5 ms·

the habitable zone, defined as the distance from a star at which you could expect temperatures on a planetary surface consistent with the presence of liquid wat

by stinos 3y ago

the habitable zone, defined as the distance from a star at which you could expect temperatures on a planetary surface consistent with the presence of liquid water I know nothing about the search for extraterrestrial lifeforms, but isn't using this definition extremely limiting? Or are the various aspects of water really such that it's extremely unlikely any other lifeform not using water could exist? Like, based on another fluid? Or maybe just no fluid at all? Or is the issue that we wouldn't be able to detect another lifeform as being one if it's not water-based?

vjjsejj 3y ago
> but isn't using this definition extremely limiting? So instead for searching something specific (we sort of understand) we should spend time and resources looking literally for nothing in particular? Also I don’t non-carbon/water lifeforms are at all compatible with our understanding of biology and chemistry?
- stinos 3y ago
  looking literally for nothing in particular That's not what I said. My last question covers this exactly: are there metrics other than 'conditions like we know them' which can be used? Maybe the answer is just 'none that we know of', again: no idea, and yes then one cannot look for it. But here on earth we can distinguish between a rock and a cell based on various metrics. They way they reflect light is perhaps different enough to tell that one is a relatively simple solid consisting of chemical bonds which are all alike, whereas the other one isn't. Or processes in the cell make the atmosphere slightly different. Now suppose some other lifeform also consists of cells but instead of water-based they use something else which happens to be liquid at 200 degrees Celcius. A rock can still look like a rock at that temperature and maybe won't alter atmosphere. Maybe something living would? Also I don’t non-carbon/water lifeforms are at all compatible with our understanding of biology and chemistry? It's not because we cannot understand it that we cannot observe it. In fact that's usually how it goes: first observ, then understand. Isn't that basically what we're doing already with a lot of telescopes/wide band radiometry/... ? Looking for things without understanding how they can exist or where they came from (the OMG particle etc)
- jerf 3y ago
  I have never seen a convincing basis for life that isn't carbon. The next best candidate is silicon, but it seems like the problems are unlikely to be overcome. Non-water may be just about possible: https://en.wikipedia.org/wiki/Hypothetical_types_of_biochemistry#Non-water_solvents https://en.wikipedia.org/wiki/Hypothetical_types_of_biochemi... Non-water solvents may run at somewhat different temperatures. However, strongly showing that an alternative is possible is a super, super uphill battle. If we weren't made of carbon and water, I'm not sure we could conclusively show that life could be built on it. But we probably couldn't disprove it. Ammonia is probably at least in the "not disproved" category. But I think most everything else is. If you seriously think about it, beyond the "my mind is so open it's falling right out", it's a pretty tall bar to find an alternative. (If you don't think so, spend some more time with organic chemistry, and consider why we call it that. There is a reason life is based on it, and there's a reason we cleave the entire chemistry world into two pieces along that division.) It is also in my opinion not that interesting a question ultimately. The big jump is the one from zero possible life substrates to one. Having a second one doesn't really change much, e.g., I don't see any compelling reason to believe there would be an "ammonia solvent" view of the universe versus a "water solvent" view of the universe or something.
dcminter 3y ago
If you have a strong theory about how silicon based life might arise then of course you could look for signs of that. What are those signs? Well, ... that depends on your theory. Versus carbon based water dependent life? We know it exists in at least one form and what its characteristics are. So we have plenty of candidates for "signs of life" in that form. It doesn't mean it's the only or even best way to search, it just means it's the one we know how to begin! Edit: You might also enjoy reading about this semi-related topic: https://en.m.wikipedia.org/wiki/Anthropic_principle https://en.m.wikipedia.org/wiki/Anthropic_principle
- daltont 3y ago
  The idea of silicon based life reminds of this old B-movie: https://www.youtube.com/watch?v=XMPLzowjKQc https://www.youtube.com/watch?v=XMPLzowjKQc
  - nightfly 3y ago
    No bones?!?
kuchenbecker 3y ago
(I am not a scientist) the assumotion is Earth is an average lifebearing plant until proven otherwise. If you find evidence of something, you are more likely to find it in the middle if the normal distribution. The problem though is choosing which population we are "average" in. Unfortunately we have a single sample for life. Its actually entirely possible we are literally the only intelligent life (or even Earth being the only planet with life) if you define the population as "life capable of wondering if they're alone in the universe". This is the anthropic principle [1] which is an observer bias implies the universe supports observers. Coupled with the fact a spacefaring civilization at 1% the speed of light could colonize the galaxy in about a million years (0.007% age of the universe) this is decently strong statistical evidence intelligent life is exceedingly rare (Fermi paradox). But to answer more directly, Carl Sagan convinced Nasa to look for life on earth. [2] We assume we are typical therefore we try and replicate this with observations of other planets. [1] https://en.m.wikipedia.org/wiki/Anthropic_principle https://en.m.wikipedia.org/wiki/Anthropic_principle [2] https://www.nature.com/articles/d41586-023-03230-z https://www.nature.com/articles/d41586-023-03230-z
- chomskyole 3y ago
  >This is the anthropic principle [1] which is an observer bias implies the universe supports observers. Coupled with the fact a spacefaring civilization at 1% the speed of light could colonize the galaxy in about a million years (0.007% age of the universe) this is decently strong statistical evidence intelligent life is exceedingly rare (Fermi paradox). Wouldn't logically follow only that it is exceedingly rare that anyone colonises a galaxy? I mean there are a lot of assumptions in that statement: It only makes intelligent life rare if you assume that intelligent life wants to colonise a galaxy and maintains the capacity to travel at 1% of the speed of light (which is still pretty fast) for 1 million years, while maintaining the desire to colonise a galaxy. Maybe after 10000 years they go "Ahhh, enough Manifest Destiny... I'll go back home...." :)
poulpy123 3y ago
It's not limiting, it's in our current knowledge the only way for life to appears. Of course, our knowledge may be limited and other ways may be discovered one day, but it the meantime it is necessary to focus time, money and energy on things that have a provable chance to succeed
retube 3y ago
Well, for life to emerge you likely need some kind of liquid medium/solvent to enable chemicals to move around and interact with each other. It's hard to imagine how that could happen in a solid/frozen environment. So, assuming a liquid solvent as a basic requirement, the next question is, what sort? There are other options beyond water, but water actually some unique properties that make it a potentially ideal medium - not least it's less dense when frozen. This meant the seas in early earth never completely froze, with liquid water remaining under the surface. If ice was heavy than water, then ice would sink, the oceans would likely have completely frozen, and no life would have evolved. More discussion here: https://sitn.hms.harvard.edu/uncategorized/2019/biological-roles-of-water-why-is-water-necessary-for-life/ https://sitn.hms.harvard.edu/uncategorized/2019/biological-r...
- ghaff 3y ago
  Ice is a really interesting material in a number of ways--especially in conjunction with liquid water. I worked in a very different area that he was also researching at the time but my thesis advisor went on to create an ice lab and was probably one of the world experts on ice. (The US Army, especially in the 80s, was very interested in all sorts of things about cold regions--for obvious reasons.)
otikik 3y ago
Once life gets intelligent enough we may be able to detect it by other means. For example, they would likely use electromagnetic waves to communicate, just like we do[1]. That's based on the physics on the constraints set by our physical universe, not on our biology. If a civilization gets really advanced they might be able to do alterations to their own star that we would also be able to see. They might build megastructures around them, changing their brightness in unusual ways [2]. [1] https://ui.adsabs.harvard.edu/abs/2007JCAP...01..020L/abstract https://ui.adsabs.harvard.edu/abs/2007JCAP...01..020L/abstra... [2] https://www.universetoday.com/153603/a-new-way-to-detect-alien-megastructures/ https://www.universetoday.com/153603/a-new-way-to-detect-ali...
- NoMoreNicksLeft 3y ago
  Non-microbial life might be plainly obvious visually. You expect it to move around, and it's probably not going to look like a landslide or the wind blowing. Even if it's sessile at the bottom of some ammonia ocean trench, who will look at it and think that it's just a geological feature? When we talk about it being difficult to detect, we're talking about the microscopic stuff, aren't we? Or trying to notice it from interplanetary/interstellar distances, I suppose.
  - otikik 3y ago
    > trying to notice it from interplanetary/interstellar distances That's the difficult part. "Visually" means different things when dealing with those distances. Often what we get from a whole star system is the equivalent of a few pixels worth of light. That's literally all we have to work with. If we are lucky we might be able to detect a planet orbiting the star if it happens to partially cover the star while we are looking at it. Still, it is surprising the amount of information that astrophysics can get from such a meager amount of data.
    - floxy 3y ago
      Maybe someday... Direct Multipixel Imaging and Spectroscopy of an Exoplanet with a Solar Gravity Lens Mission https://www.youtube.com/watch?v=NQFqDKRAROI https://www.youtube.com/watch?v=NQFqDKRAROI https://arxiv.org/abs/1802.08421 https://arxiv.org/abs/1802.08421
ravenstine 3y ago
Yes, it's limiting, but that's because anything outside of that definition of life would be purely speculative. On the other hand, we not only have an example model of how life can work, but reason to believe that other forms of life may be similar. Both carbon and oxygen have relatively unique traits that make advanced life possible without extreme inefficiency or spontaneous combustion. Why isn't there life that relies on other gases like fluorine chlorine to perform oxidization? Besides the fact that oxygen is presently far more abundant than those other gases, oxygen (specifically dioxygen or O2) has very interesting and important characteristics; it has a highly energetic double-bond that's easier to break apart than other similar molecules, and it has both a stable state (triplet ground) and a highly reactive state (singlet). Without getting into the weeds, this has to do with the quantum property of the "spin" of its electrons. Life, including the human body, uses some neat tricks to be able to transport oxygen and switch it to be highly reactive where it's needed, particularly in the Krebs/TCA cycle. If oxygen didn't have both a stable and a reactive state (while being the same molecule!), it would either be a less effective oxidizer or be too reactive to be safely transported biologically, let alone to be exposed to. So there's reason to search for life in parts of the universe we know to have oxygen – it's really an amazing molecule. Carbon might be more replaceable than oxygen (IMO), but even it so far seems to have the most advantages to life in contrast to other molecules, from what I understand. I'm sure someone will correct me on this (which I absolutely welcome!), but carbon is advantageous for life because it is almost an opposite of oxygen; it will accept multiple bonds with other molecules but doesn't need those bonds so badly that it needs to steal electrons from other molecules (oxidization) or go out of its way to lend its electrons, causing a redox reaction. It's a chill ass molecule that's really good for being a building block for far more complicated molecules. Now let's talk about water. It's actually a really good solvent and can dissolve a wide range of substances, thus making it effective as a carrier of many things. Water is a polar molecule due to the way it has an oxygen end and a hydrogen end (think of the way an H20 molecule is normally represented as being triangular), which allows it to interact with other polar molecules (including other water molecules) and reject nonpolar molecules. At the same time, it's capable of interacting with some nonpolar molecules. By being polar, it forms hydrogen bonds with other molecules. These properties are important for things like the creation of proteins, for example. Water is also a side effect of organic chemistry; both respiration and the actual burning of organic material results in CO2 (and other carbon molecules) and water. Water stores heat very well, and temperature may be an inherently limiting factor to where organic life of any kind can thrive. Oh yeah, and it also has a neutral pH. Though it doesn't conduct electricity on its own, a solution of a water and an electrolyte allows it to conduct electricity. The ability to create balances of electrolytes is another critical use of water in organic life. I'm missing a bunch of things, but the picture I'm trying to paint for you is that molecules such as water, carbon, and oxygen, aren't simply replaceable in the only example of life that we know of. Their role isn't by mere happenstance, but because they have inherent advantages over their alternatives. Searching for life elsewhere in the universe is an expensive endeavor. While we shouldn't be close-minded to other models of life, searching for life similar to our own is simply more efficient from a logical perspective. It would be one thing if Earth was the only object circling a star that has these important molecules, but we know that it's not. Something to consider is just how similar life is on our own planet. We like to think of some animals like octopuses as "alien", and while they are indeed strange in many ways, they still share properties with other animals. Take for instance the fact that octopuses have two eyes, just like other animals; in fact, the vast majority of animals have two eyes. Even spiders, which have more than two eyes, usually have eyes arranged in two rows, and many spiders have two eyes that are more dominant than the others. Eyes have evolved multiple times independently in the tree of life. While it's an interesting idea that extraterrestrials might have multiple eyes, there's reason to believe that, if we were to meet extraterrestrials that evolved on a similar planet to Earth, they would feature two eyes. Although we might not completely understand why evolutionary pressure hasn't widely promoted more than two eyes, it still appears to have an inherent advantage. This same line of thinking is why scientists have focused on searching for life similar to our own; there are properties to our form of life that are based on environments and traits that have inherent advantages. The evidence suggests that the way our life has arranged itself isn't merely a jumbling of various things that happen to function together.
simiones 3y ago
There are several aspects that make the search for non-water based life untenable. Multiple comments are pointing out the practical aspects. A separate aspect that is very important to know is that inorganic chemistry (non-carbon chemistry) is actually quite limited overall: we have a pretty good grasp of the possible inorganic compounds and their properties, at least within certain ranges of temperatures and pressures. And a huge number of those compounds require water to be present in some way. Now, organic chemistry is far richer and far harder to explore thoroughly. But, given all we know about inorganic chemistry, creating organic compounds in ways that don't involve the known inorganic ones, and doing so without water, seems extremely unlikely.
jrumbut 3y ago
The point of the rule is to be extremely limiting. There are billions and billions of stars, and it's becoming clear that a lot of them are orbited by planets. Given that there aren't obvious alien visitors or even communications reaching us regularly, it seems to be a needle in a haystack situation.
deleted 3y ago
[deleted]

by stinos 3y ago

vjjsejj 3y ago

> but isn't using this definition extremely limiting? So instead for searching something specific (we sort of understand) we should spend time and resources looking literally for nothing in particular? Also I don’t non-carbon/water lifeforms are at all compatible with our understanding of biology and chemistry?

stinos 3y ago
looking literally for nothing in particular That's not what I said. My last question covers this exactly: are there metrics other than 'conditions like we know them' which can be used? Maybe the answer is just 'none that we know of', again: no idea, and yes then one cannot look for it. But here on earth we can distinguish between a rock and a cell based on various metrics. They way they reflect light is perhaps different enough to tell that one is a relatively simple solid consisting of chemical bonds which are all alike, whereas the other one isn't. Or processes in the cell make the atmosphere slightly different. Now suppose some other lifeform also consists of cells but instead of water-based they use something else which happens to be liquid at 200 degrees Celcius. A rock can still look like a rock at that temperature and maybe won't alter atmosphere. Maybe something living would? Also I don’t non-carbon/water lifeforms are at all compatible with our understanding of biology and chemistry? It's not because we cannot understand it that we cannot observe it. In fact that's usually how it goes: first observ, then understand. Isn't that basically what we're doing already with a lot of telescopes/wide band radiometry/... ? Looking for things without understanding how they can exist or where they came from (the OMG particle etc)
jerf 3y ago
I have never seen a convincing basis for life that isn't carbon. The next best candidate is silicon, but it seems like the problems are unlikely to be overcome. Non-water may be just about possible: https://en.wikipedia.org/wiki/Hypothetical_types_of_biochemistry#Non-water_solvents https://en.wikipedia.org/wiki/Hypothetical_types_of_biochemi... Non-water solvents may run at somewhat different temperatures. However, strongly showing that an alternative is possible is a super, super uphill battle. If we weren't made of carbon and water, I'm not sure we could conclusively show that life could be built on it. But we probably couldn't disprove it. Ammonia is probably at least in the "not disproved" category. But I think most everything else is. If you seriously think about it, beyond the "my mind is so open it's falling right out", it's a pretty tall bar to find an alternative. (If you don't think so, spend some more time with organic chemistry, and consider why we call it that. There is a reason life is based on it, and there's a reason we cleave the entire chemistry world into two pieces along that division.) It is also in my opinion not that interesting a question ultimately. The big jump is the one from zero possible life substrates to one. Having a second one doesn't really change much, e.g., I don't see any compelling reason to believe there would be an "ammonia solvent" view of the universe versus a "water solvent" view of the universe or something.

dcminter 3y ago

If you have a strong theory about how silicon based life might arise then of course you could look for signs of that. What are those signs? Well, ... that depends on your theory. Versus carbon based water dependent life? We know it exists in at least one form and what its characteristics are. So we have plenty of candidates for "signs of life" in that form. It doesn't mean it's the only or even best way to search, it just means it's the one we know how to begin! Edit: You might also enjoy reading about this semi-related topic: https://en.m.wikipedia.org/wiki/Anthropic_principle https://en.m.wikipedia.org/wiki/Anthropic_principle

daltont 3y ago
The idea of silicon based life reminds of this old B-movie: https://www.youtube.com/watch?v=XMPLzowjKQc https://www.youtube.com/watch?v=XMPLzowjKQc
- nightfly 3y ago
  No bones?!?

kuchenbecker 3y ago

(I am not a scientist) the assumotion is Earth is an average lifebearing plant until proven otherwise. If you find evidence of something, you are more likely to find it in the middle if the normal distribution. The problem though is choosing which population we are "average" in. Unfortunately we have a single sample for life. Its actually entirely possible we are literally the only intelligent life (or even Earth being the only planet with life) if you define the population as "life capable of wondering if they're alone in the universe". This is the anthropic principle [1] which is an observer bias implies the universe supports observers. Coupled with the fact a spacefaring civilization at 1% the speed of light could colonize the galaxy in about a million years (0.007% age of the universe) this is decently strong statistical evidence intelligent life is exceedingly rare (Fermi paradox). But to answer more directly, Carl Sagan convinced Nasa to look for life on earth. [2] We assume we are typical therefore we try and replicate this with observations of other planets. [1] https://en.m.wikipedia.org/wiki/Anthropic_principle https://en.m.wikipedia.org/wiki/Anthropic_principle [2] https://www.nature.com/articles/d41586-023-03230-z https://www.nature.com/articles/d41586-023-03230-z

chomskyole 3y ago
>This is the anthropic principle [1] which is an observer bias implies the universe supports observers. Coupled with the fact a spacefaring civilization at 1% the speed of light could colonize the galaxy in about a million years (0.007% age of the universe) this is decently strong statistical evidence intelligent life is exceedingly rare (Fermi paradox). Wouldn't logically follow only that it is exceedingly rare that anyone colonises a galaxy? I mean there are a lot of assumptions in that statement: It only makes intelligent life rare if you assume that intelligent life wants to colonise a galaxy and maintains the capacity to travel at 1% of the speed of light (which is still pretty fast) for 1 million years, while maintaining the desire to colonise a galaxy. Maybe after 10000 years they go "Ahhh, enough Manifest Destiny... I'll go back home...." :)

poulpy123 3y ago

It's not limiting, it's in our current knowledge the only way for life to appears. Of course, our knowledge may be limited and other ways may be discovered one day, but it the meantime it is necessary to focus time, money and energy on things that have a provable chance to succeed

retube 3y ago

Well, for life to emerge you likely need some kind of liquid medium/solvent to enable chemicals to move around and interact with each other. It's hard to imagine how that could happen in a solid/frozen environment. So, assuming a liquid solvent as a basic requirement, the next question is, what sort? There are other options beyond water, but water actually some unique properties that make it a potentially ideal medium - not least it's less dense when frozen. This meant the seas in early earth never completely froze, with liquid water remaining under the surface. If ice was heavy than water, then ice would sink, the oceans would likely have completely frozen, and no life would have evolved. More discussion here: https://sitn.hms.harvard.edu/uncategorized/2019/biological-roles-of-water-why-is-water-necessary-for-life/ https://sitn.hms.harvard.edu/uncategorized/2019/biological-r...

ghaff 3y ago
Ice is a really interesting material in a number of ways--especially in conjunction with liquid water. I worked in a very different area that he was also researching at the time but my thesis advisor went on to create an ice lab and was probably one of the world experts on ice. (The US Army, especially in the 80s, was very interested in all sorts of things about cold regions--for obvious reasons.)

otikik 3y ago

Once life gets intelligent enough we may be able to detect it by other means. For example, they would likely use electromagnetic waves to communicate, just like we do[1]. That's based on the physics on the constraints set by our physical universe, not on our biology. If a civilization gets really advanced they might be able to do alterations to their own star that we would also be able to see. They might build megastructures around them, changing their brightness in unusual ways [2]. [1] https://ui.adsabs.harvard.edu/abs/2007JCAP...01..020L/abstract https://ui.adsabs.harvard.edu/abs/2007JCAP...01..020L/abstra... [2] https://www.universetoday.com/153603/a-new-way-to-detect-alien-megastructures/ https://www.universetoday.com/153603/a-new-way-to-detect-ali...

NoMoreNicksLeft 3y ago
Non-microbial life might be plainly obvious visually. You expect it to move around, and it's probably not going to look like a landslide or the wind blowing. Even if it's sessile at the bottom of some ammonia ocean trench, who will look at it and think that it's just a geological feature? When we talk about it being difficult to detect, we're talking about the microscopic stuff, aren't we? Or trying to notice it from interplanetary/interstellar distances, I suppose.
- otikik 3y ago
  > trying to notice it from interplanetary/interstellar distances That's the difficult part. "Visually" means different things when dealing with those distances. Often what we get from a whole star system is the equivalent of a few pixels worth of light. That's literally all we have to work with. If we are lucky we might be able to detect a planet orbiting the star if it happens to partially cover the star while we are looking at it. Still, it is surprising the amount of information that astrophysics can get from such a meager amount of data.
  - floxy 3y ago
    Maybe someday... Direct Multipixel Imaging and Spectroscopy of an Exoplanet with a Solar Gravity Lens Mission https://www.youtube.com/watch?v=NQFqDKRAROI https://www.youtube.com/watch?v=NQFqDKRAROI https://arxiv.org/abs/1802.08421 https://arxiv.org/abs/1802.08421

ravenstine 3y ago

Yes, it's limiting, but that's because anything outside of that definition of life would be purely speculative. On the other hand, we not only have an example model of how life can work, but reason to believe that other forms of life may be similar. Both carbon and oxygen have relatively unique traits that make advanced life possible without extreme inefficiency or spontaneous combustion. Why isn't there life that relies on other gases like fluorine chlorine to perform oxidization? Besides the fact that oxygen is presently far more abundant than those other gases, oxygen (specifically dioxygen or O2) has very interesting and important characteristics; it has a highly energetic double-bond that's easier to break apart than other similar molecules, and it has both a stable state (triplet ground) and a highly reactive state (singlet). Without getting into the weeds, this has to do with the quantum property of the "spin" of its electrons. Life, including the human body, uses some neat tricks to be able to transport oxygen and switch it to be highly reactive where it's needed, particularly in the Krebs/TCA cycle. If oxygen didn't have both a stable and a reactive state (while being the same molecule!), it would either be a less effective oxidizer or be too reactive to be safely transported biologically, let alone to be exposed to. So there's reason to search for life in parts of the universe we know to have oxygen – it's really an amazing molecule. Carbon might be more replaceable than oxygen (IMO), but even it so far seems to have the most advantages to life in contrast to other molecules, from what I understand. I'm sure someone will correct me on this (which I absolutely welcome!), but carbon is advantageous for life because it is almost an opposite of oxygen; it will accept multiple bonds with other molecules but doesn't need those bonds so badly that it needs to steal electrons from other molecules (oxidization) or go out of its way to lend its electrons, causing a redox reaction. It's a chill ass molecule that's really good for being a building block for far more complicated molecules. Now let's talk about water. It's actually a really good solvent and can dissolve a wide range of substances, thus making it effective as a carrier of many things. Water is a polar molecule due to the way it has an oxygen end and a hydrogen end (think of the way an H20 molecule is normally represented as being triangular), which allows it to interact with other polar molecules (including other water molecules) and reject nonpolar molecules. At the same time, it's capable of interacting with some nonpolar molecules. By being polar, it forms hydrogen bonds with other molecules. These properties are important for things like the creation of proteins, for example. Water is also a side effect of organic chemistry; both respiration and the actual burning of organic material results in CO2 (and other carbon molecules) and water. Water stores heat very well, and temperature may be an inherently limiting factor to where organic life of any kind can thrive. Oh yeah, and it also has a neutral pH. Though it doesn't conduct electricity on its own, a solution of a water and an electrolyte allows it to conduct electricity. The ability to create balances of electrolytes is another critical use of water in organic life. I'm missing a bunch of things, but the picture I'm trying to paint for you is that molecules such as water, carbon, and oxygen, aren't simply replaceable in the only example of life that we know of. Their role isn't by mere happenstance, but because they have inherent advantages over their alternatives. Searching for life elsewhere in the universe is an expensive endeavor. While we shouldn't be close-minded to other models of life, searching for life similar to our own is simply more efficient from a logical perspective. It would be one thing if Earth was the only object circling a star that has these important molecules, but we know that it's not. Something to consider is just how similar life is on our own planet. We like to think of some animals like octopuses as "alien", and while they are indeed strange in many ways, they still share properties with other animals. Take for instance the fact that octopuses have two eyes, just like other animals; in fact, the vast majority of animals have two eyes. Even spiders, which have more than two eyes, usually have eyes arranged in two rows, and many spiders have two eyes that are more dominant than the others. Eyes have evolved multiple times independently in the tree of life. While it's an interesting idea that extraterrestrials might have multiple eyes, there's reason to believe that, if we were to meet extraterrestrials that evolved on a similar planet to Earth, they would feature two eyes. Although we might not completely understand why evolutionary pressure hasn't widely promoted more than two eyes, it still appears to have an inherent advantage. This same line of thinking is why scientists have focused on searching for life similar to our own; there are properties to our form of life that are based on environments and traits that have inherent advantages. The evidence suggests that the way our life has arranged itself isn't merely a jumbling of various things that happen to function together.

simiones 3y ago

There are several aspects that make the search for non-water based life untenable. Multiple comments are pointing out the practical aspects. A separate aspect that is very important to know is that inorganic chemistry (non-carbon chemistry) is actually quite limited overall: we have a pretty good grasp of the possible inorganic compounds and their properties, at least within certain ranges of temperatures and pressures. And a huge number of those compounds require water to be present in some way. Now, organic chemistry is far richer and far harder to explore thoroughly. But, given all we know about inorganic chemistry, creating organic compounds in ways that don't involve the known inorganic ones, and doing so without water, seems extremely unlikely.

jrumbut 3y ago

The point of the rule is to be extremely limiting. There are billions and billions of stars, and it's becoming clear that a lot of them are orbited by planets. Given that there aren't obvious alien visitors or even communications reaching us regularly, it seems to be a needle in a haystack situation.

deleted 3y ago

[deleted]