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Thread: Is life on other planets restricted to "Earth-like" planets?

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    Is life on other planets restricted to "Earth-like" planets?

    Hello all,

    This is my first thread, but I've been lurking for some time. Such a great place to shirk my work responsibilities.

    My question regards life on other planets, and our search for them. I apologize if this is common sense to you guys, but I'm not educated in science/mathematics beyond high school.

    From what I've seen in documentaries and websites, our search for life on other planets begins with searching for other planets that share characteristics with Earth. I assume this is because we know that Earth can sustain life, therefore, due to the immensity of space, it's just more efficient to search for life on planets like ours.

    So we find some giant planet orbiting a distant star, but it's way too close to its star, so we assume there is no "life" because it's too hot.

    But my question is whether life can definitely not exist on planets completely unlike Earth? (Sorry for the multiple negative.)

    A friend of mine said "well, there might be life on other planets, but who knows what that life would be like? Perhaps life can exist on a planet with no atmosphere in forms that we are unfamiliar with. Or on a planet with a nitrogen atmosphere, with life adapted to that. Or perhaps some life-forms don't have to breathe at all?"

    Is there some reason why life cannot exist on a planet as close to its star as Mercury? Or as far as Neptune? Or without oxygen?

    Is it known with certainty that those environments cannot sustain life? (I guess I'm thinking of a chemical reason, like temperatures above X or below Y do not permit bonding beyond Helium, or something like that.)

    Or do we just not know how it would be possible?

    Thanks, and I'm sorry if this doesn't make much sense. I'll try to clarify if necessary.

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    Quote Originally Posted by EdLee
    Is it known with certainty that those environments cannot sustain life? (I guess I'm thinking of a chemical reason, like temperatures above X or below Y do not permit bonding beyond Helium, or something like that.)

    Or do we just not know how it would be possible?
    I think this is a reasonable question indeed. I don't know however if it has been addressed anywhere else in the BA forum.

    In any case, this might be of interest to you: it's about extremophiles. We are finding more and more lifeforms right here on Earth that survive and flourish in such harsh environments that we would have previously considered impossible. So, I am voting that we sooner or later will indeed discover extrasolar life that we hadn't thought possible.
    http://www.astrobiology.com/extreme.html

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    Quote Originally Posted by EdLee
    But my question is whether life can definitely not exist on planets completely unlike Earth?
    Anyone who concludes there is no life on a non-Earthlike planet is asking to be wrong. We don't know what forms life can take, and what environments are suitable for it, in all the many forms it could possibly take.

    On the other hand, if we are going to go looking for life, with our limited resources and abilities, we should look in the places where the kind of life we can understand -- and detect -- are more likely to exist. That is why places like Mars, Europa, and Enceladus are of much more interest to astrobiologists than the likes of Mercury or Neptune, or Pluto, or the surface of the Sun, or in the voids in between.
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    Quote Originally Posted by Blue Fire
    I think this is a reasonable question indeed. I don't know however if it has been addressed anywhere else in the BA forum.
    Whoa.

    Quote Originally Posted by Blue Fire
    In any case, this might be of interest to you: it's about extremophiles. We are finding more and more lifeforms right here on Earth that survive and flourish in such harsh environments that we would have previously considered impossible. So, I am voting that we sooner or later will indeed discover extrasolar life that we hadn't thought possible.
    http://www.astrobiology.com/extreme.html
    Yeah, that's exactly what I was looking for. One of the organisms mentioned is a microorganism that survives on sulfur and hydrogen in temperatures up to 190 F. Fascinating stuff. I guess it's not out of possibility that life can exist on methane alone, in temperatures up to 900 F?

    Quote Originally Posted by 01101001
    Anyone who concludes there is no life on a non-Earthlike planet is asking to be wrong. We don't know what forms life can take, and what environments are suitable for it, in all the many forms it could possibly take.
    Yeah, I figured it would be silly to declare life impossible, but with my limited knowledge, I thought it might have already been "proven." Like I said in the first post, I thought there might be things like:

    1) At a temperature above X degrees, chemical bonding beyond [whatever element] is impossible.
    2) At a temperature below Y degrees, there is no known substance that does not freeze on a cellular level.
    3) Without an atmosphere, radiation is lethal.
    4) Or whatever.

    Thanks for the replies.

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    Keep in mind there are two very separate general classes of life on Earth: plants and animals. Plants don't need oxygen, so that takes care of that, but big animals do because they need a more active metabolism. On the other hand, if all you want is to find microbes, you don't even need sunlight. So basically, you can find lots of different types of life and the environment you look for depends on the kind of life you want to find. But the super payoff is for big animals, and those might need oxygen to breathe. The main bonus in finding oxygen, however, is that life that exhales oxygen is the only way we know of to get large amounts of it into the atmosphere. Having said this, I should point out that the most difficult requirement that seems to be crucial for life is water in the liquid phase. That is very rare to find, especially on the surface where you could also have oxygen for big animals and sunlight for plants for them to eat (and to make their oxygen). So that's one of the best reasons for looking for Earthlike planets-- to hopefully find liquid oceans of water.

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    Okay, so it's the "big animals" and water requirement that constrains the "distance from the sun" and atmospheric requirements in our search for life? I assume water can't exist outside of a particular temperature range, which also requires a relatively even orbit around a star?

    Is modern science even bothering to look for microbial life on non-oxygenized planets? I'd think that would be pretty cool in itself. Single celled organisms on Venus would rock my pants, to be honest.

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    Quote Originally Posted by Ken G
    Keep in mind there are two very separate general classes of life on Earth: plants and animals. Plants don't need oxygen, so that takes care of that, .
    Hold on a sec, both plants and animals on Earth respire which requires oxygen. Most Green plants that we are commonly familiar with basically do 3 things: Photosynthesis, Respiration, and Transpiration. Photsynthesis of course uses carbon dioxide and releases oxygen, But respiration does require oxygen.
    http://www.ext.colostate.edu/pubs/garden/07710.html
    Plants take up oxygen through the stomata in their leaves and through their roots.

    Again, respiration is the burning of sugars for energy to grow and do the internal work of living. It is very important to understand that both plants and animals (including microorganisms) need oxygen for respiration.

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    That is quite interesting, and I am no biologist obviously, but this must be an evolved capability by plants. For example, do the bluegreen algaes respire? Their ancestors were here when there was very little oxygen in the atmosphere to respire in the first place.

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    Quote Originally Posted by EdLee
    Is modern science even bothering to look for microbial life on non-oxygenized planets?
    Yes, in our lifetimes NASA will (hopefully) look for microbial life under the permafrost on Mars, and under the ice sheets on Europa and Enceladus (if NASA can return to responsible priorities). But we certainly don't expect to see any fish swimming around there. Of course you are right that even a single microbe on a planet outside of Earth would be a huge finding-- it would mean that the universe is teeming with life.

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    Quote Originally Posted by Ken G
    That is quite interesting, and I am no biologist obviously, but this must be an evolved capability by plants. For example, do the bluegreen algaes respire? Their ancestors were here when there was very little oxygen in the atmosphere to respire in the first place.
    Well, I'm no biologist or botonist either. I just happened to remember some scraps of info from my botony class in college oh so many years ago. In any case, from Wikipedia:
    Cyanobacteria (Greek: κυανοs = blue) are a phylum (or "division") of Bacteria that obtain their energy through photosynthesis. They are often referred to as blue-green algae. The description is primarily used to reflect their appearance and ecological role rather than their evolutionary lineage. Fossil traces of cyanobacteria have been found from around 3.8 billion years ago (b.y.a.). See: Stromatolite. As soon as these blue-green bacteria evolved, they became the dominant metabolism for producing fixed carbon in the form of sugars from carbon dioxide. Cyanobacteria are now one of the largest and most important groups of bacteria on earth.
    As far as I know, these algae are thought to be responsible for the large amount of oxygen in our atmosphere today, but as to whether they respired back then 4 byrs ago, I am unsure.

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    Yeah, blue-green algae respire. Anything that photosynthesises "normally" will respire to make use of the compounds that were created in photosynthesis.

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    So where did the early ones get their oxygen? Do they just need trace amounts? It wouldn't seem to make sense that they would need all kinds of CO2 to undergo photosynthesis, and then just trace amounts of O2 to use the compounds they built from the CO2. That supports the idea that the CO2 they use is actually a trivial amount, but that seems to contradict what we heard above.

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    So where did the early ones get their oxygen? Do they just need trace amounts? It wouldn't seem to make sense that they would need all kinds of CO2 to undergo photosynthesis, and then just trace amounts of O2 to use the compounds they built from the CO2. That supports the idea that the CO2 they use is actually a trivial amount, but that seems to contradict what we heard above.
    I presume they got it from photosynthesis. But you can use glucose for energy without oxygen, it's just less efficent. So the first oxygen releasing photosynthesisers could have used glucose produced without needing oxygen. There is a form of photosynthesis that doesn't release oxygen, but it is less efficent. (Technically one could say there are many kinds of photosynthesis, but the term is generally only used for what we commonly see in plants.) I suppose one of the advantages of living in a mat with other cynobacteria is oxygen wouldn't diffuse out so rapidly.

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    Welcome to the board, EdLee. Your question is an excellent one that goes right to the heart of astrobiology. Given we can't get a handle on how life originally got started here on Earth, there are obviously questions about the environments in which it can start on other planets, although the mechanisms that drive evolutionary adaption once life has got a toehold may well be similar, regardless of location.

    There are reasons why chemistry and physics will likely place restraints on where life can occur. It was always thought that extremely cold environments may result in chemical reactions that are too slow for life to form, extremely hot environments act against stable chemical bonding, high radiation environments cause molecular disassociations. These are good arguments - yet, we have found life on this planet in all of those environments, as well as kilometres underground, at the bottom of the ocean, high in the atmosphere, etc, etc - all quite happily filling a niche!

    If you can, find a copy of Fred Hoyle's book, "The Black Cloud". In this fictional account, Hoyle describes a lifeform that didn't even bother forming on a planetary surface!

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    life has, Tenacity. If it can it will or all ready has.

    Yes I am of the view that life will be found when we stop looking.

    Might it find us instead of us it.

    Have we any idea what to look for?

    There are so many 'what if's' in this topic.

    In my humble view we only need to look for planets in the right heat zone of its parent star. Almost without fail i sagest life will be found.

    It will be very different from the sci fi image we have come to expect.

    Plants or what could pass for them might be found to thrive in atmospheres so different than Earths

    Numerous examples of deep ocean trenches and volcanic fracture zones seem to have life forms that deify the well ordered normality we have come to expect.


    If its Mars or Titan then we might find it. If we need to go star hunting then its decades away.

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    I wonder how common is life on Earth-like planets. Seeing as how life has been around for quite a while here it wouldn't surprise me if together with the first such planet we would find some evidence of life, such as an element that could be replenished only by living organisms.

    If we find some simple life on, say, Titan, it could be that the probability of life emerging in harsh environments is higher than we think. It all comes to the lack of evidence.

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    Quote Originally Posted by AGN Fuel
    If you can, find a copy of Fred Hoyle's book, "The Black Cloud". In this fictional account, Hoyle describes a lifeform that didn't even bother forming on a planetary surface!
    Much better example, IMO, is What Does a Martian Look Like? by Cohen and Stewart. They are very much against the "life requires sunlight, oxygen and liquid water" limitations, and speculate about life in the most unlikely environments -- some not even based on chemistry.

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    But you can use glucose for energy without oxygen, it's just less efficent. So the first oxygen releasing photosynthesisers could have used glucose produced without needing oxygen. There is a form of photosynthesis that doesn't release oxygen, but it is less efficent.
    There are no known eukaryotic anaerobic phototrophs (ie capable of anoxygenic photosynthesis) (Fenchel, 1996).
    http://www.nhm.ac.uk/research-curati...s/euk-extreme/

    I think Europa has a good chance of life, perhaps even fish-like. Deep sea creatures evolved in a volcanicly warmed water, fed by nutrient rich thermal vents. I wonder if Europa experiences plate tectonics under all that ice?

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    I've heard that the main components for life on Earth are carbon and water. But that these may be replacable with silicon and ammonia. Is this based in truth/chemical possibility? (As in, is it worth it for us to look in these types of environments as well?)

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    Quote Originally Posted by Ronald Brak
    IBut you can use glucose for energy without oxygen, it's just less efficent. So the first oxygen releasing photosynthesisers could have used glucose produced without needing oxygen.
    That's what I was wondering, I think this must have been the case. So early life on Earth did not need oxygen in their environment.

    Quote Originally Posted by Ronald Brak
    I suppose one of the advantages of living in a mat with other cynobacteria is oxygen wouldn't diffuse out so rapidly.
    That's an interesting point, that the bacteria might in effect create their own oxygen and respire it themselves, completing the CO2 cycle locally. It wouldn't be fully efficient of course, so the net result would be to have oxygen leak out and CO2 be consumed. It still still isn't clear if this is a significant source of CO2 depletion, relative to geological processes.

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    This site gives a good account of the evolution of the Earth's atmosphere over time.
    http://www.ux1.eiu.edu/~cfjps/1400/atmos_origin.html

    One thing that is noticable is that the rocks of the Earth's surface were becoming oxidised by the oxygen produced by the primitive plants; more than ten times as much oxygen has been absorbed by the crust than remains in the atmosphere. Almost as fast as the primitive plants could produce it, the oxygen was reacting with the crust to form oxides.
    So it was a long, long time before oxygen levels built up to modern values.

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    We have banded iron deposits in Australia. It appears that each year photosynthesizers gave off oxygen causing oxidation of iron which settled in a layer. Presumably there was seasonal variation in the amount of oxygen given off so the rust was deposited in layers. This continued until their wasn't enough material left to absorb oxygen and it built up in the air. If you own a Japanese car it might be made out of these old deposits of rust.

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    Thanks you two for filling in these important gaps. The picture is emerging more clearly, and I think it is now apparent that the CO2 was not primarily consumed by plants but rather by geological processes, but it is interesting that a lot of O2 may also have been consumed geologically, so plant fixing of CO2 may be significant as well.

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    Thanks you two for filling in these important gaps. The picture is emerging more clearly, and I think it is now apparent that the CO2 was not primarily consumed by plants but rather by geological processes, but it is interesting that a lot of O2 may also have been consumed geologically, so plant fixing of CO2 may be significant as well.
    Well, without photosynthsis O2 would have stayed bonded with carbon in CO2. Photosynthesis released O2 which then reacted with iron and other elements to form oxides. This wouldn't have happened without plants to release the oxygen so photosynthesis allowed the geological locking up of oxygen to take place.

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    Indeed, what I mean is, what is responsible for the elimination of the outgassed CO2. We know geology can do it after dissolving the CO2 in the oceans, but plants can also do it by converting CO2 to O2 and then having it get geologically oxidized. I think the former process is the more important, but the issue here is how much is the contribution of the latter. We are agreed that plants are the source of the free O2, particularly the bluegreen algaes.

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    Quote Originally Posted by EdLee
    I've heard that the main components for life on Earth are carbon and water. But that these may be replacable with silicon and ammonia. Is this based in truth/chemical possibility? (As in, is it worth it for us to look in these types of environments as well?)
    Silicon and carbon chemistries are quite different. For example carbon-carbon bonds are quite common, whereas silicon-silicon is not (except in elemental silicon). Carbon can be found bound to a much greater variety of atoms than silicon. Carbon chemistry is much more versatile.

    Ammonia as a solvent has some similarities to water, particularly that it has hydrogen bonding and is a pretty good solvent. But it is only a liquid at cyrogenic temperatures or very high pressures, so you get into the issue of reactions at cold temperatures that AGN Fuel mentioned.

    There has been tons of discussion about these topics in the Life in Space forum. You may wish to search that.
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    Quote Originally Posted by Ozzy
    There are no known eukaryotic anaerobic phototrophs (ie capable of anoxygenic photosynthesis) (Fenchel, 1996).
    http://www.nhm.ac.uk/research-curati...s/euk-extreme/

    I think Europa has a good chance of life, perhaps even fish-like. Deep sea creatures evolved in a volcanicly warmed water, fed by nutrient rich thermal vents. I wonder if Europa experiences plate tectonics under all that ice?
    But there are known prokaryote anaerobic phototrophs:

    http://www.bact.wisc.edu/Microtextbo...&theme=Printer

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    Quote Originally Posted by Ilya
    Much better example, IMO, is What Does a Martian Look Like? by Cohen and Stewart. They are very much against the "life requires sunlight, oxygen and liquid water" limitations, and speculate about life in the most unlikely environments -- some not even based on chemistry.
    I was always a big fan of Anne McAffrey's space microbe in the Pern series. Until I started reading those books, I had always assumed that life could not exist in an un-earthlike environment. . . . . .I personally think that life is more ubiquitous than we can imagine, and that it will take forms and use chemistries we never thought possible.

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