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Our Improbable Home

  1. Dec 3, 2009 #1
    A little blue marble, lost in the vastness of space,
    As inconspicuous as a penny dropped in a lake,
    Yet it provides a home for the human race.

    Orbiting a star of just the right temperature,
    If it were dimmer we could not survive the cold,
    And if it were hotter we would burn up for sure.

    In a nearly circular orbit; what’s the chance of that?
    If the gas giants were orbiting near or widely elliptical,
    Our little rock would be ejected like kicking a cat.

    We have a thin atmosphere while Venus, the same size,
    Has crushing pressure and a runaway greenhouse effect,
    Which makes it uninhabitable; any living thing there dies.

    We would too if we had not been struck by another planet,
    Which created the moon and blew away our atmosphere,
    When it hit the Earth a glancing blow, circled and did ram it.

    A square blow would have shattered our brittle world,
    While only the most precise angle could accomplish this.
    What’s the chance such a thing through space was hurled?

    If the moon were not there our axis would be unstable,
    The seasons would vary wildly if our planet tilted over more,
    It is only by holding steady at 23° that life here is enabled.

    Had the oceans frozen over, the ice would reflect the light,
    And never thaw. If they ever boiled away, the clouds
    Would hold the heat in and never condense. That’s right.

    Either extreme, freezing or boiling, cannot be reversed.
    Yet for billions of years we have walked this tightrope,
    Between 32° to 212° ocean temps and avoided the worst.

    It takes billions of years for intelligence to slowly evolve,
    Had this been interrupted by a large meteor or irradiation,
    Re-starting evolution, then humans would not be here at all.

    Many improbable events combined to make Earth so nice,
    The chance of failure was immeasurably close to 100%,
    The universe isn’t big enough for this to happen twice.
  2. jcsd
  3. Dec 4, 2009 #2


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    Interesting poem, but I don't agree with its anthropocentrism. It claims that Earth is "just right" for life, ignoring the fact that it was life which evolved to become just right for living on Earth.

    Some of the science is also questionable. Did the Giant Impact really strip Earth's atmosphere away? I've never seen that mentioned before now. Did the oceans really remain unfrozen throughout geological history? The Snowball Earth hypothesis:


    argues against that.
  4. Dec 4, 2009 #3
    I am not anthropocentric. My argument is based entirely on probability. I am not saying WHY we are alone - only that we are.

    Of course, life elsewhere may be so different as to be unrecognizable. But, if the word "life" is to have any meaning at all, I think we can reasonably exclude planets in very eccentric orbits, planets bathed in radiation or with temperatures well above the boiling point of water, and so forth.

    I'll get back to you regarding the formation of the moon. Regarding the Snowball Earth hypothesis,
    I was thinking of Budyko's work in the 1960's when I said that freezing the oceans over would be irreversible. However, Wikipedia states that, in 1992, Joseph Kirschvink introduced a mechanism with which to escape from an ice-covered Earth — the accumulation of CO2 from volcanic outgassing leading to an ultra-greenhouse effect.

    I was not familiar with Kirschvink's paper and Wikipedia says "aspects of the hypothesis remain controversial." Offhand, I would say that this could only have worked very early in our history, when Earth had a lot more volcanic activity.

    Actually, when I wrote the poem, the part that I was doubtful of was that boiling the oceans away could not be reversed. Water vapor is not a very potent greenhouse gas and it may be that the oceans would re-fill through precipitation. If the Earth had an ocean at the time that the moon was formed, it was certainly boiled away by the heat of the impact. When I get back to you on the formation of the moon, I will mention the current view on whether there were oceans then - I suspect that there were not.
  5. Dec 6, 2009 #4
    Can anyone tell me where we are on the big universe map? Is our Galaxy "out front-ish"? (in the Bang trajectory) ...or, as I've heard "near the middle"? And, what is in front of us? Anyone know how many light-years can be seen in front of us? ...and what's in them? I know there are more Galaxies, but do they "thin out"? Please bear with me here; NO laughing; these are important questions to me. :approve:
  6. Dec 6, 2009 #5


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    Beth Ann,

    You asked the same question in the Astrophysics forum, and I just finished typing in a rather long answer that I am sort of proud of. I worked hard to make it clear. Please try it and see how it works for you.

    Here is my answer:

    Your question was post #18 on the Astrophysics thread, and my reply was post #19.

    Here you ask identical question. It seems like a waste of space to have the same discussion going in two separate threads. How about we concentrate on the one in Astrophysics? Or should I paste a copy of my answer here in this thread?
    Last edited: Dec 6, 2009
  7. Dec 6, 2009 #6
    Thank you Marcus! I am headed over to Astrophysics now. When I posted my question, my first post BTW, I lost it and started over. It ended up here. I am navigating the Forum better now. Thanks so much for your head's up!

    I have seen your answer there - VERY impressive! I am honored! Will be responding to it when I have studied it. Thank you again!
  8. Dec 6, 2009 #7
    Aww... you beat me to it. I was going to tell her about the balloon model too. :(

    Anyway, back to the subject of the probability of encountering any little green men, I might point out that the galaxies are VASTLY farther apart than the distances between individual stars within a galaxy.

    So, for all practical purposes, it doesn't really matter if there are LGM in other galaxies because we are never going to meet them or even hear from them. Also, within our own galaxy, the stars near the center are too close together and generate too much radiation for life to exist. The stars at the far edges are composed mostly of light elements like hydrogen and helium, which cannot alone support life.

    Thus, when I say, "the universe isn't big enough for this to happen twice," if we are actually concerned about hearing from the LGM, not just speculating about their existence, we only need to consider the much smaller set of stars described above.

    So, basically, I think we are alone.
  9. Dec 6, 2009 #8

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    That is much better than your poem. That ending, "The universe isn’t big enough for this to happen twice," is far too strong.

    Wrong. A hospitable planet could exist around a cooler star, or a brighter star. All it takes is a different orbit. Because of the 100 degree range between water's melting and boiling points there is some leeway on where orbit can be. It does not need to be "perfect".

    Pretty good, actually. Planetary formation hypotheses suggest orbits should be circular. That some of the recently discovered exoplanets had markedly non-circular orbits threw astronomers for a loop.

    Again, it was the discovery of hot Jupiters that threw astronomers for a loop. Prevailing theories said that shouldn't happen.

    Recent theories saying the Moon formed in one of the Earth's triangular Lagrange points makes the Moon much less implausible than some random body that just happened to hit the Earth.

    Planetary formation theories have been thrown for a loop. What are the chances that an Earth-like will form and stay in place? The right answer is, "we don't know (yet)". We will have a much better idea in a year or two when the Kepler mission's results start coming in.

    Your poem is essentially an argument from incredulity. I agree that there certainly appear to be a lot of low probability events that led to *us*. (You missed quite a few, BTW.) The galaxy is big. The probability might be low, but to be alone in the galaxy this probability has to be very, very low. (You need to multiply this low probability with the number of stars in the galactic goldilocks zone.) To be alone in the universe, it has to be incredibly low.

    My own guess (and it is a guess; every use of the Drake equation or something like it is just a guess) is that we are alone in the galaxy. But that is just a guess.
  10. Dec 7, 2009 #9
    The other thing is that even if the chances of life forming are tiny, there are a lot of stars. If there is only a one in a billion chance of life forming, that's still about 400 stars with life. So even if the odds of life are tiny, it makes a big difference whether its one in a million or one in a trillion, because if it is one in a million then the Milky Way is just teeming with life, whereas if it is one in a trillion, we could be alone in this galaxy. However, there are a huge number of galaxies so that life has to be really, really, really improbable to be alone in the universe.

    This is something that you have to put actual numbers in things, which is what the Drake equation is all about.
  11. Dec 7, 2009 #10
    Methinks that you haven't read enough science fiction. One good thing about science fiction is that it expands the mind a bit. No you probably couldn't have anything that looks human in these conditions, but it's possible to have something like the Larry Niven's outsiders travel between galaxies.

    The problem with core stars is not so much radiation but tidal disruption of stellar systems. Also, all stars in the Milky Way have metals. One problem with these sorts of arguments is that there is a big difference between saying "life as we know it is impossible" and "life is impossible." We don't know nearly enough to make the second statement.
  12. Dec 7, 2009 #11

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    The Drake equation also leaves a lot to be desired. It kludges in time and it lumps a lot of low probability events together (or ignores them altogether).
  13. Dec 7, 2009 #12
    I wouldn't say that

  14. Dec 7, 2009 #13

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    Not to mention David Brin's sun ghosts. Or Stanley Weinbaum's silicon creatures and cart creatures. "We are v-r-r-riends! Ouch!" Or Robert Forward's cheela. Just to name a few.
  15. Dec 8, 2009 #14


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    Too many galaxies, with too many stars, with too many planets and too much time for life such as ours to be unique, IMO. I agree we are probably too distant to communicate.
  16. Dec 8, 2009 #15
    I should probably change "universe" to "galaxy" to avoid overstating my case.

    This stanza presents the least restrictive condition of those mentioned in my poem. However, it is true that the brown dwarfs are too dim for the planets surrounding them to support life. Also, very bright stars like Wolf-Rayet stars, with their tremendous stellar wind, almost certainly do not have planets that support life.

    So this stanza is a bit simplistic. But it's a poem - I can't delve into things like star classifications in a three-line stanza.

    I am not up on the recent theories.

    But my impression was that the high angular momentum of the Earth/Moon system could not be explained with Theia forming at a Lagrangian point and then catching up to Earth.

    The way I heard it, and this was some time ago, Theia struck Earth a glancing blow and set it spinning like when you graze a billiard ball with your cue ball. Theia then circled all the way around the sun and struck us solidly. Its iron core plunged through the Earth and stuck to the side of our core, making it lop-sided and causing it to wobble. Its mantle was stripped off and it and lots of earth were thrown into space, much of which fell back down but some of which formed the moon.

    Right: We don't know.

    But, as a general rule of thumb, if you lack evidence for something extraordinary, like LGM, it is a lot safer to assume that they don't exist than that they do.

    This poem was inspired by a discussion that I had at Debate Politics (I am "Onion Eater" there) where I attempted to counter statements like these:

  17. Dec 9, 2009 #16
    No it's not. The logic is that we haven't been contacted by intelligent life, therefore there are a number of possibilities. Also if you don't know, then you don't know.

    It's pretty obvious that there isn't a massive number of loud and highly advanced civilizations. However the Milky Way could be teeming with life just not intelligent life. It's also possible that intelligent life is common but that either civilizations tend to blow each other up or else evolve into something unrecognizable. Again science fiction is useful because sci-fi writers can come up with interesting wild speculation.

    The trouble with that is that if work through the numbers of the Drake equation, I'm not getting the results you are. What you are arguing is that improbabilities in the formation of the earth make life unlikely. The trouble with that is that we are getting more and more data about exo-planets, and my impression (although people are free to disagree) is that having an earth like planet is a one in a million possibility rather than a one in a trillion one. In another thread, I had to look at some papers involving planetary formation models. You can run the numbers to see what the likelihood of an earth-like planet forming is, and my impression is that the bottleneck is not there. Yes it's improbable that you'll get an earth like planet, but it's not *that* improbable (and whether you need an earth like planet for life is something that we don't know).

    Now it could be that the bottleneck is elsewhere. There could be something that make life extremely improbable even with an earth-like planet. It's also possible that life is easy but intelligence is hard. It's also possible that intelligence is easy, but that civilizations only last for a few thousand years before "something happens."

    If we've confirmed life on Mars this vastly changes the odds. Something to think about is that life has existed for about 3 billion year but multi-cellular life has existed only for 600 million. Something else to think about is that modern humans have existed for about 200,000 years but its only in the last 5000 years that we have had something approaching civilization. It may be that intelligent life is quite common in the galaxy, but that having something we'd recognize as civilization requires a set of extremely improbable climatic changes.
  18. Dec 9, 2009 #17
    One other thing. At one point I read a book on "space law" and the last chapter was on the legal status of space aliens. There are three possibilities. We meet aliens that are more advanced, the same technology, or less advanced. If they are more advanced, then it doesn't matter. We do whatever they tell us. If they are the same technology level, then we start something like diplomatic negotiations, and this isn't a hard problem.

    The really, really hard legal problem that you get into is what do you do if we start exploring other stars and find aliens that are less advanced.
  19. Dec 9, 2009 #18


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    I believe we would deal with less advanced civilization with the same dignity and respect as historical colonizing force did with indigenous populations here on earth. Evolution has hardwired us to subjugate the weak, joust with equals, and destabilize our superiors.
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