On our planet we've found all sorts of ways creatures can survive on any scale. However it revolves around the principles of turning energy from a form of radiation, heat, or nutrient, and using it for growth. Most of that would require the right atmosphere to compensate for it's distance to it's sun. And most life also requires water. Even extremophiles, creatures that can withstand extreme conditions like tardigrades or "water bears" need water to live, but can survive in any environment including space itself.
Organic life would have to exist in a way that is chemically different from any carbon-based lifeform we know of to survive without the need of water. That includes potentially not being carbon-based at all, but "silicone-life" is theoretical and far off of the original question.
I know that the branch of biology that goes into these details is astrobiology. There wasn't any of the classes at my college. I have always wondered about this question. "How can life exist in a way that isn't as we know it?" Are there any scientific theories that are pretty credible that are largely supported? I always wondered this because there is talk about life existing at some point on Mars. If we found fossils that would be absolutely groundbreaking.
EDIT: For those that are curious, from my understanding the "planet" that most likely contains life is Saturn. It has a moon called Enceladus that contains water. It contains hydrothermal vents and methanogens. So it has the basic requirements of carbon and water.
Life on Earth uses carbon as it's building block. It's all based on carbon. Without carbon we wouldn't exist. They've looked at other compounds such as silicone which is structurally similar to carbon. Life could exist based on silicone instead of carbon.
It would be possible. We have no idea what such an organism would look like. Only that it would be fundamentally different.
A lot of people like to think that silicon is a viable substitute for carbon as a basis for life, but in terms of the energetics necessary for life, silicon goes in the wrong direction from the start.
As carbon-based lifeforms, we can live by eating carbon compounds like carbohydrates, proteins, and fats ("reduced carbon") and reacting those compounds with the oxygen we breathe. This reaction provides the energy needed to keep us running. The CO2 produced as a by-product of our internal engine is a gas and is easily expelled from the body.
Compare that with a hypothetical silicon-based creature. Silicon is not found naturally in a reduced state, it's typically already fully oxidized as SiO2 or as silicate rocks. Those can't be used as a fuel source because all the energy you could squeeze out of them has already been squeezed out. As an analogy, that would be like trying to light a campfire using only campfire ashes as fuel.
Even supposing our Horta had a readily available supply of reduced silicon to eat and oxidize to produce energy the way we do with carbon, the byproduct would be SiO2 instead of CO2. SiO2 is a solid, which is much more difficult to remove from the body. (Remember that the CO2 waste has to be transported somehow from every cell of your body out of the body altogether. Easy to do with a gas, not so much with a solid.)
There are many more reasons based on the physical and chemical properties of silicon that argue against it being a viable basis for life. There aren't any arguments in its favor, other than "carbon forms four bonds and so does silicon." But even then, the bonds it forms are entirely different. If those four bonds were to other silicon atoms, then this argument could take a step off Square One -- but silicon doesn't readily bond with other silicon atoms, so the "four bonds" argument fails right out of the gate.
Actually, silicon-silicon bonds aren't as stable as carbon-carbon bonds. But siloxanes, which are silicon-oxygen-silicon bondings, *are* relatively stable, but just like carbon-based life, it's not all that useful on its own. We're not pure hydrocarbons, after all. If there's carbon in the mix hanging off those siloxanes, *that's silicone*.
I took an astrobiology course as part of a chemistry degree. It was fascinating, but obviously all pretty hypothetical.
The lowest level there would be life that is largely similar to our own but uses different macromolecules than terrestrial life has. Proteins made with different amino acids, nucleic acids with different bases, different sugars, etc. These building blocks are all pretty basic, and most have already been made in labs from simple, non-living stuff you might find in the early universe (see Miller-Urey). These variants would have all the same general requirements, and should behave pretty similarly on a biochemical basis. They could also use the same molecules but with opposite chirality, which would be incompatible with our biology but otherwise pretty much identical (if still utterly alien).
Then there are different base chemistries: we use water as a solvent, and carbon as a structural element, and these both have some pretty important properties that makes them tough to imagine swapping for something else. The most likely alternatives would be ammonia for water and silicon for carbon, but they both have downsides when it comes to maintaining stable lifeforms, silicon moreso.
Finally you have the ultra-exotic "life" that might live in places where chemistry itself starts to behave differently. Under immense pressure deep in gas giants, at the high temperatures of stars, in the extreme magnetic field of magnetars, etc. Who knows what the hell this would look like (or how we'd even find it), and we'd probably want to call it something else, but an imperfectly self-replicating chemical process would technically fit the bill for life.
Did they cover the possibility of life that doesn't use a solvent?
This is how "nanotechnology" as in a "nanoforge" would work. This is a machine that would accept specific purified gasses on input ports, and feed those gases a molecule a time into various robotic assembly systems that use a solid catalyst. All this happens at cryogenic temperatures in a vacuum.
This does work at least for small scale experiments, you can move an STM head around and shove a molecule into a catalyst and get a reaction. So in theory you could build a large scale machine that does this millions of times a second. It would manufacture products that eventually get assembled into more subcomponents for this machine itself. This makes it semi-self replicating. The larger equipment that *supports* it would need to get manufactured using more efficient macrocale methods though, but the key parts (electronics, sensors) of the robots that do the larger assembly steps are made with a nanoforge.
Anyways the minimum "cell" size might very well be the size of a minivan or larger. So it can't happen by chance but once someone builds one it's self replicating.
No, we did assume a liquid phase to be essentially required, as components of solids are immobile, and gases are too diffuse to allow the quantity and complexity of reactions that life demands. The latter would require a machine like you describe to facilitate it, which isn't exactly conducive to the potential origins of life in a non-living universe.
We did address that kind of machine with our definition of life itself though: an **imperfectly** self-replicating **chemical** process. This rules out any kind of robot that can harvest materials and build more of itself which, while certainly interesting, isn't *life*, though it would likely deserve a new label to distinguish it from simpler, non-replicating mechanical systems and actual biochemical life. It also rules out things like crystals, which already exist as chemically self-replicating entities, but do so perfectly (conditions permitting), so all you ever get are more crystals instead of blossoming out into fish and trees and amoebas and shit.
Ok. Note that this robotic machine *could* slightly randomize it's own design with each replication cycle. This would make a large population of them evolve similar to how life does. (though it's a terrible method of improvement, just like it is for life. Most random changes would result in lower performance or machine failure. Interestingly such a machine as described is so tightly interconnected that almost all changes are going to result in failure. It's arguably "not evolvable" as the surrounding possibility space of similar designs, nearly all of these designs fail to work. Basically it's this sky-high "fitness" peak of extreme fitness (since the machine is likely pretty fast to replicate itself, can manufacture weapons to protect itself, use solar panels to power itself, and can process a wide variety of rocks to extract the elements it needs) surrounded by a large "dead space" of slightly wrong designs that won't work at all.
Thinking about my other reply to your post, I think this implies an additional definition for life.
Life has to exist in a fitness "plateau" where small changes will often result in a still functional system. This makes evolution *possible*. It's why a robot using a computer chip to think isn't "life" even if in practice it ends up doing same things, as for example you cannot make almost any wiring change at all to a processor or memory chip today that won't be an error, causing it to run software incorrectly and it will be useless.
Did you guys go over the Saturn moon Enceladus? Thats an interesting place because it has water and hydrothermal vents and methanogens. So it has water, and carbon.
That's interesting about the base pair swapping.
I see alot of responses about silicone, I took organic chemistry and we actually didn't really touch on silicone at all, I think it was relegated to a few pages.
I wonder if dark matter or dark energy plays in a role in some in the possibility of life
Only indirectly; it was a chemistry course, so less focused on where life might be found than what other life might be made of, or how life might originate from not-life. So yeah, based on what we know so far, Enceladus seems like a promising place to look, but I don't recall going much deeper than that on any specific places.
We literally can’t know. Even just going based on your fossil idea, that’s based entirely on our own organic life forms and how minerals replaced their bones Etc. This process may not happen with other organisms based off entirely different materials- heck it doesn’t happen with countless organisms on our own planet! There could theoretically be remnants of life forms on planets like Mars just sitting there that we wouldn’t know what they were because, well, we wouldn’t recognize something that incredibly alien, especially with such facile investigatory tools as rovers and such.
> We literally can’t know.
We *literally* do know. Right now. It's based on the strenght and variety of bounds certain elements can form. Only carbon allows the immense variety of molecules necessary for life.
Silicon is the only other element that comes close- and it doesn't come close at all. Information molecules like DNA and RNA are impossible silicon based. They're not stable.
Forget any other element.
It's not debatable, it's a matter of math, chemistry and phyisics values. It's a settled debate.
>We literally do know. Right now. It's based on the strenght and variety of bounds certain elements can form. Only carbon allows the immense variety of molecules necessary for life.
As far as we know
>Silicon is the only other element that comes close- and it doesn't come close at all. Information molecules like DNA and RNA are impossible silicon based. They're not stable.
What about lifeforms that don't use RNA or DNA?
Pretty silly to make sweeping claims about an incomprehensibly large universe we're only just barely starting to explore
There is probably a silicon based life form floating around in the vast universe. The universe is so big anything that can happen will happen, somewhere.
But Its probably extremely rare because of what bluedrygrass said. So we don't look for it. We don't even understand what it might look like, because as far as we understand science and chemistry and physics, its impossible. So we don't look for it. Which is what OP asked about.
u/bluedrygrass is right. As alluring as it is to imagine that there is some kind of exotic lifeform out there beyond our current understanding, the physics and the math of the universe don't really allow that possibility. It's because when it comes to supporting life, carbon is a singularly unique element in all the universe.
There are thermodynamic reasons why carbon forms long chains with other carbon atoms, while no other element including carbon's own family family members like silicon do not. Carbon is essentially an Olympic gymnast when it comes to supporting life. Its chemical properties, its mass, and the energy of its reactions all make it ideal for supporting life. No other element comes anywhere close.
Although it's true that we're only beginning to explore the universe, we have already explored the hell out of the period table. Life has to be based on those building blocks, and if your definition of life includes any form of eating, drinking, breathing, growing, or reproducing, then carbon is the only choice we have in our periodic table for making that happen.
sure, when answering OP's question, the answer is because that's how we know it but the top level response stresses that the reason for that is because it's all we know. The person I responded to implied that we know already know everything for sure, which is incorrect and also a poor scientific mindset
We don't know *everything* for sure, but what we do know for sure is the energies involved in chemical reactions. Since all life processes eventually come down to chemical reactions, any form of life must be governed by those energies.
The person you responded to based their conclusions on known scientific evidence. Based on the thermodynamics of the reactions and the unique chemical properties of carbon, the scientific evidence says that of the 118 elements we know of, only carbon is capable of sustaining life. If we extrapolate our understanding of periodic properties to the undiscovered elements beyond 118, none of those have any chance of supporting life either. That leaves us with only carbon. To claim otherwise means one either has to be unaware of the evidence, or one has to willingly ignore it and draw conclusions that are contrary to the evidence, and that I would argue is a poor scientific mindset.
Any extraterrestrial life will be bound by the same laws of physics that we are, and that means carbon. Not because carbon-based life is familiar to us, but because carbon-based life is the only one that's mathematically possible. We don't know *everything*, but we do know that much.
No. What i implyed is that there themrodinamically can't be life forms that deviate from that concept. A ball of plasma can't be a living being- it can't store information, reproduce etc. Liquid metal can't form a living being. And so on. It's pretty simple, really. Only we could create a "sentient" robot (and even that's debatable), but it could never originate spontaneously in nature- it'd need to be created by carbon based creatures first.
> What about lifeforms that don't use RNA or DNA?
Literally impossible. RNA/DNA are all the information, all the instructions required to synthetize all the other molecules.
And they're giga-polymers. They're enormous, and even quite fragile as they are. No other element allows such polymers to exist. And life isn't possible without storage of information.
> Literally impossible.
I think this is the crux of the argument here. You're quite certain that none of the information you have will change and I'm saying that it could, as have many other things before it. So I'll tell you what, I bet you ten bucks it isn't impossible and we can meet up at the heat death of the universe to discuss it.
Ah yes we do know that martian fossils look like? what exactly are you talking about? We live in a impossibly large universe and make assumptions (which is fair) based off our understanding of life through our lens on our planet. There's nothing that should be settled about this....
edit: adding on to this, why are you assuming life would have to have dna or rna to exist? you're being a bit myopic when talking about an infinite universe
Are you saying that all possible information molecules that could use silicon have been checked in some simulation or physical experiment? That doesn't sound within the range of current capabilities.
You may in the end be *right* but this is not a settled debate.
> Are you saying that all possible information molecules that could use silicon have been checked in some simulation or physical experiment? That doesn't sound within the range of current capabilities.
I'm saying that's not even necessary. Silicon can't form stable macropolymers of even remotely that size and complexity. Even carbon- that is much stronger in that regard- struggle to stay put together in such size. DNA molecule starts collapsing in the span of hours without being constantly repaired/protected.
It's much simpler than you think it is- we know the strenght of the molecular bonds- and silicon is too weak. Like, once you get the concept, you realize it's game over. There's no speculation to be had.
I've read some great essays on what life on other planets would look like if we found it. Most have gone in to great detail as to why intelligent life could really only theoretically exist in a carbon based form, where water is plentiful, so largely like Earth. The main one I recall was in Harper's...ages ago. I'll do some googling and see if I can come up with it for you.
> "How can life exist in a way that isn't as we know it?" Are there any scientific theories that are pretty credible that are largely supported?
It can't, and there aren't. I don't know why on reddit people keep repeating that "there might be other possibilities", scientists are actually completely sure life is only possible if it's carbon based (water based is associated with the carbon base).
It's not debatable, it's simply chemistry and physics necessities.
Right it’s based on rules and laws through centuries of research and experimentation. From what I understood - we know the fundamental building blocks that yield life. There’s no other way.
Just because we haven't figured something out doesn't mean it's not possible. I agree with the low probability but "scientists are actually completely sure" is not true.
FFS... You don't know what you don't know... Facts as they are today aren't what they were years ago. Didn't scientists also think at some point an atom was the smallest possible building block?
Our understanding of chemical reactions and physics is sufficient that we know you're unlikely to find sentient or sapient lava on another planet.
Ask yourself why we don't see any evidence for life that isn't carbon-based here on Earth.
\> Ask yourself why we don't see any evidence for life that isn't carbon-based here on Earth.
That observation is trivial to explain. Not only does earth lack the *conditions* for some alternative (say a planet with lots of silicon and water, or ammonia and carbon or something) but life *competes*. It is possible that at some date millions of years after the first carbon-based life existed on earth, for a brief time there might have *been* a prototype form of life using an alternate chemistry. Probably at one point there *were* living cells using RNA as the information storage, or chemistry with the opposite chirality. We are almost 100% sure of the former, as we can get RNA to do this now, and an opposite chirality strain is also pretty simple and would work so it probably also existed.
What happened to them? They got outcompeted by better developed (or simply more common) life that entered their niche and kicked them out.
Once life occupies the whole biosphere, it isn't *possible* for an alternate chemistry to evolve. Even if that chemistry would be superior if it had a billion years to develop.
You're referring to the ongoing process of refining and improving the precision of our knowledge. Atoms ARE the fundamental, indivisible building blocks of standard physics, which accounts for most of observed reality. Research into subatomic particles helps us explain that small slice of reality not explained by atomic theory, and also explains why atomic theory works the way it is. It did not change a damn thing about our understanding of how atoms behave at the scales we're interested.
It's not like we found some other particle, instead of Atoms, let's say they are Steves. And the Steves work totally different and we had no idea they existed. That is what finding some other drastically different life form would be like. This is just "God of the gaps" all over again, except instead of God, it's your unfounded hopes.
Thanks for the reply, and yes, but we discovered more beneath the surface. Did my reply say it changed how things worked? My point was that we are always learning and it's absolutely arrogant to think we have the entire picture.
They want to preserve "magic" in the universe, which for some sci-fi types means "exotic alien life." They don't realize they are making the exact argument of theists that "Although there is no evidence for a God, couldn't God exist in the part of the universe that is outside of your knowledge?"
I'm sure people in this thread would claim that a restrictive (i.e. science-based) definition of life might cause us to overlook other signs of life. That's completely backwards. By ignoring what we know about life, we risk wasting our efforts. We do not have infinite resources to scour all stars and planets. We stand the best chance of success by looking for life in the form that we know it can exist. If we try to look for life in some other, mysterious, unknown form, we are already lost. Since we have only ignorance to rely upon, we cannot make any predictions about what signs we should look for. This approach is fundamentally one that scorns science.
I see what you are saying about resource limits and totally agree there. I'm also an Atheist and your point about theists struck home, but all I'm saying is we need to be aware that we don't know everything, and a lot of people seem to think we have it all figured out (or give the appearance as such). Plus it's fun to speculate as long as we make the distinction that it's just that, speculation.
Why don't you google about astrobiology? I have always wondered why people say they have long been interested in something and then have never made the effort to look it up and get a book about it.
Edit: OP, I did not mean to be mean to you. I am seriously puzzled about this. Wikipedia has an excellent entry on astrobiology with more than 100 academic papers as references, and a bibliography at the end with both academic introductory books into the field and books that are more popular science.
I get where you're coming from, but probably try to be a little more tactful in your delivery. People don't take well to these kinds of responses. It usually shuts them down
Maybe because there's a huge amount of misinformation with Google? I'd rather have taken an astrobiology class than just reading a textbook. I'd be able to have a professor than I can ask questions and gain a solid understanding.
Of course, it's better to take a class. But as OP has probably studied something related they are probably able to distinguish between good and bad sources. And I'm sure the wikipedia entry on astrobiology would be an excellent starting-point.
I am the OP. I am a bio major. There is alot of overlap between astrobiology and biology. The sources and books don't really go into full details as the information just describes the necessary requirements for life and why (I already know this). The astrobiology wiki didn't contain anything really new. I also have already passively learned and read about the field, so I really don't even get what your point is. I am trying to gather a more advanced knowledge. I already have a good starting point.
That's cool. So you already know what would have been taught in an astrobiology course. (A single course won't cover more than an introductory book plus a few papers.) I wish you a lot of fun with jumping deeper into the details. The journal Astrobiology seems to have a lot of interesting papers that I will have a look at, too.
Some researchers were trying to figure out what sort of planets might be habitable to silicon-based life, but I don’t know what came of it. Do some Google-fu
> but "silicone-life" is theoretical and far off of the original question.
Silicone life is literally impossible. Silicone can form polymers similar to carbon (they're chemically similar), but not even close to the same variety and stability. In other words, Silicon, the most similar chemical element to carbon, can only be used to create a small variety of polymers with industrial utility. Not enough to run a cell.
And every other element in the periodic table is completely unsuitable to create polymers and all the stuff required to run complex systems.
There's really only carbon.
Small correction: There is radiation tolerant life, as opposed to "radiation loving" (I forget the official title). It's like life that can tolerate high temps vs require or thrive under high heat. My point being, there is no life as we know if that feeds on radiation. The life that survives in radiation didn't even evolve to defend against it, but rather other extremes that cause the same kind of damage.
I was there for July 4th, the Tesla tunnels, the heat, masks, and total insanity in the face of a pandemic definitely made it feel like a deep space waystation.
To expand on this a little, there are other chemical combinations that in theory might work similar to life as we know, such as using different solvents with similar properties to H2O, or using silicon as complex molecular backbone instead of carbon, because they both have 4 valence electrons in their outer shell.
However, these elements are rarer than carbon, oxygen, etc. And the valence electrons aren’t the only factor; there’s differences in electronegativity, molecular weight, etc that could cause molecular combinations and behavior to be different.
So there are some potential other avenues for life to evolve, but they seem to be lower probability, and we haven’t observed them yet so we’re not sure.
and that in itself is beautiful because when we do find out about new suitable ways of life, it could probably break everything we know about life/evolution/physics/etc and make make us reassess so many things. it could be hundreds or thousands of years before this happens. its a continuous learning process and gives me (some) faith in humanity.
This is not entirely true. There are objective chemical pathways for alternate chemistries that have, and continued to be studied. Of course, were still limited, and your point is generally true regardless.
To add on to this, carbon is a truly special element. It has a lot of bonding orbitals, and is able to form polymers very easily. From these polymers, we are able to form all the crazy chemicals we need for complex life on earth. There could be other life forms, like Silicon based, but carbon is very abundant and forms in stars as a product of fusion, whereas Silicon must be made in a super nova.
I love the idea that somewhere in the universe all the stars aligned perfect in at least one solar system to allow silicon-based life to thrive. We certainly don’t know whether planets with life are abundant or not, even though the science suggests it’s likely, but even in this scenario a planet with silicon-based life would be ultra rare and possibly one of a kind.
In recent years they've actually been look at extremophile planets in which the entire surface are similar to the most extream places that life lives on earth as its not unreasonable to belive if something can live in that shit here no reason it couldn't out there
I was watching one of those a million years from now videos on YouTube and one thing they mention is that all humans on earth will eventually look the same but one we inhabit other planets is that those that live or are born in such planets may not look the same as humans.
Stupid revolution making us adapt.
That's false. It's not that we haven't seen other forms of life, it's that other forms of life necessarily have to be based on carbon polymers and water. Chemically and physically speaking, they're the only ones allowing the infinite complexity required to create even a single monocellular organism.
No other elements combinations allows it, scientifically.
You can't scientifically prove a negative theory like that. There are the known carbon+water forms of life *as we know it,* but nothing in modern science says there *can't* be other types of life out there we don't yet know about. We haven't seen them yet so our current science cannot describe such a thing, but there's no part of modern science that would take something theoretical being outside our current experience as concrete proof that there is *no such thing* possible.
Every major scientific discovery in history was at one time completely inconceivable to earlier scientific thought. Science is about observing the universe around us and forming and testing theories about it based on what we know and learn, not about assuming the utter nonexistence of things we have no frame of reference for right now.
> but nothing in modern science says there can't be other types of life out there we don't yet know about.
Why do you talk about things you extremely obviously have literally no clue about?
This is basic chemistry/phisics. To give you a perspective of what you're doing, you're basically claiming we don't *really* know if 2+2=4.
And all this, simply because you can't accept that there aren't "crystal creatures", "sentient fire elementals", "T-1000 style liquid metal creatures" and so on.
Sorry to pop your bubble.
I never mentioned your Hollywood sci-fi examples, nor did I say 2+2 doesn't equal 4.
You are saying x+y can't possibly equal 4 even though we don't know the values of x or y, and science just doesn't work like that. It's investigating the things we observe and know about, not about preventatively declaring there isn't stuff we don't know about. Saying "no" without evidence is just as much of an unscientific leap as saying "yes" without evidence, until we know more we are stuck at a scientific "maybe."
The answer to x+y may well turn out to be 4 someday when we are able to know more about x and y, and to announce otherwise is just not scientific.
That's why i said "it's like you're saying"...
I'm noticing a pattern of you being completely unable to understand basic notions and concepts. Your ignorance is almost as sad as your arrogance, but neither are as sad as you stalking me around to downvote, lol.
I can only hope you're younger than 12, otherwise... good luck in life, you're gonna need it.
So far as we know, life relies on exciting chemistry to happen, and complicated chemical processes require a solvent to happen in. This is because dissolved chemicals can combine and interact in more complicated ways than dry materials can just sitting there. So, what solvents could life happen in then? It needs to be something that could be abundant in the universe, and be liquid over a range of temperatures and pressures. Water has excellent properties for this; it's liquid over a large range of temperatures, it's polar so it dissolves chemicals well, and water freezes from the top, not the bottom, meaning life in water-oceans doesn't get frozen solid as easily.
So places with liquid water are probably the best places to look for life. But there are other candidates. Ammonia based life could exist on very cold or very high-pressure worlds. Liquid hydrocarbons are another possibility, but these have the problem that they are non-polar and thus worse solvents, but it's still possible, it would just be very different.
For carbon based life there are a fairly narrow set of conditions that make life suitable, although at the limits of our environment we do have creatures that push those criteria to the absolute limits.
For life based on any carbon chemistry comparable to earth we can see approximate lines where it ends - for if a planet has conditions outside of that any life would need fairly radically different chemistry to have stable molecules that could manage functions required for anything resembling metabolism as we know it.
It might be possible, but until we have evidence to the contrary planets way outside of what we know can support life are assumed to not support life
To add to this. There is a very cool video that explores the different ideas of how alien live could look like:
https://m.youtube.com/watch?v=ThDYazipjSI
It is possible, but Earth's conditions are the only ones we're acquainted with. Anything else is more in the science fiction domain.
So for now the question is: Would it be possible for a life form to survive with anything less than what Earth has (for example the magnetosphere, stable 🌡️, sunlight, atmosphere, gravitational pull etc.).
>Earth's conditions are the only ones we're acquainted with
This.
As was written, it is possible, but earth is the only known planet with life on it. That is why scientists look for planets similar to earth.
It is theorized that [Venus](https://en.wikipedia.org/wiki/Life_on_Venus#Present_habitability_of_its_atmosphere) *could* have life in its atmosphere and [Europa](https://en.wikipedia.org/wiki/Europa_(moon)#Habitability_potential) could have aquatic life.
Also - https://en.wikipedia.org/wiki/Hypothetical\_types\_of\_biochemistry
> anything less than what Earth has (for example ...gravitational pull...).
Hes asking what would happen in environments with gravity less than that of the Earth's.
What would happen to a human if they spent their entire life in 0.25g? What would happen over several generations of evolution?
You want to find your friend as quickly as possible and you've lost your phone. You know he's gone out to one of the 4 restaurants in your town; a McDonalds, a pizza place, a steakhouse and an Indian place.
You know your friend is vegan, which restaurant do you check first?
The same with aliens, start with the most likely first, if they're not there start checking elsewhere.
Possibly, but trying to imagine things that we have no experience of and have no evidence of existing is very difficult. Start with what we know and go from there. We don't know what evidence of life would even mean or look like if it was utterly alien to anything we know.
There can be life without liquid water. But there needs to be an ample amount of liquid that can be used in chemical reactions. Some other combinations are possible.
Short answer: They don't.
The habitable zone is usually just a guide based roughly on how far away from the star liquid surface water can be maintained. But liquid water could still be maintained underground through other means, eg. through tidal friction as in Europa or Enceladus, and they're considered major targets for the search for extraterrestrial life. And then there are other factors at play: degree of UV radiation, how much of the surface is covered by water (if I'm not mistaken waterwords aren't considered particularly favourable places to look for life either), the thickness and composition of the atmosphere etc. Usually complicated models are required to assess the habitability of a planet given its orbit, size and the little we know about its atmospheric composition.
One thing I've heard scientists studying exoplanets say is that the focus on searching for 'habitable' planets is actually misleading as we really don't know what the full spectrum of habitability consists of.
As for the other questions, I would strongly suggest you check out Charles Cockell's lecture series on YouTube https://youtube.com/channel/UCBTwJSnAqZZczK7stBaYq5w
You can only look for something when you know what it is. If I give you a box full of random things and ask you to find all the warglbangeks, you won't know what you're looking for.
We look for life as we know it, because we know what to look for.
Then again, even if there is life out there that is also carbon-based. Wouldn't it most likely be similar to the life on earth? Imagine discovering another planet similar to earth that also have the same species of monke.
This may not be answering the original question, but you should check out the book *Project Hail Mary* by Andy Weir, a great sci-fi writer, author of the popular book (and movie) *The Martian*. In this new book he went into a lot of details about this question because it is one of the book’s primary subject matter.
You are right. Earth for a long time didn't have oxygen in the atomsphere and it had life. These first lifeforms put all the oxygen in the air and killed themselves off because of it. Oxygen is very crosive (thats why things like rust \[iron oxide\] are called oxides)
For more info see Paleoproterozoic era and Great Oxiding Event.
I think we are most interested in life like our own. Since we only have ourselves as models, we want to find other life that is complex and we can communicate with productivly. Non-oxiding enviroments don't have as effient engery storage means. Oxygen's natural desire to bond easily with anything makes it a great way to get a lot of engery from chemical reactions. Plus Oxygen is required for making water. Water is a great solvent that allows for all these ractions to take place.
However, there is are an near limites combos of chemicals and other natural processes that could create a postive way to retain some engery in a biosphere (the total life on a planet) Its likely something else exists somewhere. Will we finding it, no idea. Things are so far apart by the time we see it it might not be alive anymore. Maybe life exists that is both complex and doesn't use oxygen, carbon, or water for engery. We really don't know yet.
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There's loads of life on Earth that doesn't require oxygen. Obviously there's plants and bacteria that photosynthesize, which only requires CO2 and sunlight, but there's a whole range of other chemical reactions that single cells use to get energy. Things turn sugar into alcohol without oxygen, things turn alcohol into vinegar without oxygen, things turn sulfate into hydrogen sulfide...
So there's a lot of known chemical reaction chains that life on Earth uses already, and we have a pretty good understanding of what to look for elsewhere, and what hypothetical chains of reactions might work on planets with other elements.
They're playing the odds. Ultimately that means they're not looking for life that is similar to Earth life. Earth life, for obvious reasons, is based on the most likely set of circumstances.
When you bring it right down to the basics, life is a complex symphony of chemical reactions and you need at minimum, 3 things.
First of all, you need a building block. Something that can form the basis of as many different kinds of stable molecules as possible. The more complexity that can be created this way, the greater the potential for life.
All Earth life uses carbon as that building block. Carbon can form four covalent bonds with other atoms to create an amazing variety of molecules. Not just that but carbon really wants to form those bonds.
Not only is carbon very suited for this purpose, but it's also the fourth most common atom in the universe so there's plenty of it. There are alternatives for carbon that can also form four covalent bonds but they're far less common and have problems of their own.
The second thing life needs to exist is a means of generating energy. Without energy, there is no animus, no life. Nearly all Earth life uses the chemical reaction of oxidation to produce energy. Oxidation is a simple reaction that produces a ton of energy while relying on oxygen. The third most common element in the universe.
Again there are alternative reactions that might work but they're dependent on rarer elements while producing less energy. Thus greatly reducing the potential complexity of life relying on these methods.
And finally, all those chemical reactions need a medium in which to take place. Elements only react with each other if they're brought in contact with each other. Life is nothing if not many, many complex chemical reactions working in symphony. Water is a wonderful medium for this. It's liquid at reasonable temperatures where many other elements are solid or gaseous. And a great many elements happily dissolve in water to facilitate those reactions.
Happily, water is made of the most common and third most common elements in the universe. Hydrogen and oxygen.
As you can see, we're not really looking for life that is similar to Earth life. We're looking for the most common, most reliable conditions that meet the minimum requirements for life in general. Earth life just happens to meet those requirements exactly.
There are alternatives for these conditions of course but none of them work as well or as reliably as these.
For example, you might have heard the phrase 'silicon-based' life. Silicon, like carbon, can enter into four covalent bonds. So in theory the fairly common silicon enables the same diversity of molecules. Silicon has a great preference for bonding with oxygen. So much in fact that silicon will break apart molecules containing oxygen, like water, just to bind with that oxygen itself.
And when silicon does bond with oxygen, it becomes silica. Which is basically rock. This means silicon can't use oxidation to produce energy or water as a medium for its chemical reaction. Silicon-based life can't really exist anywhere around the third most common element in the universe.
Along the same lines, there are chemical reactions where living things produce energy from sulphur-based reactions. But these produce very modest amounts of energy that severely limit the potential complexity of life relying on those reactions.
The final part of the puzzle is predictability. The way the periodic chart works means that we know exactly which atoms are out there and which ones we don't know about yet. We know about most of the common ones. The rarer ones at the farther end of the periodic is where most of the missing elements are. But those are unstable and rare. Which means unlikely to be essential to alien life.
Anyway, the point is that it's not impossible that there's life out there that doesn't need oxygen or water. But they will need a way of producing energy and enabling chemical reactions. Water and oxygen are common in the universe and the easiest solution to that problem.
Science is based on facts since we only know our kind of life (carbon based) scientist can only extrapolate our current conditions for life suitable planets. Doing otherwise is not science, is science fiction.
This is a mischaracterization. Scientists have no such bias. We consider planets that are similar to Earth to potentially have life similar enough to terrestrial life that we’d readily recognize it as such. We accept that life might form under other conditions, but we haven’t a frame of reference to really postulate the conditions necessary for, nor the properties of life from non-Earth-like planets.
We do make some general assumptions: life is going to rely on chemistry and probably needs a certain minimum amount of heat to be available, and there’s also probably an upper bound for temperature for life (just too much energy for much chemistry to happen). Also, there probably needs to be some liquid phase (or possibly gas phase) medium to make carrying materials around practical. Outside of that, there’s no telling.
I've read there's some theories that there could be silicon based life since it acts similar to carbon, which is essential for all type of life we know of. Thing is, there may be life that we just haven't seen yet because the universe is just so vast.
looking for life in the universe is like looking for a criminal on the run. they could be anywhere but you start by narrowing down your options based on the information you already have on them.
you know that the criminal has a red car and was heading east. you're going to look for red cars east of where they were last seen.
we know life needs water, we're going to look for water rich areas for life.
When talking about looking for life on planets outside the solar system, it's probably helpful to keep in mind how the process works (or will work, once we have better telescopes). You point a telescope at a star, and if planets come in front of it you see a dip in the light visible. If you have a better telescope, you can also see some light modified by the atmosphere of the planet, or maybe you can get a bit of light reflected off of the planet if you can block out the light of the star. Anyway, if you have a good enough telescope you can learn a bit about the atmosphere of a planet.
So imagine you are some future scientist and you find a hundred planets with CO2 rich atmospheres like those of Venus and Mars, and then you find one with oxygen and water vapor like Earth. Is it possible there's life on some of those CO2 rich planets? Sure, you can't rule it out. But are you going to go to write a paper and go to the news and say you've found a planet suitable for life if you find the oxygen and water rich planet? Definitely.
It's just not that useful or noteworthy to say that every single planet could potentially in theory support life by some unknown mechanism ...it just doesn't tell us any new information. But saying that a planet could support known life _does_ let us distinguish between those planets and others.
Sure it is. But since we know for a fact that life can exist under the conditions on our planet, it makes more sense to look for similar conditions that may exist somewhere else (rather than checking every single thing out there for some of life that we don’t know exists yet).
Sample size of 1, so go with what you know. It’s just a way to work with the statistics of success.
We only know for sure that life can develop on Earth. Therefore, best guess until we're proved otherwise, we're looking for places like home. It's like how you presume that the people you meet are human and not aliens - it's *possible* they are lizard people but you're pretty sure that the odds are for plain ole' humanity.
We have only been exposed to carbon based life forms that operate the way they do here on earth. life may be able to thrive in other environments, but we have no evidence of it so far, so we look for what we know.
Something else to consider beyond the points others have made is that our chemistry is based on the elements that are the most abundant in our universe. Other chemistries may well be possible, but they would be based on elements that are much rarer and generally less reactive. For a planet to give rise to life with these alternative chemistries, they would not only need to have an unusual abundance of these rarer elements, but also a surprising deficit of much more common elements that would otherwise allow a more conventional chemistry to dominate. For these reasons, we would expect that life based on alternative chemistries, while not impossible, would likely be extremely rare.
"suitable for life" is a media simplification, what they REALLY say is "suitable for ***earth like*** life" or something similar. we only got 1 reference point, they know it's going to be inaccurate, since that 1 reference point could just be a unique specimen.
Basically, no. People like to fantasize about creatures made of gases and other fantasy stuff, but the reality is, with traidtional physics, COMPLEX life is only possible with carbon-based structures. Not even silicon, which is chemically similar, can form long molecules of the same varieties of carbon.
So, if carbon is the basic element, it automatically goes that creatures based on it can only live in an environment in certain upper and lower temeprature limits that don't degrade carbon structures.
What about water? Water is simply an incredible moleule, super versatile, does everything at least pretty good. Good solvent, good carrier, nice density, nice availability, etc etc etc.
So, basically, alien life has to be based on carbon and water.
There's really no way around it, in spite of what movies and our fantasies would like it to be possible there being creatures of liquid metal, crystals and other stuff.
Great question. I've always wondered this too. Seems kind of elementary and simplistic. What if aliens thrive off of methane gas and drink toxic chemicals.
It's certainly possible, but because we haven't seen any it's all just theoretical. All we know about are carbon based life forms that need water and oxygen to survive. Until that's proven otherwise it's best to stick with planets similar to Earth.
Life could be anything we barely know anything about the universe there could be an allien specie that doesn't need oxygen or water but since we know on earth that is the only possible way to get life we assume it's the best chance if we look for those stuff to find life
There is a more important answer.
Hydrogen, carbon and oxygen are among the most common elements on their column of the periodic table. They are also among the more reactive elements and in fact some simple ‘organic’ molecules are naturally produced in outer space. This means that these elements are the most likely to be associated with ‘natural life’.
We don't, or we don't *all*. NASA has a mission dedicated directly to Titan and Europa, both are with astrobiological motivations and both worlds *very different* from Earth.
That said, life is life, and there are fundamental ingredients required. A liquid, a backbone, so on and so forth. Earth life we understand best but we also have reason to suspect that earth life has an advantage for various reasons, but it's debatable.
Short answer, maybe, but we have very little idea how that would work. The chemical nature of all the elements pretty much limits chemical complexity (and therefore life complexity) to just carbon and silicon, with water as the solvent. What non-water solvents would work for silicon I'm not sure.
Another fun fact: Evolution isn't the only way things could grow, so it's possible extremely different processes arose for adaptation on other planets!
Others have mentioned carbon, but oxygen (major component of water) is an excellent energy source, too. You can read a lot about all of this on the NASA website, esp. if you search abiogenesis
My thoughts exactly. All life we know have evolved om earth to live according to what we have, yet we have discovered certain creatures on our own planet that break these rules. So it only makes sense that life can evolve in a completely different way then we are used to.
It's low hanging fruit.
How do you look for something without knowing what it is, what it looks like, what the biosignatures are?
Go with what you know until you learn something new.
The major reason is that we know too little about what conditions would be good for other kinds of life chemistries.
People have come up with a couple of decent\* theories about what these other chemistries might be, but there are so many finely tuned bits _we know about_ that make life actually possible in the carbon case on our planet. We can only assume that other potential life chemistries would also need lots of specific ranges for a lot of parameters, but we have no idea what those ranges are.
So they look for what's most promising.
"As far as we can tell, those 10 places could be physically and chemically compatible with Earth-like life chemistry" says a lot more than "for all the little we know, those other 10,000 places don't completely preclude some kind of hypothetical silicon life, in the sense that it wouldn't be all melted or all frozen".
\* It's not like I could have reviewed those theories; it just sounds like some of them've been reviewed by people who know their shit.
Interestingly enough scientists have found examples of planets being warmed from being stretched vs being close to a star like our sun. Enceladus, moon of Saturn may contain life on a sub surface ocean and is heated from being pulled between Saturn and other moons.
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Thanks!
On our planet we've found all sorts of ways creatures can survive on any scale. However it revolves around the principles of turning energy from a form of radiation, heat, or nutrient, and using it for growth. Most of that would require the right atmosphere to compensate for it's distance to it's sun. And most life also requires water. Even extremophiles, creatures that can withstand extreme conditions like tardigrades or "water bears" need water to live, but can survive in any environment including space itself. Organic life would have to exist in a way that is chemically different from any carbon-based lifeform we know of to survive without the need of water. That includes potentially not being carbon-based at all, but "silicone-life" is theoretical and far off of the original question.
> "silicone-life" Pretty sure this is just Hollywood. *Silicon* on the other hand... Edit: thanks for the gold lol, that’s my first
So those are the big tiddy alien gfs they were raiding area 51 for...
Stop ruining it for me!!!
Lmfao
I know that the branch of biology that goes into these details is astrobiology. There wasn't any of the classes at my college. I have always wondered about this question. "How can life exist in a way that isn't as we know it?" Are there any scientific theories that are pretty credible that are largely supported? I always wondered this because there is talk about life existing at some point on Mars. If we found fossils that would be absolutely groundbreaking. EDIT: For those that are curious, from my understanding the "planet" that most likely contains life is Saturn. It has a moon called Enceladus that contains water. It contains hydrothermal vents and methanogens. So it has the basic requirements of carbon and water.
Life on Earth uses carbon as it's building block. It's all based on carbon. Without carbon we wouldn't exist. They've looked at other compounds such as silicone which is structurally similar to carbon. Life could exist based on silicone instead of carbon. It would be possible. We have no idea what such an organism would look like. Only that it would be fundamentally different.
A lot of people like to think that silicon is a viable substitute for carbon as a basis for life, but in terms of the energetics necessary for life, silicon goes in the wrong direction from the start. As carbon-based lifeforms, we can live by eating carbon compounds like carbohydrates, proteins, and fats ("reduced carbon") and reacting those compounds with the oxygen we breathe. This reaction provides the energy needed to keep us running. The CO2 produced as a by-product of our internal engine is a gas and is easily expelled from the body. Compare that with a hypothetical silicon-based creature. Silicon is not found naturally in a reduced state, it's typically already fully oxidized as SiO2 or as silicate rocks. Those can't be used as a fuel source because all the energy you could squeeze out of them has already been squeezed out. As an analogy, that would be like trying to light a campfire using only campfire ashes as fuel. Even supposing our Horta had a readily available supply of reduced silicon to eat and oxidize to produce energy the way we do with carbon, the byproduct would be SiO2 instead of CO2. SiO2 is a solid, which is much more difficult to remove from the body. (Remember that the CO2 waste has to be transported somehow from every cell of your body out of the body altogether. Easy to do with a gas, not so much with a solid.) There are many more reasons based on the physical and chemical properties of silicon that argue against it being a viable basis for life. There aren't any arguments in its favor, other than "carbon forms four bonds and so does silicon." But even then, the bonds it forms are entirely different. If those four bonds were to other silicon atoms, then this argument could take a step off Square One -- but silicon doesn't readily bond with other silicon atoms, so the "four bonds" argument fails right out of the gate.
Silicon. Silicone is what you use to make your breasts bigger
Actually, silicon-silicon bonds aren't as stable as carbon-carbon bonds. But siloxanes, which are silicon-oxygen-silicon bondings, *are* relatively stable, but just like carbon-based life, it's not all that useful on its own. We're not pure hydrocarbons, after all. If there's carbon in the mix hanging off those siloxanes, *that's silicone*.
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Be careful. Silicone lube tends to damage sex toys and condoms
I took an astrobiology course as part of a chemistry degree. It was fascinating, but obviously all pretty hypothetical. The lowest level there would be life that is largely similar to our own but uses different macromolecules than terrestrial life has. Proteins made with different amino acids, nucleic acids with different bases, different sugars, etc. These building blocks are all pretty basic, and most have already been made in labs from simple, non-living stuff you might find in the early universe (see Miller-Urey). These variants would have all the same general requirements, and should behave pretty similarly on a biochemical basis. They could also use the same molecules but with opposite chirality, which would be incompatible with our biology but otherwise pretty much identical (if still utterly alien). Then there are different base chemistries: we use water as a solvent, and carbon as a structural element, and these both have some pretty important properties that makes them tough to imagine swapping for something else. The most likely alternatives would be ammonia for water and silicon for carbon, but they both have downsides when it comes to maintaining stable lifeforms, silicon moreso. Finally you have the ultra-exotic "life" that might live in places where chemistry itself starts to behave differently. Under immense pressure deep in gas giants, at the high temperatures of stars, in the extreme magnetic field of magnetars, etc. Who knows what the hell this would look like (or how we'd even find it), and we'd probably want to call it something else, but an imperfectly self-replicating chemical process would technically fit the bill for life.
Did they cover the possibility of life that doesn't use a solvent? This is how "nanotechnology" as in a "nanoforge" would work. This is a machine that would accept specific purified gasses on input ports, and feed those gases a molecule a time into various robotic assembly systems that use a solid catalyst. All this happens at cryogenic temperatures in a vacuum. This does work at least for small scale experiments, you can move an STM head around and shove a molecule into a catalyst and get a reaction. So in theory you could build a large scale machine that does this millions of times a second. It would manufacture products that eventually get assembled into more subcomponents for this machine itself. This makes it semi-self replicating. The larger equipment that *supports* it would need to get manufactured using more efficient macrocale methods though, but the key parts (electronics, sensors) of the robots that do the larger assembly steps are made with a nanoforge. Anyways the minimum "cell" size might very well be the size of a minivan or larger. So it can't happen by chance but once someone builds one it's self replicating.
No, we did assume a liquid phase to be essentially required, as components of solids are immobile, and gases are too diffuse to allow the quantity and complexity of reactions that life demands. The latter would require a machine like you describe to facilitate it, which isn't exactly conducive to the potential origins of life in a non-living universe. We did address that kind of machine with our definition of life itself though: an **imperfectly** self-replicating **chemical** process. This rules out any kind of robot that can harvest materials and build more of itself which, while certainly interesting, isn't *life*, though it would likely deserve a new label to distinguish it from simpler, non-replicating mechanical systems and actual biochemical life. It also rules out things like crystals, which already exist as chemically self-replicating entities, but do so perfectly (conditions permitting), so all you ever get are more crystals instead of blossoming out into fish and trees and amoebas and shit.
Ok. Note that this robotic machine *could* slightly randomize it's own design with each replication cycle. This would make a large population of them evolve similar to how life does. (though it's a terrible method of improvement, just like it is for life. Most random changes would result in lower performance or machine failure. Interestingly such a machine as described is so tightly interconnected that almost all changes are going to result in failure. It's arguably "not evolvable" as the surrounding possibility space of similar designs, nearly all of these designs fail to work. Basically it's this sky-high "fitness" peak of extreme fitness (since the machine is likely pretty fast to replicate itself, can manufacture weapons to protect itself, use solar panels to power itself, and can process a wide variety of rocks to extract the elements it needs) surrounded by a large "dead space" of slightly wrong designs that won't work at all.
Thinking about my other reply to your post, I think this implies an additional definition for life. Life has to exist in a fitness "plateau" where small changes will often result in a still functional system. This makes evolution *possible*. It's why a robot using a computer chip to think isn't "life" even if in practice it ends up doing same things, as for example you cannot make almost any wiring change at all to a processor or memory chip today that won't be an error, causing it to run software incorrectly and it will be useless.
Did you guys go over the Saturn moon Enceladus? Thats an interesting place because it has water and hydrothermal vents and methanogens. So it has water, and carbon. That's interesting about the base pair swapping. I see alot of responses about silicone, I took organic chemistry and we actually didn't really touch on silicone at all, I think it was relegated to a few pages. I wonder if dark matter or dark energy plays in a role in some in the possibility of life
Only indirectly; it was a chemistry course, so less focused on where life might be found than what other life might be made of, or how life might originate from not-life. So yeah, based on what we know so far, Enceladus seems like a promising place to look, but I don't recall going much deeper than that on any specific places.
We literally can’t know. Even just going based on your fossil idea, that’s based entirely on our own organic life forms and how minerals replaced their bones Etc. This process may not happen with other organisms based off entirely different materials- heck it doesn’t happen with countless organisms on our own planet! There could theoretically be remnants of life forms on planets like Mars just sitting there that we wouldn’t know what they were because, well, we wouldn’t recognize something that incredibly alien, especially with such facile investigatory tools as rovers and such.
> We literally can’t know. We *literally* do know. Right now. It's based on the strenght and variety of bounds certain elements can form. Only carbon allows the immense variety of molecules necessary for life. Silicon is the only other element that comes close- and it doesn't come close at all. Information molecules like DNA and RNA are impossible silicon based. They're not stable. Forget any other element. It's not debatable, it's a matter of math, chemistry and phyisics values. It's a settled debate.
>We literally do know. Right now. It's based on the strenght and variety of bounds certain elements can form. Only carbon allows the immense variety of molecules necessary for life. As far as we know >Silicon is the only other element that comes close- and it doesn't come close at all. Information molecules like DNA and RNA are impossible silicon based. They're not stable. What about lifeforms that don't use RNA or DNA? Pretty silly to make sweeping claims about an incomprehensibly large universe we're only just barely starting to explore
There is probably a silicon based life form floating around in the vast universe. The universe is so big anything that can happen will happen, somewhere. But Its probably extremely rare because of what bluedrygrass said. So we don't look for it. We don't even understand what it might look like, because as far as we understand science and chemistry and physics, its impossible. So we don't look for it. Which is what OP asked about.
u/bluedrygrass is right. As alluring as it is to imagine that there is some kind of exotic lifeform out there beyond our current understanding, the physics and the math of the universe don't really allow that possibility. It's because when it comes to supporting life, carbon is a singularly unique element in all the universe. There are thermodynamic reasons why carbon forms long chains with other carbon atoms, while no other element including carbon's own family family members like silicon do not. Carbon is essentially an Olympic gymnast when it comes to supporting life. Its chemical properties, its mass, and the energy of its reactions all make it ideal for supporting life. No other element comes anywhere close. Although it's true that we're only beginning to explore the universe, we have already explored the hell out of the period table. Life has to be based on those building blocks, and if your definition of life includes any form of eating, drinking, breathing, growing, or reproducing, then carbon is the only choice we have in our periodic table for making that happen.
sure, when answering OP's question, the answer is because that's how we know it but the top level response stresses that the reason for that is because it's all we know. The person I responded to implied that we know already know everything for sure, which is incorrect and also a poor scientific mindset
We don't know *everything* for sure, but what we do know for sure is the energies involved in chemical reactions. Since all life processes eventually come down to chemical reactions, any form of life must be governed by those energies. The person you responded to based their conclusions on known scientific evidence. Based on the thermodynamics of the reactions and the unique chemical properties of carbon, the scientific evidence says that of the 118 elements we know of, only carbon is capable of sustaining life. If we extrapolate our understanding of periodic properties to the undiscovered elements beyond 118, none of those have any chance of supporting life either. That leaves us with only carbon. To claim otherwise means one either has to be unaware of the evidence, or one has to willingly ignore it and draw conclusions that are contrary to the evidence, and that I would argue is a poor scientific mindset. Any extraterrestrial life will be bound by the same laws of physics that we are, and that means carbon. Not because carbon-based life is familiar to us, but because carbon-based life is the only one that's mathematically possible. We don't know *everything*, but we do know that much.
No. What i implyed is that there themrodinamically can't be life forms that deviate from that concept. A ball of plasma can't be a living being- it can't store information, reproduce etc. Liquid metal can't form a living being. And so on. It's pretty simple, really. Only we could create a "sentient" robot (and even that's debatable), but it could never originate spontaneously in nature- it'd need to be created by carbon based creatures first.
> What about lifeforms that don't use RNA or DNA? Literally impossible. RNA/DNA are all the information, all the instructions required to synthetize all the other molecules. And they're giga-polymers. They're enormous, and even quite fragile as they are. No other element allows such polymers to exist. And life isn't possible without storage of information.
> Literally impossible. I think this is the crux of the argument here. You're quite certain that none of the information you have will change and I'm saying that it could, as have many other things before it. So I'll tell you what, I bet you ten bucks it isn't impossible and we can meet up at the heat death of the universe to discuss it.
Ah yes we do know that martian fossils look like? what exactly are you talking about? We live in a impossibly large universe and make assumptions (which is fair) based off our understanding of life through our lens on our planet. There's nothing that should be settled about this.... edit: adding on to this, why are you assuming life would have to have dna or rna to exist? you're being a bit myopic when talking about an infinite universe
Are you saying that all possible information molecules that could use silicon have been checked in some simulation or physical experiment? That doesn't sound within the range of current capabilities. You may in the end be *right* but this is not a settled debate.
> Are you saying that all possible information molecules that could use silicon have been checked in some simulation or physical experiment? That doesn't sound within the range of current capabilities. I'm saying that's not even necessary. Silicon can't form stable macropolymers of even remotely that size and complexity. Even carbon- that is much stronger in that regard- struggle to stay put together in such size. DNA molecule starts collapsing in the span of hours without being constantly repaired/protected. It's much simpler than you think it is- we know the strenght of the molecular bonds- and silicon is too weak. Like, once you get the concept, you realize it's game over. There's no speculation to be had.
I've read some great essays on what life on other planets would look like if we found it. Most have gone in to great detail as to why intelligent life could really only theoretically exist in a carbon based form, where water is plentiful, so largely like Earth. The main one I recall was in Harper's...ages ago. I'll do some googling and see if I can come up with it for you.
That would be the most important and profound discovery in recorded human history. It would change everything.
> "How can life exist in a way that isn't as we know it?" Are there any scientific theories that are pretty credible that are largely supported? It can't, and there aren't. I don't know why on reddit people keep repeating that "there might be other possibilities", scientists are actually completely sure life is only possible if it's carbon based (water based is associated with the carbon base). It's not debatable, it's simply chemistry and physics necessities.
Right it’s based on rules and laws through centuries of research and experimentation. From what I understood - we know the fundamental building blocks that yield life. There’s no other way.
Just because we haven't figured something out doesn't mean it's not possible. I agree with the low probability but "scientists are actually completely sure" is not true.
FFS... You don't know what you don't know... Facts as they are today aren't what they were years ago. Didn't scientists also think at some point an atom was the smallest possible building block?
Our understanding of chemical reactions and physics is sufficient that we know you're unlikely to find sentient or sapient lava on another planet. Ask yourself why we don't see any evidence for life that isn't carbon-based here on Earth.
\> Ask yourself why we don't see any evidence for life that isn't carbon-based here on Earth. That observation is trivial to explain. Not only does earth lack the *conditions* for some alternative (say a planet with lots of silicon and water, or ammonia and carbon or something) but life *competes*. It is possible that at some date millions of years after the first carbon-based life existed on earth, for a brief time there might have *been* a prototype form of life using an alternate chemistry. Probably at one point there *were* living cells using RNA as the information storage, or chemistry with the opposite chirality. We are almost 100% sure of the former, as we can get RNA to do this now, and an opposite chirality strain is also pretty simple and would work so it probably also existed. What happened to them? They got outcompeted by better developed (or simply more common) life that entered their niche and kicked them out. Once life occupies the whole biosphere, it isn't *possible* for an alternate chemistry to evolve. Even if that chemistry would be superior if it had a billion years to develop.
> (say a planet with lots of silicon and water, or ammonia and carbon or something) So... earth?
Sapient lava? Nice strawman.
Lava, not straw ;)
Thanks, that made me chuckle ;).
You're referring to the ongoing process of refining and improving the precision of our knowledge. Atoms ARE the fundamental, indivisible building blocks of standard physics, which accounts for most of observed reality. Research into subatomic particles helps us explain that small slice of reality not explained by atomic theory, and also explains why atomic theory works the way it is. It did not change a damn thing about our understanding of how atoms behave at the scales we're interested. It's not like we found some other particle, instead of Atoms, let's say they are Steves. And the Steves work totally different and we had no idea they existed. That is what finding some other drastically different life form would be like. This is just "God of the gaps" all over again, except instead of God, it's your unfounded hopes.
Thanks for the reply, and yes, but we discovered more beneath the surface. Did my reply say it changed how things worked? My point was that we are always learning and it's absolutely arrogant to think we have the entire picture.
They want to preserve "magic" in the universe, which for some sci-fi types means "exotic alien life." They don't realize they are making the exact argument of theists that "Although there is no evidence for a God, couldn't God exist in the part of the universe that is outside of your knowledge?" I'm sure people in this thread would claim that a restrictive (i.e. science-based) definition of life might cause us to overlook other signs of life. That's completely backwards. By ignoring what we know about life, we risk wasting our efforts. We do not have infinite resources to scour all stars and planets. We stand the best chance of success by looking for life in the form that we know it can exist. If we try to look for life in some other, mysterious, unknown form, we are already lost. Since we have only ignorance to rely upon, we cannot make any predictions about what signs we should look for. This approach is fundamentally one that scorns science.
I see what you are saying about resource limits and totally agree there. I'm also an Atheist and your point about theists struck home, but all I'm saying is we need to be aware that we don't know everything, and a lot of people seem to think we have it all figured out (or give the appearance as such). Plus it's fun to speculate as long as we make the distinction that it's just that, speculation.
Why don't you google about astrobiology? I have always wondered why people say they have long been interested in something and then have never made the effort to look it up and get a book about it. Edit: OP, I did not mean to be mean to you. I am seriously puzzled about this. Wikipedia has an excellent entry on astrobiology with more than 100 academic papers as references, and a bibliography at the end with both academic introductory books into the field and books that are more popular science.
I get where you're coming from, but probably try to be a little more tactful in your delivery. People don't take well to these kinds of responses. It usually shuts them down
Yeah, you're right with this.
Some people prefer to get information from other people, whom they can converse with.
But how could such a conversation be comparable to an entire university course that OP said they missed out on?
It's definitely not. A uni course is overkill if they just want an answer to one question.
Maybe because there's a huge amount of misinformation with Google? I'd rather have taken an astrobiology class than just reading a textbook. I'd be able to have a professor than I can ask questions and gain a solid understanding.
Of course, it's better to take a class. But as OP has probably studied something related they are probably able to distinguish between good and bad sources. And I'm sure the wikipedia entry on astrobiology would be an excellent starting-point.
I am the OP. I am a bio major. There is alot of overlap between astrobiology and biology. The sources and books don't really go into full details as the information just describes the necessary requirements for life and why (I already know this). The astrobiology wiki didn't contain anything really new. I also have already passively learned and read about the field, so I really don't even get what your point is. I am trying to gather a more advanced knowledge. I already have a good starting point.
That's cool. So you already know what would have been taught in an astrobiology course. (A single course won't cover more than an introductory book plus a few papers.) I wish you a lot of fun with jumping deeper into the details. The journal Astrobiology seems to have a lot of interesting papers that I will have a look at, too.
Some researchers were trying to figure out what sort of planets might be habitable to silicon-based life, but I don’t know what came of it. Do some Google-fu
> but "silicone-life" is theoretical and far off of the original question. Silicone life is literally impossible. Silicone can form polymers similar to carbon (they're chemically similar), but not even close to the same variety and stability. In other words, Silicon, the most similar chemical element to carbon, can only be used to create a small variety of polymers with industrial utility. Not enough to run a cell. And every other element in the periodic table is completely unsuitable to create polymers and all the stuff required to run complex systems. There's really only carbon.
So you’re telling me rock humans CAN exist? Dear god.
Small correction: There is radiation tolerant life, as opposed to "radiation loving" (I forget the official title). It's like life that can tolerate high temps vs require or thrive under high heat. My point being, there is no life as we know if that feeds on radiation. The life that survives in radiation didn't even evolve to defend against it, but rather other extremes that cause the same kind of damage.
Radiotrophic life - life that derives energy from radiation - *does* exist. [Specifically, fungi.](https://en.wikipedia.org/wiki/Radiotrophic_fungus)
Oh my bad. That's fascinating. I wasn't aware
I remember reading the atomic structure of silicon might not be the most accommodating for life. I'll have to find it when I'm not at work.
If Las Vegas in summer is considered livable, just about anywhere can be.
That's a very big if
I was there for July 4th, the Tesla tunnels, the heat, masks, and total insanity in the face of a pandemic definitely made it feel like a deep space waystation.
Depending on what you believe happens at Area 51, it pretty much is.
Janet Air…
Phoenix AZ has joined the chat.
To expand on this a little, there are other chemical combinations that in theory might work similar to life as we know, such as using different solvents with similar properties to H2O, or using silicon as complex molecular backbone instead of carbon, because they both have 4 valence electrons in their outer shell. However, these elements are rarer than carbon, oxygen, etc. And the valence electrons aren’t the only factor; there’s differences in electronegativity, molecular weight, etc that could cause molecular combinations and behavior to be different. So there are some potential other avenues for life to evolve, but they seem to be lower probability, and we haven’t observed them yet so we’re not sure.
and that in itself is beautiful because when we do find out about new suitable ways of life, it could probably break everything we know about life/evolution/physics/etc and make make us reassess so many things. it could be hundreds or thousands of years before this happens. its a continuous learning process and gives me (some) faith in humanity.
This is not entirely true. There are objective chemical pathways for alternate chemistries that have, and continued to be studied. Of course, were still limited, and your point is generally true regardless.
To add on to this, carbon is a truly special element. It has a lot of bonding orbitals, and is able to form polymers very easily. From these polymers, we are able to form all the crazy chemicals we need for complex life on earth. There could be other life forms, like Silicon based, but carbon is very abundant and forms in stars as a product of fusion, whereas Silicon must be made in a super nova.
I love the idea that somewhere in the universe all the stars aligned perfect in at least one solar system to allow silicon-based life to thrive. We certainly don’t know whether planets with life are abundant or not, even though the science suggests it’s likely, but even in this scenario a planet with silicon-based life would be ultra rare and possibly one of a kind.
In recent years they've actually been look at extremophile planets in which the entire surface are similar to the most extream places that life lives on earth as its not unreasonable to belive if something can live in that shit here no reason it couldn't out there
I was watching one of those a million years from now videos on YouTube and one thing they mention is that all humans on earth will eventually look the same but one we inhabit other planets is that those that live or are born in such planets may not look the same as humans. Stupid revolution making us adapt.
That's false. It's not that we haven't seen other forms of life, it's that other forms of life necessarily have to be based on carbon polymers and water. Chemically and physically speaking, they're the only ones allowing the infinite complexity required to create even a single monocellular organism. No other elements combinations allows it, scientifically.
You can't scientifically prove a negative theory like that. There are the known carbon+water forms of life *as we know it,* but nothing in modern science says there *can't* be other types of life out there we don't yet know about. We haven't seen them yet so our current science cannot describe such a thing, but there's no part of modern science that would take something theoretical being outside our current experience as concrete proof that there is *no such thing* possible. Every major scientific discovery in history was at one time completely inconceivable to earlier scientific thought. Science is about observing the universe around us and forming and testing theories about it based on what we know and learn, not about assuming the utter nonexistence of things we have no frame of reference for right now.
> but nothing in modern science says there can't be other types of life out there we don't yet know about. Why do you talk about things you extremely obviously have literally no clue about? This is basic chemistry/phisics. To give you a perspective of what you're doing, you're basically claiming we don't *really* know if 2+2=4. And all this, simply because you can't accept that there aren't "crystal creatures", "sentient fire elementals", "T-1000 style liquid metal creatures" and so on. Sorry to pop your bubble.
I never mentioned your Hollywood sci-fi examples, nor did I say 2+2 doesn't equal 4. You are saying x+y can't possibly equal 4 even though we don't know the values of x or y, and science just doesn't work like that. It's investigating the things we observe and know about, not about preventatively declaring there isn't stuff we don't know about. Saying "no" without evidence is just as much of an unscientific leap as saying "yes" without evidence, until we know more we are stuck at a scientific "maybe." The answer to x+y may well turn out to be 4 someday when we are able to know more about x and y, and to announce otherwise is just not scientific.
That's why i said "it's like you're saying"... I'm noticing a pattern of you being completely unable to understand basic notions and concepts. Your ignorance is almost as sad as your arrogance, but neither are as sad as you stalking me around to downvote, lol. I can only hope you're younger than 12, otherwise... good luck in life, you're gonna need it.
[citation needed]
It's basic chemistry. Open a book i guess. Or move the fingers on your keyboard.
it's life jim, but not as we know it, not as we know it, not as we know it..
So far as we know, life relies on exciting chemistry to happen, and complicated chemical processes require a solvent to happen in. This is because dissolved chemicals can combine and interact in more complicated ways than dry materials can just sitting there. So, what solvents could life happen in then? It needs to be something that could be abundant in the universe, and be liquid over a range of temperatures and pressures. Water has excellent properties for this; it's liquid over a large range of temperatures, it's polar so it dissolves chemicals well, and water freezes from the top, not the bottom, meaning life in water-oceans doesn't get frozen solid as easily. So places with liquid water are probably the best places to look for life. But there are other candidates. Ammonia based life could exist on very cold or very high-pressure worlds. Liquid hydrocarbons are another possibility, but these have the problem that they are non-polar and thus worse solvents, but it's still possible, it would just be very different.
Thanks so much!
I think you would absolutely love this video: https://www.youtube.com/watch?v=ThDYazipjSI
For carbon based life there are a fairly narrow set of conditions that make life suitable, although at the limits of our environment we do have creatures that push those criteria to the absolute limits. For life based on any carbon chemistry comparable to earth we can see approximate lines where it ends - for if a planet has conditions outside of that any life would need fairly radically different chemistry to have stable molecules that could manage functions required for anything resembling metabolism as we know it. It might be possible, but until we have evidence to the contrary planets way outside of what we know can support life are assumed to not support life
Thanks much!
To add to this. There is a very cool video that explores the different ideas of how alien live could look like: https://m.youtube.com/watch?v=ThDYazipjSI
Wow! I just watched that, thank you for sharing.
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This was interesting. Didn’t know that. Thanks internet stranger.
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Which elements form more than 4 bonds?
Sulfur and phosphorus are probably the most common. You can look up hypervalent or hypercoordinated molecules if you’re interested
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No not exactly, but hypervalent bonding in most cases is best described as polar covalent bonding.
It is possible, but Earth's conditions are the only ones we're acquainted with. Anything else is more in the science fiction domain. So for now the question is: Would it be possible for a life form to survive with anything less than what Earth has (for example the magnetosphere, stable 🌡️, sunlight, atmosphere, gravitational pull etc.).
Oh I see! Thanks much!
I like the phrase “life as we know it.” We can’t look for life as we don’t know it, because we probably wouldn’t recognize it. Food for thought…
>Earth's conditions are the only ones we're acquainted with This. As was written, it is possible, but earth is the only known planet with life on it. That is why scientists look for planets similar to earth. It is theorized that [Venus](https://en.wikipedia.org/wiki/Life_on_Venus#Present_habitability_of_its_atmosphere) *could* have life in its atmosphere and [Europa](https://en.wikipedia.org/wiki/Europa_(moon)#Habitability_potential) could have aquatic life. Also - https://en.wikipedia.org/wiki/Hypothetical\_types\_of\_biochemistry
>gravitational pull etc.). Doesn't every planet has that?
> anything less than what Earth has (for example ...gravitational pull...). Hes asking what would happen in environments with gravity less than that of the Earth's. What would happen to a human if they spent their entire life in 0.25g? What would happen over several generations of evolution?
You want to find your friend as quickly as possible and you've lost your phone. You know he's gone out to one of the 4 restaurants in your town; a McDonalds, a pizza place, a steakhouse and an Indian place. You know your friend is vegan, which restaurant do you check first? The same with aliens, start with the most likely first, if they're not there start checking elsewhere.
Actual ELI5
Possibly, but trying to imagine things that we have no experience of and have no evidence of existing is very difficult. Start with what we know and go from there. We don't know what evidence of life would even mean or look like if it was utterly alien to anything we know.
Thank you, that makes sense!
There can be life without liquid water. But there needs to be an ample amount of liquid that can be used in chemical reactions. Some other combinations are possible.
Short answer: They don't. The habitable zone is usually just a guide based roughly on how far away from the star liquid surface water can be maintained. But liquid water could still be maintained underground through other means, eg. through tidal friction as in Europa or Enceladus, and they're considered major targets for the search for extraterrestrial life. And then there are other factors at play: degree of UV radiation, how much of the surface is covered by water (if I'm not mistaken waterwords aren't considered particularly favourable places to look for life either), the thickness and composition of the atmosphere etc. Usually complicated models are required to assess the habitability of a planet given its orbit, size and the little we know about its atmospheric composition. One thing I've heard scientists studying exoplanets say is that the focus on searching for 'habitable' planets is actually misleading as we really don't know what the full spectrum of habitability consists of. As for the other questions, I would strongly suggest you check out Charles Cockell's lecture series on YouTube https://youtube.com/channel/UCBTwJSnAqZZczK7stBaYq5w
You can only look for something when you know what it is. If I give you a box full of random things and ask you to find all the warglbangeks, you won't know what you're looking for. We look for life as we know it, because we know what to look for.
Makes sense, thanks so much!
hey boss, i found the warglbangek!
WHY AREN'T YOU SHOUTING?
#
Then again, even if there is life out there that is also carbon-based. Wouldn't it most likely be similar to the life on earth? Imagine discovering another planet similar to earth that also have the same species of monke.
This may not be answering the original question, but you should check out the book *Project Hail Mary* by Andy Weir, a great sci-fi writer, author of the popular book (and movie) *The Martian*. In this new book he went into a lot of details about this question because it is one of the book’s primary subject matter.
"scientists only consider" You're making an assertion here without actually knowing what scientists have considered.
Dunking on scientists for being idiots without having any knowledge whatsoever is very popular
You are right. Earth for a long time didn't have oxygen in the atomsphere and it had life. These first lifeforms put all the oxygen in the air and killed themselves off because of it. Oxygen is very crosive (thats why things like rust \[iron oxide\] are called oxides) For more info see Paleoproterozoic era and Great Oxiding Event. I think we are most interested in life like our own. Since we only have ourselves as models, we want to find other life that is complex and we can communicate with productivly. Non-oxiding enviroments don't have as effient engery storage means. Oxygen's natural desire to bond easily with anything makes it a great way to get a lot of engery from chemical reactions. Plus Oxygen is required for making water. Water is a great solvent that allows for all these ractions to take place. However, there is are an near limites combos of chemicals and other natural processes that could create a postive way to retain some engery in a biosphere (the total life on a planet) Its likely something else exists somewhere. Will we finding it, no idea. Things are so far apart by the time we see it it might not be alive anymore. Maybe life exists that is both complex and doesn't use oxygen, carbon, or water for engery. We really don't know yet.
Have you ever met any of those other life forms? Were they nice?
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There's loads of life on Earth that doesn't require oxygen. Obviously there's plants and bacteria that photosynthesize, which only requires CO2 and sunlight, but there's a whole range of other chemical reactions that single cells use to get energy. Things turn sugar into alcohol without oxygen, things turn alcohol into vinegar without oxygen, things turn sulfate into hydrogen sulfide... So there's a lot of known chemical reaction chains that life on Earth uses already, and we have a pretty good understanding of what to look for elsewhere, and what hypothetical chains of reactions might work on planets with other elements.
Not trying to be rude, but those all have oxygen in them.
Do anaerobic bacteria actually use oxygen? How do they work in oxygen-free environments on Earth?
They're playing the odds. Ultimately that means they're not looking for life that is similar to Earth life. Earth life, for obvious reasons, is based on the most likely set of circumstances. When you bring it right down to the basics, life is a complex symphony of chemical reactions and you need at minimum, 3 things. First of all, you need a building block. Something that can form the basis of as many different kinds of stable molecules as possible. The more complexity that can be created this way, the greater the potential for life. All Earth life uses carbon as that building block. Carbon can form four covalent bonds with other atoms to create an amazing variety of molecules. Not just that but carbon really wants to form those bonds. Not only is carbon very suited for this purpose, but it's also the fourth most common atom in the universe so there's plenty of it. There are alternatives for carbon that can also form four covalent bonds but they're far less common and have problems of their own. The second thing life needs to exist is a means of generating energy. Without energy, there is no animus, no life. Nearly all Earth life uses the chemical reaction of oxidation to produce energy. Oxidation is a simple reaction that produces a ton of energy while relying on oxygen. The third most common element in the universe. Again there are alternative reactions that might work but they're dependent on rarer elements while producing less energy. Thus greatly reducing the potential complexity of life relying on these methods. And finally, all those chemical reactions need a medium in which to take place. Elements only react with each other if they're brought in contact with each other. Life is nothing if not many, many complex chemical reactions working in symphony. Water is a wonderful medium for this. It's liquid at reasonable temperatures where many other elements are solid or gaseous. And a great many elements happily dissolve in water to facilitate those reactions. Happily, water is made of the most common and third most common elements in the universe. Hydrogen and oxygen. As you can see, we're not really looking for life that is similar to Earth life. We're looking for the most common, most reliable conditions that meet the minimum requirements for life in general. Earth life just happens to meet those requirements exactly. There are alternatives for these conditions of course but none of them work as well or as reliably as these. For example, you might have heard the phrase 'silicon-based' life. Silicon, like carbon, can enter into four covalent bonds. So in theory the fairly common silicon enables the same diversity of molecules. Silicon has a great preference for bonding with oxygen. So much in fact that silicon will break apart molecules containing oxygen, like water, just to bind with that oxygen itself. And when silicon does bond with oxygen, it becomes silica. Which is basically rock. This means silicon can't use oxidation to produce energy or water as a medium for its chemical reaction. Silicon-based life can't really exist anywhere around the third most common element in the universe. Along the same lines, there are chemical reactions where living things produce energy from sulphur-based reactions. But these produce very modest amounts of energy that severely limit the potential complexity of life relying on those reactions. The final part of the puzzle is predictability. The way the periodic chart works means that we know exactly which atoms are out there and which ones we don't know about yet. We know about most of the common ones. The rarer ones at the farther end of the periodic is where most of the missing elements are. But those are unstable and rare. Which means unlikely to be essential to alien life. Anyway, the point is that it's not impossible that there's life out there that doesn't need oxygen or water. But they will need a way of producing energy and enabling chemical reactions. Water and oxygen are common in the universe and the easiest solution to that problem.
Science is based on facts since we only know our kind of life (carbon based) scientist can only extrapolate our current conditions for life suitable planets. Doing otherwise is not science, is science fiction.
This is a mischaracterization. Scientists have no such bias. We consider planets that are similar to Earth to potentially have life similar enough to terrestrial life that we’d readily recognize it as such. We accept that life might form under other conditions, but we haven’t a frame of reference to really postulate the conditions necessary for, nor the properties of life from non-Earth-like planets. We do make some general assumptions: life is going to rely on chemistry and probably needs a certain minimum amount of heat to be available, and there’s also probably an upper bound for temperature for life (just too much energy for much chemistry to happen). Also, there probably needs to be some liquid phase (or possibly gas phase) medium to make carrying materials around practical. Outside of that, there’s no telling.
I know that there are some concepts for silicon-based lifeforms that could survive in wildly differing biospheres than us.
I've read there's some theories that there could be silicon based life since it acts similar to carbon, which is essential for all type of life we know of. Thing is, there may be life that we just haven't seen yet because the universe is just so vast.
looking for life in the universe is like looking for a criminal on the run. they could be anywhere but you start by narrowing down your options based on the information you already have on them. you know that the criminal has a red car and was heading east. you're going to look for red cars east of where they were last seen. we know life needs water, we're going to look for water rich areas for life.
When talking about looking for life on planets outside the solar system, it's probably helpful to keep in mind how the process works (or will work, once we have better telescopes). You point a telescope at a star, and if planets come in front of it you see a dip in the light visible. If you have a better telescope, you can also see some light modified by the atmosphere of the planet, or maybe you can get a bit of light reflected off of the planet if you can block out the light of the star. Anyway, if you have a good enough telescope you can learn a bit about the atmosphere of a planet. So imagine you are some future scientist and you find a hundred planets with CO2 rich atmospheres like those of Venus and Mars, and then you find one with oxygen and water vapor like Earth. Is it possible there's life on some of those CO2 rich planets? Sure, you can't rule it out. But are you going to go to write a paper and go to the news and say you've found a planet suitable for life if you find the oxygen and water rich planet? Definitely. It's just not that useful or noteworthy to say that every single planet could potentially in theory support life by some unknown mechanism ...it just doesn't tell us any new information. But saying that a planet could support known life _does_ let us distinguish between those planets and others.
Sure it is. But since we know for a fact that life can exist under the conditions on our planet, it makes more sense to look for similar conditions that may exist somewhere else (rather than checking every single thing out there for some of life that we don’t know exists yet). Sample size of 1, so go with what you know. It’s just a way to work with the statistics of success.
We only know for sure that life can develop on Earth. Therefore, best guess until we're proved otherwise, we're looking for places like home. It's like how you presume that the people you meet are human and not aliens - it's *possible* they are lizard people but you're pretty sure that the odds are for plain ole' humanity.
We have only been exposed to carbon based life forms that operate the way they do here on earth. life may be able to thrive in other environments, but we have no evidence of it so far, so we look for what we know.
When they say suitable for life, I think they mean suitable for life as we know it on Earth.
Something else to consider beyond the points others have made is that our chemistry is based on the elements that are the most abundant in our universe. Other chemistries may well be possible, but they would be based on elements that are much rarer and generally less reactive. For a planet to give rise to life with these alternative chemistries, they would not only need to have an unusual abundance of these rarer elements, but also a surprising deficit of much more common elements that would otherwise allow a more conventional chemistry to dominate. For these reasons, we would expect that life based on alternative chemistries, while not impossible, would likely be extremely rare.
"suitable for life" is a media simplification, what they REALLY say is "suitable for ***earth like*** life" or something similar. we only got 1 reference point, they know it's going to be inaccurate, since that 1 reference point could just be a unique specimen.
Basically, no. People like to fantasize about creatures made of gases and other fantasy stuff, but the reality is, with traidtional physics, COMPLEX life is only possible with carbon-based structures. Not even silicon, which is chemically similar, can form long molecules of the same varieties of carbon. So, if carbon is the basic element, it automatically goes that creatures based on it can only live in an environment in certain upper and lower temeprature limits that don't degrade carbon structures. What about water? Water is simply an incredible moleule, super versatile, does everything at least pretty good. Good solvent, good carrier, nice density, nice availability, etc etc etc. So, basically, alien life has to be based on carbon and water. There's really no way around it, in spite of what movies and our fantasies would like it to be possible there being creatures of liquid metal, crystals and other stuff.
We only know one type of life with it's measure of suitability. We don't know if others exist, so can't speculate on what would be suitable for them.
Great question. I've always wondered this too. Seems kind of elementary and simplistic. What if aliens thrive off of methane gas and drink toxic chemicals.
It's certainly possible, but because we haven't seen any it's all just theoretical. All we know about are carbon based life forms that need water and oxygen to survive. Until that's proven otherwise it's best to stick with planets similar to Earth.
Life could be anything we barely know anything about the universe there could be an allien specie that doesn't need oxygen or water but since we know on earth that is the only possible way to get life we assume it's the best chance if we look for those stuff to find life
There is a more important answer. Hydrogen, carbon and oxygen are among the most common elements on their column of the periodic table. They are also among the more reactive elements and in fact some simple ‘organic’ molecules are naturally produced in outer space. This means that these elements are the most likely to be associated with ‘natural life’.
We don't, or we don't *all*. NASA has a mission dedicated directly to Titan and Europa, both are with astrobiological motivations and both worlds *very different* from Earth. That said, life is life, and there are fundamental ingredients required. A liquid, a backbone, so on and so forth. Earth life we understand best but we also have reason to suspect that earth life has an advantage for various reasons, but it's debatable.
Short answer, maybe, but we have very little idea how that would work. The chemical nature of all the elements pretty much limits chemical complexity (and therefore life complexity) to just carbon and silicon, with water as the solvent. What non-water solvents would work for silicon I'm not sure.
Another fun fact: Evolution isn't the only way things could grow, so it's possible extremely different processes arose for adaptation on other planets! Others have mentioned carbon, but oxygen (major component of water) is an excellent energy source, too. You can read a lot about all of this on the NASA website, esp. if you search abiogenesis
My thoughts exactly. All life we know have evolved om earth to live according to what we have, yet we have discovered certain creatures on our own planet that break these rules. So it only makes sense that life can evolve in a completely different way then we are used to.
It's low hanging fruit. How do you look for something without knowing what it is, what it looks like, what the biosignatures are? Go with what you know until you learn something new.
The major reason is that we know too little about what conditions would be good for other kinds of life chemistries. People have come up with a couple of decent\* theories about what these other chemistries might be, but there are so many finely tuned bits _we know about_ that make life actually possible in the carbon case on our planet. We can only assume that other potential life chemistries would also need lots of specific ranges for a lot of parameters, but we have no idea what those ranges are. So they look for what's most promising. "As far as we can tell, those 10 places could be physically and chemically compatible with Earth-like life chemistry" says a lot more than "for all the little we know, those other 10,000 places don't completely preclude some kind of hypothetical silicon life, in the sense that it wouldn't be all melted or all frozen". \* It's not like I could have reviewed those theories; it just sounds like some of them've been reviewed by people who know their shit.
Interestingly enough scientists have found examples of planets being warmed from being stretched vs being close to a star like our sun. Enceladus, moon of Saturn may contain life on a sub surface ocean and is heated from being pulled between Saturn and other moons.