I do not understand the consistent assumption that life on other planets must take a chemical and physical form similar to our own. Given the myriad of unique chemical compositions and the seemingly infinite ways in which information may be stored and propagated as molecular data, I feel it is myopic to constrain our search for life to such a narrow chemistry.
Sure, I understand that given limited resources it is sensible to look for what we know works, but I guess what I'm getting at is that articles which infer potential for life on other planets based on earth-bound extremophiles are probably not as substantial as they may seem.
In addition to the other fine points: A fundamental job of a life form is to pump entropy out faster than the environment can pump it in. This tends to eliminate any hot environments, such as stars, the insides of active planets, and one of my personal favorite science fiction life forms, neutron stars and other such exotica.
A life form must also be able to develop to some reasonable level of complexity in some reasonable period of time in order to be alive. This tends to eliminate old stand-by favorites such as nebula, very cold planets like Pluto, and the interstellar medium. It doesn't matter if they got a 12 billion year head start on us if they "live" a billion times slower than we do. (And if you start doing the math on how cold things are and how often particles interact in even a nebula, let alone an interstellar medium, it doesn't take long to see that a billion times slower is actually me being quite generous.) It is likely that at the time scales an interstellar life form would live at, the interstellar medium is too frequently disrupted by novae and such for anything to live in it.
So you pretty much end up limited down to roughly the temperature range we're familiar with pretty quickly just based on thermodynamic considerations. Given that you also have to add in the fact that chemistry must be able to support the life form, you end up with the result that life really isn't going to be that hard to identify, even if it's based on ammonia or something. It'll still be pretty obvious that something life-based is taking place.
One of the other things about this conversation is I think "a bit too much science fiction" can lead people to think that Earth life is somehow pedestrian or something, but in reality, Earth life is incredibly diverse and has covered almost every imaginable niche on this rather diverse little planet, and that makes the odds of meeting something that has literally no Earth analog pretty long. We could meet a planet of ammonia-based silicon life forms and it could very well be the case that we'd still recognize every player in the ecosystem just fine on sight, because we've got a lot of terrestrial data points as to what might constitute a viable life form. "Life forms that could exist in this universe that looks nothing like Earth life" may very well be a nearly empty set. (Or at least life that could naturally evolve. One could hypothesize a machine civilization in a lot of places, but it's hard to see how such a thing could evolve directly and biologically.)
>In addition to the other fine points: A fundamental job of a life form is to pump entropy out faster than the environment can pump it in.
Yes. From a chemistry perspective, this requires a membrane, or barrier, that separates "life" from "non-life". It also requires mass transport across that barrier in both directions, which would presume liquid inside that barrier. A third variable is the mass of the planet, which dictates which molecules can be present in the atmosphere, and the temperature, which dictates physical states.
The requirement for a barrier and mass transport via liquid already imposes restraints on the chemistry that rule out most of the periodic table. Metals lose their electrons too easily. After metals, you're not left with much to work with. Carbon compounds in water is still the best idea we have. Simple long chain carboxylic acids spontaneously make membrane-like micelle structures in water. If anyone has any better ideas, there's a Nobel Prize for the first person to create life from non-life. Go for it.
A lot of the chemical reactions required to maintain life as we know it involve charged intermediates. These are not generally soluble in the gas phase as they have very low vapor pressures.
The reason we know that life on Mars (if it exists) must be similar to life on Earth is because both have been peppered with rocks from the other knocked off by meteorite impacts (from memory something like 100kg from Mars to Earth per year and 1 kg the other way around).
Some of these rocks have been ejected under benign enough conditions (and protected during transit) to enable microbes to make the journey alive. I think if we do find life on Mars we might be rather disappointed to find that it is exactly like life on Earth.
There is an interesting hypothesis that life arose on Mars and was later transferred to Earth. One of the reasons for thinking this is that life seems to have appeared almost instantly on Earth as soon as it cooled down enough for there to be liquid water. Mars being further out from the sun (and smaller) likely had liquid water well before the Earth.
And considering we still have no signs of mega-engineering structures, interstellar probes or otherwise interstellar broadcasters, it would increase the likelihood for a great filter to be ahead of us. Constraining the last terms of the equation.
Aside from the point others have made that carbon based chemistry is a likely candidate for life, there is another factor I think:
We already know that carbon based can exist and we know what it looks like. Since our means for searching for life in the rest of the universe are still very limited, it is best to focus on what we know, instead of searching for a different form of life of which we
* don't know what it looks like
* don't know whether it can exist in the first place
I think the field is potentially wide even within carbon chemistry. Genetics (which seems to have a single origin) could feasibly exist as a completely different yet still carbin-centric "implementation."
If you assume that physics works the same on other planets then carbon-based life is simply the most likely simply from the chemistry we know about.
Speculation about say Si based life isn't new but nobody has come up with how that type of organic chemistry might plausibly work.
There's an entire Wikipedia article on alternative biochemistries [1].
Quoting that page, with regards to Si: However, silicon has several drawbacks as an alternative to carbon. Silicon, unlike carbon, lacks the ability to form chemical bonds with diverse types of atoms as is necessary for the chemical versatility required for metabolism. Elements creating organic functional groups with carbon include hydrogen, oxygen, nitrogen, phosphorus, sulfur, and metals such as iron, magnesium, and zinc. Silicon, on the other hand, interacts with very few other types of atoms [...]
I’d really hope that physics work the same on other planets. If that were not the case, we’d have a fundamental problem with our understanding of the universe.
That would actually be really exciting. In trying to resolve where we had gone wrong, we'd learn even more about how the universe works. Our current theories still have holes (no pun intended)
but then, do chemists really know how carbon based life works really? if it weren't for the living examples, wouldn't it be just as hard to work out how proteins form and so on?
It isn’t about knowing how carbon life works but understanding basic chemistry which we do.
Carbon is the atomic floozy it will hook up with a lot of other elements and can form relatively strong bonds, and it can have upto 4 bonds per carbon atom nominally.
This makes carbon the star of its little own chemistry show.
Silicon is similar to carbon in this regard which is why it is in the same column in the periodic table however it bonds with fewer elements and it’s chem requires more energy and is often less stable.
This is what the grandparent question always skips over.
"Why not alternatives to organic life?"
Underestimates the fact that, as near as we can tell, building life from organic molecules is easier (and substantially so).
Given that life seems to be a relatively rare event sequence, and that organic life is the easiest form we've been able to figure out from chemistry (ne physics)... betting on another kind is a long shot.
Granted, maybe we've missed something obvious. Maybe our understanding is critically flawed. But from everything we know right now, one can't just handwave and say "Well, many kinds of life are possible."
Indeed, currently there is no replacement for organic chemistry for life because it seems that we can't find another combination that would be as flexible and stable as carbon.
Silicon can replace carbon in a few scenarios but it isn't likely enough for any plausible mechanism that would support life to be based on silicon alone.
This is why likely all life is carbon based, some of the other elements like phosphors vs arsenic have more wiggle room but there is simply no "glue" other than carbon and this is simply due to physics not lack of imagination or knowledge.
Yes not possible not enough complexity or stability for that matter and the energy levels required are well explosive the smaller you go the more energy is needed since you essentially go form gravity (macro) to electroweak (molecular and atomic) to the strong nuclear force (subatomic) and there is a pretty big increase in magnitude of the forces that govern interactions between each phase.
Heck even within the unified electroweak force there is a huge range e.g. burning coal releases he energy in the stored covalent carbon bonds which are tied by electromagnetism part of the electroweak force but if you start breaking protons a part from neutrons which is the weak force you get a nuclear bomb.
Life needs bonds that are cheap to create, cheap to break and ones that don’t explode once you do break them.
To bolster your other response: in college chemistry classes are split into two kinds: “organic chemistry”, and “everything else”. But he term “organic” in this context means “compounds with carbon atoms”. There are so many ways that carbon bonds to other atoms, and in such complex configurations that there are classes reserved for this single element. And as undergrads will tell you, these are the hard classes!
Organic chemistry isn't required. I can easily come up with one mechanism: What if AI is a common development for advanced organic life forms, after which those organic life forms die out, but silicon or otherwise non organic life forms continue on and move out to other planets?
I don't know about the physical form - the physical form of life on earth is pretty diverse IMO.
But as far as the chemical form goes, my limited layman's understanding of why life is most likely to be carbon-based, is as follows.
The universe is mostly made up of hydrogen, helium, oxygen, and carbon/nitrogen. Other elements also feature, but those four are typically the most abundant. If there is any life at all in the universe, the likelihood is it will comprise of these common elements. Humans, for example, are mostly (96%!) hydrogen, oxygen, carbon and nitrogen (see https://en.wikipedia.org/wiki/Composition_of_the_human_body).
But what about helium! Helium is uninteresting because it is not very reactive, so the ways it can interact and "be stored and propagated as molecular data", as you very nicely put it, is quite limited.
Which brings us to carbon, which is quite reactive. It reacts with itself and gives you diamonds, graphite, buckyballs, graphene, nanotubes. It reacts with the most abundant chemical in the universe and gives you hydrocarbons. Sprinkle some nitrogen and oxygen in the mix - also very common elements - and you got yourself proteins and fats and carbohydrates and amino acids and whatnot.
So just based on that, it's more likely than not that life would be made up of hydrogen, oxygen, carbon and nitrogen, with carbon in the central role. That is not to say that there couldn't be silicon-based life, for instance, but it's just not as common an element as carbon.
I think the problem is knowing how to define "life" and then working back to other possible implementations. As Douglas Adams framed it, "does it go squish if you step on it?"
As far as I know, the current thinking is still that life emerged (or arrived) only once on earth. One version of genetics, one implementation of life's basics, the eath-life basics. This makes it hard to generalize. It's also hard to speculate on the likelihood of such a seed event, on the basis of a single occurance. We have very little information, in Drake's equation terms, on the likelihood of life emerging given earth-like circumstances. Earth is obviously life-supporting. Why did it only happen once here, and so early in the game... As soon as the planet cooled.
For example, we could define life in terms of evolution, any evolutionary system with potential for a lot of complexity in the long term. ...we only have one example of this, earth life.
I think it's natural to start here, with earth life because that's all we know about and our speculations are very... speculative. At least we know what we're looking for this way. Earth-life can survive X conditions, therefore life can. What conditions can other types survive, if they do or even can exist? Who knows.
A lot of mystery still. Even the emergence or pan-spermia question is wide open. Even within this information bubble of earth life.. we don't know how extreme life can get. We just know about stuff that evolved to survive the extreme conditions found on earth. It's plausible (even probable) that earth-life has the potential to go even further, if earth provided more extreme niches to adapt to.
But... the universe is filled with water, so that's a likely medium for chemical reactions. And if you want long molecules to actually do interesting things, you need carbon in there somewhere. So the most likely chemistry is at least somewhat similar.
And for Mars, which is made out of similar stuff as Earth, and periodically trades rocks with us? If there's life on Mars, it probably shares ancestry with you and me.
As you mentioned, for the life as we understand it you need complex yet flexible enough molecules. And looks like carbon because of its chemical properties fits the bill for the best basis for them.
There are some fundamental chemistry reasons why, no other element is like carbon in it's ability to bind and react in so many ways, so carbon based is the most likely thing. Maybe not the only.
> I do not understand the consistent assumption that life on other planets must take a chemical and physical form similar to our own.
I always pondered about this as well. I came to the conclusion that, given the sheer number of celestial bodies (e.g. planets) in the universe, restricting our gaze to only the ones that can sustain life as we know it (i.e. carbon-based bags-of-mostly-water) restricts the search space considerably.
I don't know whether that actually increases our likelihood of finding extraterrestrial life; I certainly do hope so.
There are two spaces to consider. One is the range of habitats available, the other is the range of chemistry complex and rich enough to form viable metabolic pathways.
The problem is the OPs comment about restricting ourselves to a narrow range of chemistries is a bit naive. Carbon chemistries are so rich and so complex it's a whole field of study alongside all other forms of chemistry put together. That's not just because it happens to be interesting because we are made of it, it's just a fact of chemistry and physics that carbon chemistry is fantastically diverse and scalable compared to all other known and theoretical chemistries.
A lot of somewhat simplistic conjecture starts from the idea that the universe is vast and unknown and could contain 'anything'. But the fact is we can see the universe through our telescopes and other instruments. We know what all the elements are and what their properties are so it seems rational to use that information to optimise our search.
After all, if there are unknown elements and unknown chemistries or structural paradigms out there that might also be viable, since we don't know what they are, we also don't know what to look for to find them, so that avenue of speculation doesn't actually take us anywhere useful.
Sure, I understand that given limited resources it is sensible to look for what we know works, but I guess what I'm getting at is that articles which infer potential for life on other planets based on earth-bound extremophiles are probably not as substantial as they may seem.
I suppose time will tell.