[JoeCoder_]

> 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%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."

[vorg]

> 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_]

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.

[kamaal]

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.

[Tossrock]

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.

[roywiggins]

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]

> 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).

[Grishnakh]

>So carbon wins over silicon.

Hortas disagree with you.

[aagha]

> "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?

[nknezek]

Yes, there is a reason. We've thoroughly explored the periodic table through our studies of nuclear processes (lots of government funding for anything nuclear weapons related). It turns out you can create a ton of elements we don't see in nature, but almost none of them are stable: you smash atoms together to make a new one, but the new one flys apart after a <second (and that's actually a long lifetime for these atoms, many have lifetimes of nanoseconds). There is a potential "island of stability" around element 120-130, but even there lifetimes are predicted to be less than a minute.

In addition to the stability argument, there are also energy requirements. It turns out fusion (stars) only gives energy up to iron, then it requires energy to make bigger atoms. In other words, you get energy back out when you split big atoms (I.e. nuclear reactors). In nature, all elements past iron are created only in the spectacular energies of supernovas, which occur in less than a second.

Basically, we have a really good grasp on the elements that can exist. We're only missing the details on a few things that occur on nanosecond and less timescales.

[phkahler]

>> In addition to the stability argument, there are also energy requirements. It turns out fusion (stars) only gives energy up to iron, then it requires energy to make bigger atoms.

That's actually a common misunderstanding. If you fuse hydrogen (or even lithium) with iron, you can get a higher numbered element and some excess energy. See the chart on this page:

https://en.wikipedia.org/wiki/Nuclear_binding_energy

While iron is at the top of the curve, that just means you won't see iron-iron fusion. There really is no reason the lighter (common) elements can't fuse with the heavier elements.

This is also the idea the LENR (low energy nuclear reaction, formerly known as cold-fusion) guys are considering. If you fuse Hydrogen with Nickel62 to produce Coppper63 you could get some energy out. Notice that Nickel is already heavier than Iron. Some claim to have seen this copper production in hydrogen-nickel cells. The claims are not really relevant - the math supports it as a possibility. Weather it can happen on earth or in a star is open for debate. One key question is how the excess energy would get out as heat, and there are ideas about that.

I for one find it amusing that people don't think something like this is where all the naturally existing heavy elements came from.

[scentoni]

Wrong, it really does mean that it takes energy input to fuse iron and hydrogen. Naturally existing heavy elements are the result of supernovas, where a percentage of the energy of the dying star is converted to fusing elements beyond iron.

[smaddox]

Indeed. Oblig. hyperphysics: http://hyperphysics.phy-astr.gsu.edu/hbase/nucene/nucbin.htm...

[Analog24]

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.

[danbruc]

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...

[2] https://en.wikipedia.org/wiki/Island_of_stability

[nknezek]

Good answer, but I think you mean Fe, not Pb.

[Analog24]

Good catch! It has been corrected in my comment.

[gherkin0]

> Is there a reason that other (alien) elements can't exist that we've never been exposed to?

Yes, because you form new chemical elements by adding protons (and stabilizing neutrons) to the nucleus, and humans have found or synthesized the first 118 of these. The ones that don't occur in naturally on earth are short-lived an unstable.

[roywiggins]

If there was a lot of exotic elements out there, we'd have seen it with our telescopes. It would point to new physics, for a start, since we don't think very heavy elements are likely to form naturally at all, let alone be stable long enough to be observed in any quantity.

Even if such elements are out there in the universe in tiny quantities, we'd run into a billion carbon-based biospheres before we found any.

[saalweachter]

To elaborate on the first point, we can identify elements by the spectrum of light they produce. We discovered helium in the sun before we discovered it on Earth.

(This only works if the element is common enough and hot enough to emit light that is seen from Earth.)

[chadgeidel]

Physics is the reason that other elements likely don't exist. One can only arrange protons, neutrons, and electrons in a fixed number of "stable" ways.

Of course, this is today's understanding. We may be wrong. :-)