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> There is nothing in the manual that requires liquid water, gravity, a planetary surface, carbon, or any of the other ingredients that define life on Earth. O

> There is nothing in the manual that requires liquid water, gravity, a planetary surface, carbon, or any of the other ingredients that define life on Earth. On carbon, this is from [a recent Astrobiology textbook](http://books.google.com/books?id=x83omgI5pGQC&q=%22there%20may%20very%20well%20be%20only%20a%20single%20element%22 http://books.google.com/books?id=x83omgI5pGQC&q=%22there%20m...) which probably does count as a manual : ) "There are, after all, only a finite number of elements in the periodic table, and many of these are very poorly suited to support life for any of a fair list of reasons. Consequently, many of the 90-odd naturally occurring elements can be ruled out. So many, in fact, that in the end there may very well be only a single element--carbon, the basis of all life on earth--that is able to support the complex chemistry presumably required to create any self-replicating chemical system. The easiest way to appreciate the special, perhaps even unique, qualities of carbon is to compare it with silicon, its closest cousin. Many of the properties that suit carbon so well to its central role in Terrestrial life are shared or even exceeded by silicon. For example, silicon, like carbon, is tetravalent--that is each atom forms four bonds, allowing for the formation of a rich array of complex molecular structures. And, while silicon-silicon bond is weaker than a carbon bond, the discrepancy is only about 25%. Consistent with this, both silicon and carbon can form long molecular chains, For example, compounds of silicon and hydrogen, called silanes, with up to 28 consecutive silicon-silicon bonds have been reported in the scientific literature. Likewise, while carbon is the fourth most common element in the Solar System as a while, silicon is many orders of magnitude more common on the surface of Earth. Indeed, silicon is second only to Oxygen in terms of its abundance in the Earth's crust. Nevertheless, silicon simply cannot support the same rich chemistry as its "upstairs" neighbor in the periodic table. The problem lies in both the thermodynamics (equilibrium stabilities) of silicon's interactions with other atoms and the kinetics (rates) of these reactions... So carbon wins over silicon. But what of the 90 or so other naturally occurring elements? They fare even worse than silicon."

aagha 11y ago
> "There are, after all, only a finite number of elements in the periodic table..." That's because we've only discovered or figured out how to make a finite number of them. Is there a reason that other (alien) elements can't exist that we've never been exposed to?
- Analog24 11y ago
  The structure of elements/atoms is well understood based on their subatomic constituents. Naively, you might think that can you just keep combining increasingly larger numbers of electrons, protons, and neutrons to create new elements. However, the stability of an atom becomes problematic when the size of the nucleus approaches the interaction length of the strong force (i.e. the nucleus is too large for the strong force to hold it together). These elements are unstable and therefor not relevant as far as organic chemistry is concerned. Furthermore, the formation of elements in the Universe is also a fairly well understood process. For elements lighter than Fe it generally occurs through nuclear fusion in the center of stars. For elements larger than Fe it generally occurs through the r-process and s-process. With these we can model nucleosynthesis extremely well and it gives us a very good idea of the elemental composition of the Universe. That being said, there could be some crazy unknown element out there but it would contradict almost everything know about atomic physics.
  - nknezek 11y ago
    Good answer, but I think you mean Fe, not Pb.
    - Analog24 11y ago
      Good catch! It has been corrected in my comment.
  - danbruc 11y ago
    To add some visualization, you can have a look at the isotope chart [1] from Wikipedia showing the half-live times of the known isotopes to get the big picture. The distinct area towards the top is called island of stability [2] and contains long-lived but nonetheless unstable elements. A second island of stability is suspected even further up in yet uncharted territory but nobody expects additional stable elements beyond lead. [1] https://upload.wikimedia.org/wikipedia/commons/8/80/Isotopes_and_half-life.svg https://upload.wikimedia.org/wikipedia/commons/8/80/Isotopes... [2] https://en.wikipedia.org/wiki/Island_of_stability https://en.wikipedia.org/wiki/Island_of_stability
kamaal 11y ago
There are many assumptions and a story built on assumptions in the quote you mentioned. Like, only elements we know about exist, and no matter what conditions might be carbon is an absolute must for life.
- roywiggins 11y ago
  The quote doesn't assume you need carbon, the quote says that carbon is much better at the sort of chemical processes we associate with life than anything else. Nobody thinks all life absolutely must be made out of carbon. But if you're looking for life, it seems very likely that most life is made out of carbon; possibly nearly all of it. You might as well start looking there. We have limited resources, after all.
- yoha 11y ago
  > only elements we know about exist The interesting thing with elements is that they describe almost all conventional matter in a very predictable pattern and we can pretty much enumerate them. Some scientists are even trying to make the next elements (Ununennium for instance), even though there are incredibly unlikely to occur in nature. Granted, elements only describe a fraction of all matter, but we do not really expect anything complex to last long as plasma (stars), and we do not know much of dark matter anyway. > no matter what conditions might be carbon is an absolute must for life GP was answering this exact point. In short, it's just that the other elements do not look as promising as carbon for building complex molecules (whether we know them or not).
- Tossrock 11y ago
  That's not an assumption, it's a fact. There are a finite number of arrangements of protons and neutrons which can yield atoms. Each sequentially increasing count of protons is a new element. Adding neutrons to a given count of protons makes an isotope. We know which of these can exist in nature, and which can't due to their short half-lives / instability.
Grishnakh 11y ago
>So carbon wins over silicon. Hortas disagree with you.
vorg 11y ago
> in the end there may very well be only a single element--carbon, the basis of all life on earth--that is able to support the complex chemistry presumably required to create any self-replicating chemical system This assumes the fine structure constant has the same value throughout the entire Universe for all time. Tentative results from recent observations suggest it could increase in one direction and decrease in the other along one of the spatial dimensions of the Universe.
- JoeCoder_ 11y ago
  The fine structure constant may not have the same value throughout the universe, but I'm still curious if its a measurement error. Changing it by much would prevent the synthesis of carbon in stars. But are you suggesting a different value would allow some other atom to take on properties as useful as carbon? If so I would like to read more about it. Regardless, if there is a gradient, any meaningful change in the fine structure constant beyond the range at which we could observe anything in the universe, so it still makes sense to only assume carbon-based life.

aagha 11y ago

> "There are, after all, only a finite number of elements in the periodic table..." That's because we've only discovered or figured out how to make a finite number of them. Is there a reason that other (alien) elements can't exist that we've never been exposed to?

Analog24 11y ago
The structure of elements/atoms is well understood based on their subatomic constituents. Naively, you might think that can you just keep combining increasingly larger numbers of electrons, protons, and neutrons to create new elements. However, the stability of an atom becomes problematic when the size of the nucleus approaches the interaction length of the strong force (i.e. the nucleus is too large for the strong force to hold it together). These elements are unstable and therefor not relevant as far as organic chemistry is concerned. Furthermore, the formation of elements in the Universe is also a fairly well understood process. For elements lighter than Fe it generally occurs through nuclear fusion in the center of stars. For elements larger than Fe it generally occurs through the r-process and s-process. With these we can model nucleosynthesis extremely well and it gives us a very good idea of the elemental composition of the Universe. That being said, there could be some crazy unknown element out there but it would contradict almost everything know about atomic physics.
- nknezek 11y ago
  Good answer, but I think you mean Fe, not Pb.
  - Analog24 11y ago
    Good catch! It has been corrected in my comment.
- danbruc 11y ago
  To add some visualization, you can have a look at the isotope chart [1] from Wikipedia showing the half-live times of the known isotopes to get the big picture. The distinct area towards the top is called island of stability [2] and contains long-lived but nonetheless unstable elements. A second island of stability is suspected even further up in yet uncharted territory but nobody expects additional stable elements beyond lead. [1] https://upload.wikimedia.org/wikipedia/commons/8/80/Isotopes_and_half-life.svg https://upload.wikimedia.org/wikipedia/commons/8/80/Isotopes... [2] https://en.wikipedia.org/wiki/Island_of_stability https://en.wikipedia.org/wiki/Island_of_stability

kamaal 11y ago

There are many assumptions and a story built on assumptions in the quote you mentioned. Like, only elements we know about exist, and no matter what conditions might be carbon is an absolute must for life.

roywiggins 11y ago
The quote doesn't assume you need carbon, the quote says that carbon is much better at the sort of chemical processes we associate with life than anything else. Nobody thinks all life absolutely must be made out of carbon. But if you're looking for life, it seems very likely that most life is made out of carbon; possibly nearly all of it. You might as well start looking there. We have limited resources, after all.
yoha 11y ago
> only elements we know about exist The interesting thing with elements is that they describe almost all conventional matter in a very predictable pattern and we can pretty much enumerate them. Some scientists are even trying to make the next elements (Ununennium for instance), even though there are incredibly unlikely to occur in nature. Granted, elements only describe a fraction of all matter, but we do not really expect anything complex to last long as plasma (stars), and we do not know much of dark matter anyway. > no matter what conditions might be carbon is an absolute must for life GP was answering this exact point. In short, it's just that the other elements do not look as promising as carbon for building complex molecules (whether we know them or not).
Tossrock 11y ago
That's not an assumption, it's a fact. There are a finite number of arrangements of protons and neutrons which can yield atoms. Each sequentially increasing count of protons is a new element. Adding neutrons to a given count of protons makes an isotope. We know which of these can exist in nature, and which can't due to their short half-lives / instability.

Grishnakh 11y ago

>So carbon wins over silicon. Hortas disagree with you.

vorg 11y ago

> in the end there may very well be only a single element--carbon, the basis of all life on earth--that is able to support the complex chemistry presumably required to create any self-replicating chemical system This assumes the fine structure constant has the same value throughout the entire Universe for all time. Tentative results from recent observations suggest it could increase in one direction and decrease in the other along one of the spatial dimensions of the Universe.

JoeCoder_ 11y ago
The fine structure constant may not have the same value throughout the universe, but I'm still curious if its a measurement error. Changing it by much would prevent the synthesis of carbon in stars. But are you suggesting a different value would allow some other atom to take on properties as useful as carbon? If so I would like to read more about it. Regardless, if there is a gradient, any meaningful change in the fine structure constant beyond the range at which we could observe anything in the universe, so it still makes sense to only assume carbon-based life.