Is life an extremely rare thing in the universe?

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Yeah, 99% of species are gone. Who can say where they came from.

But it's also true that once life has a foothold, it's more difficult for a new form to move in. For the same reason that species need opportunities to take over a niche. If the niche is occupied it's very difficult to move in. The species you are trying to displace has been doing it longer than you.

lack imagination= sarcasm

To keep things in to perspective, in fact, proteins are surprisingly unstable. This is hidden, because the cell replace them fast enough.

I think, you are restricting your self to "normal" conditions ( 273°K, 1atm).

if you reduce the temperature enough, carbon based molecules become too rigid. At these lower temperature, other molecules, that possibly don't last very long, become dynamic enough for use in a biochemistry.

If you increase the temperature enough, carbon based molecules disintegrate. But other molecules that where rigid, become more dynamic.

If you start playing with the pressure also. The possibilities explode so much, that we can't check them all exhaustively.

Availability needs to also be considered. Titanium is much beater then rocks, for making spears, thus Neanderthals surely did spears out of titanium. Right?

Which will probably lead you to a long search for something far more difficult to find.

Many proteins certain do have inherent instabilities--that's why they are capable of doing the things that they do. But proteins are made up of very long chains of amino acids connected by peptide bonds. The peptide bonds are relatively weak, but the amino acids themselves are extremely stable. That way a cell can sythesize a protein, the protein can do its thing, break down and then make those same amino acids available for the next process.

Now if we start reducing temperatures to a point where hydrocarbons lose their flexibility, there are no other candidates to replace them. Because low temperature necessarily means a lower energy state, then there is less energy in the environment with which chemical processes can fix, store and release energy. If it's too cold for carbon based life, then it is probably too cold for any other form of chemical life. Ditto for lower pressures of gasses--low partial pressures of various gasses means that there is less of the element avaialble to participate in chemical reactions.

On the other hand when temperatures and pressures rise, different potentials create themselves. But in high pressure, high temperature envrionments, where things are sufficiently dense and hot enough to prevent the formation of hydrocarbons, the question becomes what else is abundant enough to take its place, and can these replacements create stable molecules in which to fix and release energy in these environments. Given the energy states that would be required for the creation of complex, energy fixing molecules based on metals, for example, the envrionments would have to be so hot, that they would also be rapidly cooling, which would probably provide insufficient time for complex molecules to be created in sufficient numbers to become components of a systematic energy cycle. (And if the environment is not rapidly cooking, it would probably have to have an inbuilt energy source, such as a star, and the potential for the creation of complex molecules of any type, with any element, are virtually nil.)

And of course availability has to be considered--that was rather my point about such analagous systems not arising in environments in which H,C,N and O are not relatively abundant. But since these elements are vastly more abundant in the universe than any of the candidate analogs, I stilll maintain that life will continue arise most often on the basis of hydrocarbons than on any other basis. I don't dispute that their other possibilities--but they appear to be so remote as to be effectively meaningless.

If you reduce the temperature, everything else reduces its energy with it. I mean, you consider stuff that have same order of magnitude energies.

I was rather thinking higher pressures.

Seriously, have you seen how complicated is the phase chart of just watter? They are like 12 kinds of ice alone, or something like that. Hydrogen becomes metallic after a certain pressure. If you consider chemistry in different P and T, among the 100 natural elements, the possibilities are gigantic, you cant exhaustively check that an alternate biochemistry can't be viable. We our selves, are alive inside a very narrow range of T and P. Its not like C biochemistry is different at that then all other possibilites. Simply checking how a protein folds is a very hard computational problem, checking completely exotic alternate biochemistries is even harder. Its not reasonable to proclaim that alternate biochemistries don't exist. The possibilities are huge, our ignorance is very big, in practice its very hard to check.

Venus is rather hot. A gas giant close of its star can be hot, with out being too hot. Inside of a planet, interesting things could be happening. In the galaxy, abundance of elements vary dependent where you are on the galactic radius.

C biochemistry is viable at a very narrow set of conditions. The viability of C biochemistry is mostly an illusion, earth is a rather special place. If you checked, there isn't that much liquid water in the universe. Its not an easy thing to colonize anything in our solar system. Most of the universe is deadly. Other potential biochemistries have similar problems.

I'm talking about the reality of programs in the near term.

You can't go to an engineer and ask him to design a detector for something when you don't even know what you are looking for. If we have additional theories that require testing in the the future, and something concrete can be built, then we could look for signs of something unfamiliar. But that's not the next step.

That is a bit of a dead end.

Carbon on earth is found in the air and in the oceans, silicon is locked up in the rocks and hard to get at trees here don't grow out of the ground, they grow out of the air and just happen to be anchored to the ground.

How exactly would an organism function if it needed to gobble up 3 kilos of rock per minute to get at the silicon you need to respire?

By contrast with a liquid or gas you can just form a membrane and float around merrily, the easy availability makes a big difference.

Well respected scientists have until recently doubted that exoplanets or the Oort cloud existed and firmly claimed that Pluto was indeed a planet as well as the outmost object in our solar system. As of 2012, 778 exoplanets have been identified. Whatever was needed to create life on earth, originated from the space; thus, it's reasonable to assume that the earth isn't unique.

that was just a definition change. A lot of people resented it.

But the thing that does not change at different temperatures or pressures is the energy and the length of various chemical bonds. So when evironments change in orders of magnitude the underlying chemical bonds do not.

One of the challenges of higher energy states and higher pressures is that molecules formed from heavier elements are necessarily larger--a Si atom is larger than a C atom, therefore the bonds that it forms are longer, and they require less energy to break. At root, an silane molecule is less stable than a methane molecule because its bonds are longer, have a lower binding energy, and are therefore easier to break. They simply will not retain energy nearly as efficienty as C-H bonds.



You rather prove my point. There is nothing like water for its complexity--and that makes it a unique medium for supporting organic chemistry. If we were to suppose an alternative biochemistry, the very first question to resolve would be, "What is the solvent in which all of this chemistry takes place?" There is nothing else like it that we have observed in the natural universe.

And let me repeat: I have never said that alternative biochemistries don't exist. I have said that they are extremely unlikely to exist.



Venus is nothing close to being hot enough to support alternative biochemistries. In K terms it is, perhaps, twice as hot as the Earth--hardly close to an energy state that would support the sustained creation of heavier, complex energy storing molecules.

As for elemental abundance, again I think you are being somewhat misguided. The most common elements in the universe, in descending order of abundance are: Hydrogen (73.9% by mass), Helium (24%), Oxygen (1%) and Carbon (0.5%). Neon and Iron are the only other two elements that break the 0.1% figure. When we contemplate how elements are produced, this stands to reason. Vast amounts of hydrogen are out there, because they are the fundamental material of stars. Helium is the end product of hydrogen fusion, which is the end state for most stars. Heavier mass stars will involve Carbon, Nitrogen and Oxygen atoms in their fusion cycles (which accounts for their relative abundance when compared with lighter elements like Lithium, Beryllium and Boron). It is only as stellar masses proceed much higher that helium fusion into carbon, neon, oxygen, silicon and, finally, iron are possible. It is only in supernovae that proton bombardment of lighter elements can produce trace amounts of elements heavier than iron.

No matter how you slice it, there's just not enough of the other elements to outweigh the major building blocks of carbon-based biochemistry. All of the theoretical possibility in the world will not change the fundamental nature of matter in the observable universe.


If we look at the mechnisms for creation and distribution of matter, the preponderance of Hydroge

I'm not sure what you say about pressure and bonds its correct. My understanding is, that if you squeez things together, they'll get more compact. As long they are bathed in a thing that is fluid at that pressure, you are potentially good to go. Maybe having this inside a gas giant.

I'll avoid specific examples. If the molecules look too fragile, then reducing the T, in general should make them look beater. They may retain energy less efficiently, but there is also less energy around.



Water is studied to death. I don't think that its really that special( or is bad luck). They didn't bother with other stuff yet, in been so thorough. I don't believe that the fluid is that important. In our biochemistry, all reaction evolved with water around. So we can't simply replace it in a living cell.

My point was, that phase charts can be extremely complicated. Water is simply well studied( i hope). It shows how ugly they can get. I wanted to show, that the possibilities (considering our ignorance) are staggeringly huge. There's ample of room for alternate biochemistries.

Our biochemistry is nothing special regarding sensitivity to P-T. We are viable in a restricted range of P-T.

I don't think we can say they are unlikely to exist. Unless water is really special, and its unlucky that we study it so well, and distorts what to expect for the other molecules.




Venus was just an example. Its bigger problem is the lack of fluid. I'm aware of the CNO cycle. Despite this, the local abundance of elements on earth, are in opposition to those of the universe.

No, when you squeeze things together, the molecules get closer together, but the size of the atom does not change unless it charge changes--if it loses an electron, for example, it will shrink, if it gains one it will expend. But ions are inherently unstable and will generally bond with an atom of the opposite charge, at which points the radii of the atoms involve will revert to their normal state. And since the size of the atom does not change, and the charge of the electron does not change, the binding energy of the bond does not change. These are some pretty fundamental constants.



Reducing T will inhibit their creation--you must have enough energy in the environment for chemical reactions to occur--because that energy cannot be created from nowhere. And no matter what you do to the temperature, C-H is always a more efficient bond than Si-H.



Water most assuredly is special. Water is the only solvent that works for our biochemistry because water is a simple, abundant, polarized molecule. It is composed of two of the three most abundant elements in the universe. Its composition is enormously simply and occurs at low energy states. It is negatively charged on one side and positively charged on the other, and that means that the molecules behave in particular ways under temperature (it's complex phase chart), and its molecules are effective at dissolving some agents but not others.

There is nothing else--nothing--that we have observed in the natural universe that behaves in the way that water does. And given the high abundance of Hydrogen and Oxygen in the Universe, there is nothing else that is ever likely to be created in the natural universe that will behave in anything like a comparable way.



But that local abundance has not influenced our biochemistry. There is plenty of silicon in our biosphere--but rather than creating silanes, it got locked into solid mineral compositions, and almost all of the hydrogen got hitched to carbon, instead.

I don't know how many more ways I can say this and make it clear to you: I don't dispute the theoretical possibility of what you claim. My only dispute is with its likelihood. If you can conceive of an environment in which carbon and hydrogen are prevented from bonding--whether that is due to scarcity or environment, then I am prepared to consider what might arise in its place. But I am willing to be that in any such environment, the conditions would also mitigate against the creation of any alternative biochemistries as well.

If you squeeze them hard enough (i mean really, really hard). They "see" an extra force, you "kick" them. Electrons and nucleus will rearrange. The atoms will be forced to get closer. All chemistry change rules.

Just an examples. Hydrogen becomes metallic if pressed hard enough. And all the different forms of ice in the phase chart? In the liquid outer core of the earth, they think there's a new form of iron.

If you have atoms in a fluid, at these pressures, you could have a working biochemistry.



You said that they were fragile. You need as much energy to brake them or to create them, its like a bank account. If the T is at the right range, they are strong enough for what ever use.

Hell yea, a Si based organism, living at a range of negative T will be more fragile then us. At those T, C will be unbreakable, can't use it.

I don't want to go in too specific examples. The possibilities are so insanely diverse, and our knowledge so poor. We're not going to start a Ph.D in silanes.



Water is abundant on earth, its important for a bunch of industries. They put the money to study it to death.



silanes: P-T conditions could be not good enough. Fluid not adequate enough. etc.......

I think we need to discuss more the P-T issue. You underestimate the level of change that a P-T variation can bring. You underestimate the amount of change of P-T i'm talking about. Inside gas giants for example, at various distances from there star. They suspect, that at one of the planets they discovered, its raining iron, that hot it is there.

You also need to see the practical limitations of our knowledge. Of course we have studied to death chemistry in our P-T range. Thats even more true for biochemistry. Why bother spending money in chemistry at liquid nitrogen temperatures? Or develop solvents working in liquid nitrogen? Or catalysts? Or crystals? Or whatever?

I think, you are doing a methodological mistake here. You should pull your self out of the details, and see the big picture in this.

No, it suggests no such thing. Each planet in our Cosmos is unique. They may be worlds very similar to Earth, but I can assure you they're not exact duplicates of our planet. That being said, I see no reason why life should have evolved independently multiple times on Earth because once life began the first time, it soon colonized pretty much every possible habitat on Earth. And that includes the atmosphere, the bottom of the Oceans, and deep underground(inside volcanoes and within bedrock). There are only 2 elements whose chemistry allows the formation of large, complex, stable molecules which life requires: Carbon and Boron. The latter is far too scarce here on Earth and once Carbon based life began, living organisms quickly consumed ALL available carbon compounds that formed without life and altered the Earths geochemistry so drastically that the conditions that allowed life to start can no longer reoccur.

Exactly!

There is one other element that has a similar size and chemistry as Carbon and is capable of forming long, stable molecules. And that element is Boron. The conditions on Earth were not appropriate for the evolution of Boron based life. Partly because Boron is just so damn scarce here in our solar system, and in the Cosmos in general. The conditions necessary for the formation of Boron based life would be a nascent star with a nitrogen rich proto-solar disk bombarded by intense radiation from a nearby supernova(spallation) resulting in nitrogen being transmuted into Boron.