how either H or He could be used for transporting nutrients.
Ammonia is another reasonable candidate in addition to water. But water is a much more common molecule. Another interesting candidate is CO2. It is less common than water, but it is more stable against solar wind (Venus lost all its water because the solar wind removed the hydrogen). It would certainly be interesting to see how a gas could be used to transport nutrients. I can’t imagine a way. I don’t believe that you can dissolve anything in CO2 gas.
I believe that the above considerations mean that water is by far the most likely candidate for a fluid. Hence, most life forms should be where liquid water is.
Concluding thoughts
In this post I’ve argued that most chemical life in the universe should be carbon-based. I’ve said that it is likely to be made of hydrogen, oxygen, nitrogen and/or iron. Finally, I’ve argued that it should use water to transport nutrients and waste products.
Should we be surprised that these building blocks would be almost identical to our own? I have made every effort to avoid a homo-centric mentality. But might this be nothing more than a carbon-based life form being to unimaginative to conceive something different?
I propose a different explanation. Earthlings are typical life forms. We are made of carbon because most life forms in the universe are made of carbon. We use water because everyone in the universe uses water.
I believe that we are made of carbon, hydrogen, oxygen, nitrogen, iron and water for no other reason than because those are the most probable building blocks. And if we want to find in the universe, we should search for places where liquid water is present.
