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Does life here on Earth indicate life elsewhere?

  1. Jun 5, 2008 #1
    What does the presence of life on Earth imply about the chance of life existing on other worlds?

    Can one assume, with high probability, that despite Earth's seeming singularity of life, there is life elsewhere in our (assumed finite) universe? In our (assumed infinite) universe?

    A possible definition of life would be entities that reproduce nucleic acid through assimilating their chemical environment.
  2. jcsd
  3. Jun 5, 2008 #2
    Good question; I'll start with the details: the universe is definitely finite. Also, one can't really say earth seems to be the only location with life, as life on other planets necessarily wouldn't have been discovered unless they came by (which is practically impossible even theoretically, not to mention statistically).

    As for your actual question: with the discovery of many near earth mass extrasolar planets close to and in the habitable region of their solar systems, it seems very probable that there exists other life - in respect to planetary requirements. Based on our current knowledge, there is no reason to think that life would be unique to earth, although it is certainly difficult to form.
  4. Jun 5, 2008 #3


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    The discovery that fairly complex life appeared on earth within 100Myr of it cooling and it's existance in such extreme conditions as deep sea vents and deep in crust rocks suggests that life appears where ever it is energetically possible and so might be a lot more common than once thought.
  5. Jun 6, 2008 #4
    Strong words.
    Based on where science & physics is so far, is the universe finite: yet travels by the speed of light, that may possibly not be exceeded, which is therefor in an infinite speed in comparison with reach.

    @Topic, I honestly got no strong opinion about this, except: that I find our life (all creatures on Earth) not as anything close to a proof or fact of other existence, yet very possible and hopefully yes.
  6. Jun 10, 2008 #5
    You don't really know this for sure. You don't know if the universe if either finite or infinite. Also, nobody yet knows if there is life elsewhere. It is definitely 'possible', because one can just assume that if life is here, then there could be living things elsewhere. But, we don't have any information. Like, for all we know, we could even be the first (or only) ones living. You just never know. On the other hand, there could be heaps of other living things out there. We don't know that either.
  7. Jun 25, 2008 #6


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    It implies that life does exist in other sun systems. The stage of life's development may not match ours or may be of a completely different evolution. However... when we see that we have a sun and every other system has a sun.... and we see that every other system has planets like our own... and we see that all the other conditions found in our solar system exist in other systems throughout the the galaxies and clusters of galaxies... then we can assume that life is a component of some of these systems as it is a part of ours.
    Last edited: Jun 25, 2008
  8. Jun 26, 2008 #7
    How about this...

    h = the proposition that life is fairly common in the universe (it occurs in a non-negligible fraction of the solar systems)

    e = event that life is observed on earth.

    Plug these into the Bayes formula

    P(h|e) = P(e|h)P(h)/P(e)

    where P(e) = P(e|h)P(h) + P(e|~h)P(~h)

    From the definition of h, P(e|~h) << P(e|h), so if you started out relatively indifferent to the hypotheses h and ~h (ignoring for the moment that you know there is life on earth), the above equation for P(e) becomes

    P(e) is slightly greater than P(e|h)P(h) or

    P(e|h)/P(e) slightly less than 1/P(h).

    Plugging this into Bayes' formula gives

    P(h|e) is slightly less than 1.

    Of course, this depends on you being relatively indifferent to the propositions h and ~h, which may not be the case. If you assign P(h) << P(~h), then you can't neglect the second term in the equation for P(e).

    P.S. After thinking about it some more, I'm pretty sure there's a serious flaw in the above argument, but I'll see what others think...
  9. Jun 26, 2008 #8


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    Why so limiting?
  10. Jun 26, 2008 #9
    Just "possible," it errs on the side of doubtfulness. In retrospect, it is too limiting.
  11. Jun 26, 2008 #10


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    I've heard that crystals are reproductive in some way or another. I thought this was only when they're under pressure but people say the continue to grow once they're out of the ground.

    There may be some type of life form arising from that but we do not see it here. Crystals have proven to be very useful in terms of storing data and transmitting info... they are also part and parcel to the encapsulation of viral RNA. Perhaps there is the possibility of a more evolved form of crystal with free-associative powers, logic and what we'd call "intelligence" lurking in the neither regions of the "old" universe.
  12. Jun 26, 2008 #11
    I don't think its a safe assumption. In reality earth is a small sample size.
    Because my left hand looks the way it does, and there's 6+ billion other people in the world, does that mean someone else has the exact same left hand? What if it were a billion billion? Its not answerable.
    You would have to look at the number of configurations/pieces it takes to make this hand unique. Then you have a gauge of the statistics.
    For life, until we create it in the lab, we're not sure how many molecules have to be this way with this much energy and blah blah blah, and we have no idea the probability of its occurrence. Nor do we know how many different configurations can be used to make "life".
  13. Jun 27, 2008 #12
    I think you're close.....but the word 'does' should be 'could'.
  14. Jun 27, 2008 #13


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    I think you have to include time in the equation that explores whether or not life on earth indicates life occurring elsewhere.

    So much time has gone by since the theoretical BB that life could have developed and evolved to incredible complexities then completely destroyed by one factor or another.

    I'm not sure if this makes it more probable or less probable that life has or is occurring elsewhere as it is today on earth.
  15. Jun 28, 2008 #14
    That's right. I agree that life *could* have developed elsewhere. We don't know if it 'did' develop elsewhere.
  16. Nov 12, 2008 #15
    Life on earth doesn't indicate life elsewhere, but one can assume that, if the universe contains forms of life, then there is a possible chance of the existence of other forms of life in other places. We can also assume that life will exist and has existed, because time and space are relative. Now it depends...(guilty until proven inocent or inocent until proven guilty).
  17. Nov 12, 2008 #16
    The central difficulty is that we do not yet know how life arose. mgb phys made the pertinent observation about life arising rapidly (in geological terms) after the formation, or rather the surface cooling, of the Earth. His conclusion is that this suggests life arises naturally and rapidly. There are two further possiblilities.
    1. Life arrived from space.
    2. This rapid abiogenesis was a unique, chance event.

    The first of these alternatives increases the probability of life elsewhere. The second reduces it to zero. Until we have established how life arose, we have no way of reasonably selecting between these alternatives.
    The question remains fascinating, worthy of study, but is currently unanswerable.
  18. Nov 12, 2008 #17
    Life is comparable to a highly contageous planetary illness IMO. Whenever possible, it will colonise a host planet so i'd be truly surprised if it's not widespread in one form or another on most(if not ALL) relatively habitable planets.
  19. Nov 12, 2008 #18


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    Well, a metaphor can only carry you so far. And that's all this is really, a metaphor. The mechanisms for infecting an organism and the mechanisms for colonizing a planet are unrelated (though again, you could find some nice metaphors).
  20. Nov 12, 2008 #19
    In that case why have none of the other planets in the solar system caught a cold from the Earth? In the absence of evidence to support your hypothesis I would have to downgrade it to interesting speculation. (Or nice metaphor.)
  21. Nov 13, 2008 #20
    Which part of my post didn't you get? The whole post said:

    May i suggest that you first familiarise yourself with the concept "Habitable Zone" before jumping to wrong conclusions:

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