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What are the ways we can search for alien life?

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  1. Sep 23, 2015 #1
    Are we alone AND what are the ways we can search alien life?
     
    Last edited by a moderator: Sep 23, 2015
  2. jcsd
  3. Sep 23, 2015 #2

    Chalnoth

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    I think it's more a question of how close life is, rather than whether or not there is life beyond the Earth.

    Our best bet for finding life outside of our solar system is by looking at the atmospheres of planets orbiting other stars. As far as we know, life is the only thing that can maintain an atmosphere that has out-of-equilibrium chemistry. For our own Earth, life is the only reason why we have free oxygen in the atmosphere: without life, our atmosphere would be almost entirely composed of nitrogen, carbon dioxide, and water vapor. If we were to detect oxygen in the atmosphere of a planet orbiting some other star, that would be a pretty good indication that there's life there.

    This is pretty difficult to do, but not impossible. We would detect the chemistry by looking at the spectral lines of either emission or absorption of from the planet's atmosphere.
     
  4. Sep 23, 2015 #3

    Chronos

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    While we have no credible evidence of ET, absense of evidence is not evidence of absense. The best known example of the search for ET is undoubtedly SETI - which focuses on detection at radio wavelengths. Of course we have no idea if radio is a technology widely deployed by intelligent ET's, it's just a reasonable guess.
     
  5. Sep 23, 2015 #4
    There are several ways we could find life, astronomers recently analyzed the infrared signatures of thousands of galaxies looking for the heat created by mega civilizations, they found nothing out of the ordinary. Astronomers have been listening for alien broadcasts for the better part of a century and only had a few blips that were unexplained (and none of them repeated.) There are also visual surveys of places where probes might be found (L points, surface of the moon, polar orbit...) Again, none have been found.

    I think the most likely way we'll find life is with a probe. Lots of people believe that life isn't just abundant in deep space, but also much closer to home. Earth's early life was pelted with cataclysm after cataclysm and it held on. The early solar system was probably like a giant mixing bowl with ingredients being blasted away from one planet and pulled into another. We might find life on one or many of the icy moons further out in the solar system.

    Are we alone? Can't say. Are we likely alone? No, it's likely we're not even alone in our own solar system.
     
    Last edited: Sep 23, 2015
  6. Sep 23, 2015 #5
    I think if there were other intelligent life within the solar system we would have had a definite indication of some sort by now.
    Most of the planets have surface conditions which would one way or another be lethal for carbon based life similar to that on Earth.
    Life based on something other than carbon and the presence of liquid water seems to be increasingly unlikely simply from the point of view of chemistry alone. Highly complex reproducing molecules just don't seem credible any other way.
    Only Mars could be possibly be survivable given an enclosed habitat and adequate shielding from radiation - Mars' atmosphere doesn't cut it as a radiation shield like Earth's does, and it completely lacks any effective magnetic field.
    Primitive life might be possible in the subsurface water oceans of some gas giant moons, but there is no evidence at present suggesting that this is so.
     
    Last edited: Sep 23, 2015
  7. Sep 23, 2015 #6
    I made no specifications about the intelligence of in solar system life. No evidence is not the same as evidence of nothing. It's highly likely that we are the only intelligence species in this solar system.

    Your physics is also incorrect. Earth's atmosphere has little to do with radiation. It protects against UV, but its the magnetic field that deflects the majority of dangerous particles, that's what Mars lacks. It does however have a warm interior and we know that life on Earth is perfectly happy under kilometers of rock.

    There is plenty of evidence of organics, Cassini flew through the jets of Enceladus and confirmed their presence, while not indicative of life, it's a pretty good first indicator. Organic molecules are also present in the surface of Europa.

    I think it'd be very difficult to have a body that contains such large amounts of water like Europa and Enceladus in such close proximity to primordial Earth and not cross contaminate in the violence of the heavy bombardment.
     
  8. Sep 23, 2015 #7
    Yes I was thinking of the Earth atmosphere as a blocker of high intensity UV, and also of gamma and X rays, all of which can damage organic molecules. (I had already been updating my earlier post to mention Mars having no effective magnetic field as well, sometimes crossposting happens.)

    I thought you had been referring to intelligent life since that seemed to be the context of the OP question.
    As for primative life, I agree there are possibilities for the gas giant Moons, It certainly can't be ruled out, it's a case of insufficient data to conclude anything at the moment so we will have to see what future explorations reveal.
    In the past Mars seems to have definitely had surface water and probably a more substantial atmosphere so previous existence of life remains a possibility even if there is nothing there now.
     
    Last edited: Sep 23, 2015
  9. Sep 23, 2015 #8

    Chalnoth

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    I'm not sure there's any reason yet to believe that there is life beyond the Earth within our own solar system (aside from the microbes we've sent out from the Earth in our probes). There are some tantalizing possibilities of life in places like Mars, Titan, and Europa, but so far no evidence.

    I don't think that's accurate. High-energy cosmic rays are not deflected at all by the Earth's magnetic field. Those tend to impact the upper atmosphere, and result in a relatively low level of high-energy radiation that we're exposed to every day. The upper atmosphere also reduces incoming ultraviolet radiation.

    The magnetic field of the Earth mostly has an impact on the relatively low-energy solar wind, but it doesn't prevent the solar wind from striking the Earth: it just deflects the particles to strike the Earth near its magnetic poles.

    "Good indicator" is a bit strong. There's a pretty wide gulf between the comparatively simple organic molecules we've detected and life.
     
  10. Sep 23, 2015 #9
    I specified good FIRST indicator for that reason. I agree, them being there is not a good indicator of life, it is however a good indicator of a potentially inhabited world.
     
  11. Sep 23, 2015 #10

    mfb

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    The atmosphere is much more important in terms of shielding. Compare the radiation levels from cosmic radiation received on at sea level (shielded by atmosphere and magnetic field) with those on the ISS (shielded by the magnetic field and a thin spacecraft hull) and on the way to moon (shielded only by the spacecraft hull). Sea level to ISS is a factor of roughly 1000, while ISS to moon is a factor of about 2-3.
     
  12. Sep 23, 2015 #11
    Oh. The magnetic field is what protects the atmosphere from the solar wind, but the atmosphere itself is what protects the planet's surface from high energy radiation?
     
  13. Sep 23, 2015 #12

    DaveC426913

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    Sorry, can you clarify?

    You seem to have said that organic molecules are a good indicator of a potentially inhabited world. I don't think that's what you meant to say.
     
  14. Sep 23, 2015 #13
    Inhabited in the sense that there is anything at all living on it, not intelligent species. And organic molecules being a good indicator of POTENTIAL, not actually inhabited. In such a way that seeing ice caps and dark channels on the Martian landscape was a good indicator that Mars was potentially inhabited in 1900.
     
  15. Sep 23, 2015 #14

    DaveC426913

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    Except that they weren't a good indicator. (As on the Moon, there is also ice, but certainly no life, therefore ice is not a good indicator of life)

    Just like Enceladus, organics are not a good indicator of potential habitation.
    So I'm not sure why you're saying they are.
     
  16. Sep 23, 2015 #15

    mfb

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    The atmosphere would protect the surface from solar wind as well, if necessary.

    There is a different effect: the magnetic field is protecting the atmosphere from solar wind. While this doesn't seem to be a must (see Venus), a magnetic field certainly helps to maintain a thick atmosphere. And this atmosphere is necessary for shielding. At least for mammals and many other complex species. Some other organisms are much more radiation hard, or live deep enough to not care about the atmosphere at all.
     
  17. Sep 23, 2015 #16
    Again, I said nothing about indicating habitation. I only said it indicates potential. Enceladus indicates potential, Anthe doesn't. That's all I mean by indicating, the way I was using it, any object with liquid water and simple organics has potential, as well as Titan and places with potential for exotic life. I only meant to illustrate that the building blocks of known life are known to exist there. I should have been more careful with wording.


    @mfb, Why doesn't the solar wind rip Venus' atmosphere off? I know the composition of the atmosphere is mostly fairly heavy molecules, is Venus's gravity enough to maintain it or is there something else going on there?
     
  18. Sep 23, 2015 #17

    mfb

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    If you figure it out completely, write a paper...
    The interaction of the solar wind with the atmosphere seems to generate a magnetic field that acts like a shield in some way.
     
  19. Sep 23, 2015 #18

    Chalnoth

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    Organic molecules are a necessary condition for organic life, but nowhere near sufficient.

    You also need some sort of out-of-equilibrium energy process to sustain any life, as well as an environment that is sufficiently gentle to prevent immediate breakdown of molecules complex enough for self replication.

    I do expect that the formation of life is essentially inevitable given the right conditions, I'm just not sure those conditions existed anywhere but the Earth within our own solar system. I strongly support searches for life elsewhere in the solar system, I'm just not sure we yet have cause for optimism.
     
  20. Sep 23, 2015 #19
    Agreed.

    I think Europa is the best candidate for life. It's internal climate has probably been very consistent for a long time or at least very slowly changing. Jupiter and the other moons' tidal forces provide a lot of energy. Jupiter however would pull in a lot of comets and asteroids and some would slam into it, causing periodic mass extinction events, driving evolution. The reason I prefer Europa over Enceladus is it's color. It's got a very young surface which indicates either geysers, snow, and/or tectonics. Enceladus has that too, but it's got a a beautiful white surface. That to me shows that the water spewing out of those vents don't have a ton of stuff in it, it's got enough heat to produce liquid water, but it seems like that water is very pure, all the sediment having settled at the bottom. Europa, is brownish, which shows that the water has something stirring it. The tides as well as the seafloor volcanos keep sediments and organics constantly circulating, giving them more chances to combine with other things and produce self replicating molecules. Down side: no lightning, no massive discharges of electricity that can jumble atoms.

    Side note: Mars, does have a liquid water table. It's very deep, but there is a point at which the pressure creates enough heat to create liquid water. That water is usually stuck in rocks, but on earth, the deepest ground water still contain traces of life. It's possible that extremophiles like them were all that were left after huge bombardments that baked the surface.
     
  21. Sep 23, 2015 #20

    ogg

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    Unfortunately, many confuse ET life with ET Intelligent life. And what is an even bigger blunder, people (including some commenters here) confuse intelligent life with "technological civilization". They are not the same, obviously. A very recent paper suggested a mechanism which allows for abundant Oxygen in the atmosphere of planets WITHOUT life. That is, it puts into question whether we can use O2 as a proxy for life. Further work is required to firm this idea up. But our best guess is that if we ever find a planet has water, oxygen, and hydrocarbons in its atmosphere, and is in or near its star's habitable zone (goldilocks zone) and is a rocky (Earth-like) planet, then it would be an extrememly good candidate for life to exist. From what we understand now, we think the question is not "if" but "where" life exists. But this popular opinion should be put into context. Factually, we do NOT know what the requirements for life to evolve are, so anybody who claims that life "must" exist elsewhere is not basing it on scientific facts alone, but on some wishful thinking, too. The argument for life is that there are so many rocky planets that even if the chances of life evolving are miniscule, then there still must be many planets with life on them. This can be informally thought of as follows: since life exists on Earth, the probability of life on any planet is greater than zero. Given the number of rocky planets is very large, we can calculate the number of planets (or satellites) having life as P x N, where P is the probability of any one planet to harbor life, and N is the number of rocky planets. We know this number is greater than 1, and it would be extraordinary that P and N were so well matched (there doesn't seem to be any reason they would be) if it were less than 2. (It could be 1.00001 or 1.9, but there are so many MORE numbers above 2, that it isn't likely to fall below 2.). The problem with this type of argument, is that it assumes that P isn't related in some way to N, but the way the Universe works, we know that they are related, in a very complex way. It isn't good science, no matter how plausible it is, to claim that any relationship that may exist should be ignored. It is an argument of ignorance. If you read about the Drake Equation (see, for example Wikipedia), you can see one way to look at the question in a more scientific way. A few years ago, I went though the terms of that equation, and decided that the logic was flawed and the equation was both obsolete and inadequate, but I don't have a real serious issue as using it as a "rough draft" allowing us to begin to address many of the issues that need to be addressed before we are able to establish a good estimate for that number. To answer your questions specifically: If by "we" you mean "life on Earth", then we are alone. Its not very likely we will ever communicate with life in other star systems. AND, so far, we haven't found life anywhere else in our Solar System. These mean that right now we are alone. (But implied is that this could change someday). To your second question, the two realistic methods are 1. using large telescopes to analyze the compostion of atmospheres in extraterrestrial planets and 2. Sending probes to some of the jovian satellites and getting below the ices to analyze some of the liquids present. (today the cost of such a mission would be enormous.). Definitive evidence of life requires physical evidence, which requires either telescopes far more advanced than anything we can build today, or interstellar probes which are also far beyond what we can build. (Here "life" means "life as we know it, or similar enough to be detected using the same analytical methods"). If your question is "Do other technological civilizations exist right NOW, somewhere in the Universe?" then that question is not scientific - in several ways. We can make it more scientific by placing limits on the distance (and time) between us and any life "of interest". My opinion is that distances further than 100 - 500 light years SHOULD NOT BE of much interest for technological life, but that looking for life signs at any distance for any types of life (from bacteria to plants to civilizations) is reasonable. With the qualification that, as we know, technological civilizations can change a "natural" system drastically which could possibly help in their detection.
     
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