A case for life from silicon

[Torco]

We're all carbon based here, right? we're, for the most part, water, sugars, aminoacids and lipids put together all fancy like. This is because carbon, unlike many other molecules, is great at polymerizing: that's what we call it when atoms get into these big, long chains that can all sorts of atoms on the sides of them, and that diversity affords for the different kinds of things sugars, aminoacids and lipids can do, like write this post. Right. so apparently the consensus is, while silicon *can* inprinciple make similarly complex chemistry, it's really not as good: the silicon-oxygen bond is too strong, i'm told (i don't know how to calculate such things, so I'll have to believe them)... but then again, free oxygen is only very common on earth because our plants breathe it out: I don't think any exoplanets have rhododendrums, and it's safe to say that there could be planets (indeed, certainly most planets we know are like this) that don't have a lot of free oxygen flying around to capture silicon into rock. sure, oxygen is common, but stellar abundances vary: carbon is about as common as oxygen on our system, bit less, but in others it's much more common than it.

Like, okay, full disclosure here, I'm of the opinion, without great evidence to that effect, that if we find lifeforms not of our genesis, so to speak, they're going to be weird as fuck. not just weirdly shaped fish or weirdly shaped insect-like things the way we portray them: but like weird. I don't think we'll find many people with rubber foreheads, or that many things you could say "man, kinda looks like a cow".

known exoplanets are probably weird because of some biases we have in our techniques, like planets that are big and near their star are easier to find and so we find more of them, granted, but they nevertheless *are there*. the hot neptunes, the super-earths, the really really hot jupiters... the condition at the surface of such a planet as, say, a hot neptune (which is a common enough type of exoplanet, apparently), or what you could find in a planet made of about 50/50 rock and gas (could be 30/70 too) (which there's no reason, prima facie, why there couldn't be) ... these are very different than those on the surface of our planet. you would have hundreds, possibly a thousand kelvin. at a thousand atmospheres or more of pressure, with god knows what's down there. do we know what kind of weird polymers silicon can -or can't- make under those conditions? cause I get the feelings the most studied area of chemistry is the chemistry of silicon at about 1atm, at about 0 to 100 celsius. I'm pretty sure the kinds of chemical reactions that can happen with silica at, i don't know, venus like conditions, or the bottom of neptune like conditions, if it has one, are probably understudied.

like, my case is not there's definitely silicon-based life that's possible, I don't know... but it seems... well, something to look out for. or not that unrealistic conworlding, if one were to launch oneself into such a cool endeavour.

[Xhin]

Well, Silicon dioxide's boiling point is only 2,950 C. That's definitely doable on exoplanets close to the sun or within molten cores -- neptune/earth/uranus cores are all around 5,000 C. You do however run into the problem that both potassium and magnesium are also boiling at those temperatures, and they are essential to photosynthesis -- though presumably in such a hot environment they aren't the basis of chemical energy and structural synthesis. However, whatever way you're generating chemical energy, you do run into the problem of most common metals being boiling at that temperature -- here's a handy periodic table of boiling points:



My thinking with alien life is probably that it's everywhere and we generally have no idea what we're looking at. We don't exactly send probes down to Jupiter to see if the ammonia clouds are self-replicating. From an alien perspective, earth probably looks like a caustic planet whose chemical reactions pull metals out of the ground and produce light. Very rarely, these chemical reactions will build up somewhere and send metal complexes out of the atmosphere but they usually don't get very far and usually return to the planet. These alien astronomers refer to the phenomenon as "Terran Flares" but they're no more than a curiosity -- such a planet could never support life, ammonia is too uncommon and can't crystallize properly in that environment.

[bradrn]

I'm pretty sure the kinds of chemical reactions that can happen with silica at, i don't know, venus like conditions, or the bottom of neptune like conditions, if it has one, are probably understudied.

On the other hand, geochemists spend quite a bit of time studying the reactions of silicon at several thousand atmospheres and hundreds of degrees Celsius, and it shows that silicon makes some very interesting polymers indeed: you might know them as quartz, feldspar, mica, garnet, zircon, olivine, pyroxene, beryl, etc. And even at the surface of the Earth those can react easily to form clays. Those are all based on repeated Si–O–Si chains, though; unlike carbon, silicon doesn’t bond to itself too well. Life might be based on reactions similar to serpentinisation, which (according to Wikipedia) releases up to 40 kJ of heat per 18 g of water which reacts. It would probably require a very slow metabolism, though, and I can’t quite imagine what a silicate analogue of enzymes would look like.

My thinking with alien life is probably that it's everywhere and we generally have no idea what we're looking at. We don't exactly send probes down to Jupiter to see if the ammonia clouds are self-replicating. From an alien perspective, earth probably looks like a caustic planet whose chemical reactions pull metals out of the ground and produce light. Very rarely, these chemical reactions will build up somewhere and send metal complexes out of the atmosphere but they usually don't get very far and usually return to the planet. These alien astronomers refer to the phenomenon as "Terran Flares" but they're no more than a curiosity -- such a planet could never support life, ammonia is too uncommon and can't crystallize properly in that environment.

I tend to agree with this perspective. (Except for the second sentence: NASA is planning flybys of Europa, potentially sampling its water plumes.)

[Torco]

It's almost certain that under hot neptune or super earth conditions photosynthesis would not operate like it does here, especially cause it's unlikely that light will be able to make its way down there, but life didn't begin with photosynthesis, and so it kind of falls under the "ammonia doesn't cristalize properly there, there can't be life there" umbrella.

it's true that it's hard to imagine enzymes made from silicon... but then again, do we know of naturally occurring enzymes? I don't know but I imagine the enzymes we've seen are all (?) of biotic origin.

[rotting bones]

Are we still detecting exoplanets by distortions when they pass in front of their star, and so on? We can't actually see their surfaces. If we could, we could pinpoint nodes of organization by studying their entropic effects. Personally, I'm a hard reductionist and I don't subscribe to the relativisms fashionable in contemporary science. (Search for "Classical Econophysics" with the quotes on pdfdrive.)

[bradrn]

Are we still detecting exoplanets by distortions when they pass in front of their star, and so on? We can't actually see their surfaces.

Indeed. However, we can take their spectra (mostly infrared, I believe), especially now that we have the JWST.

[rotting bones]

Are we still detecting exoplanets by distortions when they pass in front of their star, and so on? We can't actually see their surfaces.

Indeed. However, we can take their spectra (mostly infrared, I believe), especially now that we have the JWST.

Yes, I think we're concentrating on known metabolic processes to reduce reliance on speculation. Now, if we were able to grow artificial life in the lab by other methods, that would lend credibility to its existence on the relevant categories of exoplanets.

As for earthlike planets, are the best prospects still in the Teegarden system? The most promising candidate there was tidally locked, I think.

[Torco]

I think the most promising system is proxima, simply because a) it has some planets around what we understand to be the habitable zone, though that doesn't make it that special and b) it's really really close.... or, well, relatively less mind-bogglingly far away. we could even image a planet there with webb, i understand. there could be a dyson sphere in andromeda and we wouldn't be able to learn a lot from it. i think there's a few other close-by candidates too.

[rotting bones]

Google suggests the Kepler system as a place with superhabitable worlds. But that's like 2000 light years away.

[Torco]

Yeah! kepler's super fun, if it looks like people think it does. the best thing is that because all those planets are real close to each other, in theory interplanetary travel is a lot easier, so it's a cool place to set like a space opera thing.