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I Speculative Term in Drake's Equation

  1. Apr 5, 2016 #1
    The Drake Equation has been around since 1961,
    and there have been some later extrapolations as well. The probability of there being life somewhat similar to Earth life somewhere else in our Milky Way galaxy seems to be strongly dependent on the equation factor
    fl = the fraction of planets that could support life that actually develop life at some point.​
    This seems to me the most speculative of all the terms in the Drake equation. For example, the following article discusses the possibility that the Earth's unusual moon may have played an essential role in life occurring on our planet.
    I have unsuccessfully tried to find any source that discusses the probability of an exoplanet "that could support life" having an unusual moon like the Earth's.
    One way of describing the unusual nature of the Earth's moon is that it is the only moon in our solar system that has a larger angular momentum than its planet's rotation.​
    Quite a few computer simulation models have been developed to explain the moon's origin. Can someone cite a reference that discusses the statistics from such models so that some ballpark estimate might be made for the probability that an exoplanet might have a moon like ours?
  2. jcsd
  3. Apr 5, 2016 #2


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    I think it is worse than this. All of the last four terms in Drake's equation (fl, fi, fc, L) are completely unknown - we can't even begin to estimate them. I hear people say things like, "what if each of these terms is only 1 in 1000...". Sure, but what if each of these terms is 1 in 10^20? We really have no idea.
  4. Apr 5, 2016 #3
    Hi phyzguy:

    I appreciate that the probabilities for the factors relating to intelligent life are even more speculative than for life. However, my current interest is about any life at least somewhat similar to Earth life, say perhaps carbon and water based and cellular. For this issue, fl seems the most speculative.

  5. Apr 5, 2016 #4


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    Most of the terms in the Drake equation are difficult to reliably quantify. While it is widely accepted the moon probably played an important role in the emergence of life on earth, the nature of its influence is still uncertain. You could also make the case there is something fairly unique about every large moon in the solar system, but, earth remains the only planet known to harbor life. So any unique properties of earth's moon looks to me like a red herring until we have clear evidence some particular factor was vital to the emergence of life on earth. Factors that are merely favorable towards life only suggest life would be delayed by their absence. My suspicion is the moon probably was favorable, but, not vital to the emergence of life on earth.
    Last edited: Apr 5, 2016
  6. Apr 5, 2016 #5
    Hi Chronos:

    As I said in Post #1, I agree that the role of the moon is speculative. However, I would like to know, if it is possible to do so, what value for fi could be ballpark estimated assuming that the role of the moon is vital for life to get started. I am hoping that at least one participant in the PF will know enough about the computer simulations of the moon's origins to help me make an estimate of fi under this assumption.

  7. Apr 5, 2016 #6
    Even if it turns out that the moon was vital to start life on this planet - that would tell you nothing about the value you're looking for. Because it doesn't tell you anything about whether it's vital for other planets too.
  8. Apr 5, 2016 #7
    Hi Jando:

    I find that the descriptions of the reason given in various sources, e.g. the Scientific American article I cited, seem to all have the characteristic of being necessary for one or more of the pre-life chemical processes to actually occur on any planet that could support life. Which of the particular reasons given in the S.A. article for the moon's necessary role do you find the be restricted to just the Earth?

  9. Apr 5, 2016 #8
    Awfully sorry, but all of them. :) The article only talks about this solar system, this earth and this moon. Whether that has any relevance for other systems, other planets and other moons is completely unknown.
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