Adapted for the Internet from:

Why God Doesn't Exist

    1.0   Statistically, we can't be the only life in the Universe

    The astronomer Harlow Shapley estimated that if one out of a trillion planets meets the tests for life, in an
    observable universe with 100 million trillion stars there should be some 100 million planets teeming with
    intelligent life. [1] Most astronomers will probably agree with him. The experts extrapolate these
    conclusions from the elevated number of Sun-like stars that they observe in our galaxy:

    “ the Milky Way, is 100,000 light years across and contains approximately a hundred
       billion stars… About 10% of the stars in the Milky Way galaxy are Sun-like, and there
       are about a thousand such stars within 100 light-years of the Sun.” [2]

    The insinuation is that, from a statistical perspective, it is next to impossible that ours is the only planet in
    the Universe that has developed intelligence, let alone life. Spectral analysis has confirmed that the most
    important elements of life a we know it – carbon, hydrogen, oxygen, and nitrogen – are found in most
    stars. These statistics give organizations such as the Search for Extraterrestrial Intelligence (SETI)
    confidence to spend your tax dollars in an effort to talk with someone other than an earthling over the
    phone.

    [Lately, SETI relies more on private donations. Government funds dried up because SETI
     failed to produce tangible results. However, taxes sometimes have a stealthy way of
     reaching organizations such as SETI through contracts. Nevertheless, if government
     money is not being diverted to SETI at this moment, this doesn't mean that it won't flow
     in the future as it did in the past. Therefore, it is worthwhile to determine whether SETI
     has any reason to exist in the first place.]


    2.0   But statistically, we are the only life in the Solar System or that we know of!

    However, probability can also be used to play devil’s advocate:

    •        The lifebelt is constrained to an insignificant 0.00004 % of cross-sectional area of the Solar
    System.
    •       The mass of our planet including the life upon it is only about 0.000003 % of the total mass
    of the Solar System. Watson [3] estimates that life comprises only about 0.0000001 % of the
    mass of the Earth.

    These numbers objectively show what a scarce resource life really is in our own neighborhood! If we now
    consider that the Sun was burning for millions of years before life arose on Earth and will probably
    continue exhausting its fuel after life has ceased, we have to place these numbers in an even bleaker
    context. While inanimate matter is eternal, life only has a minute to make its case in court. Hence, left to
    statistics, it could just as well be argued that it is overly optimistic to expect e-mails from outer space
    within our lifetimes.


    3.0   Life demands tight specifications

    In fact, when we look at the tolerances known to be necessary for life, we end up with a very
    disheartening picture about the possibility of life existing elsewhere. You should conclude that it is
    statistically more improbable to discover a system teaming with life within the next million years. By
    merely considering the hundreds of celestial bodies drifting mindlessly throughout the Solar System, life
    does not seem to be a very popular phenomenon. As a minimum, life requires an atmosphere, and only
    massive objects such as planets and moons may generate the necessary gravity to hold one. Gravity, in
    turn is a function of mass, but then both gravity and mass are like the story of Goldilocks: the planet can't
    be too big and it can't be too small. It has to be just right.

    Life also has an absolute respect for the God of Temperature. And temperature essentially comes down to
    how far a planet is from the Sun! Just to get a feel for how sensitive climate is to distance, consider the
    extreme temperature changes between Summer and Winter in the northern and southern hemispheres.
    This temperature gradient is due mostly to the tilt of our planet. Hart calculated that life on Earth could
    only have if the Earth maintained a distance between 0.958 and 1.004 astronomical units (AU) (Earth = 1
    AU). [4] A little too close and we would have fried. A little too far and we would have frozen.

    And then it is also a good bet that life cannot do without water and other chemicals such as carbon and
    nitrogen. For all we know, life may require volcanic activity, planetary spin, a magnetic field, or a particular
    tilt with respect to the Sun.

    The point is that there are a number of environmental conditions that life simply cannot do without (Fig. 1).
    [5] Researchers refer to the regions within a solar system and within the galaxy as ‘habitable zones.’ [6]
    Gonzalez reviews habitable zones from a universal and a local perspective and concludes that they are
    extremely scarce. [7]  

    Could life have developed if the Earth was located far from the ecliptic or if the Sun was near the center of
    the Milky Way?

    Your guess is as good as that of the statisticians. Many factors have to come together in order for life to
    evolve. If life were easy to make, the Earth would not be the only celestial object teaming with life among
    thousands of planets, moons, asteroids, and other objects in our own vicinity. That’s why it is baffling to
    see people like Carl Sagan, who has estimated that the probability of life evolving on an Earth-like planet
    is one in 1x102,000,000,000, [8] waiting for a call from outer space. This is irrational enthusiasm. Why
    would any statistician continue playing dice against odds like that?
Life requires
tight specifications
Man! The atmosphere on
Earth stinks. And the
pressures are intolerable!
Who can live in this place?

    4.0   It seems that we can predict the appearance of life from the distribution and abundance of chemical
    elements

    It could be argued that the nebular theory of stellar and planetary formation renders life inevitable. All that
    God needs to do is throw the right amount of starting material into the pot and stir. According to the
    nebular theory, our solar system has its origin in a mass of swirling gas and dust, the remnants of an
    earlier supernova. Consequently, the development of an Earth-like planet would be solely a function of
    the composition of the primordial soup. Gravity does the rest. Gas and dust condense into a disk that
    begins to spin. The disk gradually sifts elements by mass. Planetesimals form. And life finally arises
    somewhere at 150 million kilometers from the star. Hence, if the starting composition of gas is like the one
    that gave origin to our solar system, we should end up with the same Sun-like star and a handful of
    planets in every single experiment.


    5.0   The problem is that we don't know the ratios or distributions of chemical elements within the Solar
    System

    However, even if we look at planet formation in such favorable light, we still would be unable to reach
    conclusions about the possibility of ET life. The trouble is determining the exact composition of stellar
    systems. Our solar system is essentially comprised of two chemical elements (90% hydrogen and 10%
    helium).The remaining elements of the periodic table contribute less than 1%, most of which is taken up
    by oxygen, carbon, nitrogen, neon, magnesium, silicon, and iron. It is difficult to estimate the exact
    percentage of these trace elements in our own vicinity since much depends on diverse theories of
    the internal structures of bodies. There are even disagreements regarding the internal structure of the
    Sun and the Earth. Hence, we are far from the ability to determine the percentages of heavy elements in
    distant stars with any useful accuracy. If we theorize, in addition, that the distribution of these ingredients
    in the primordial mixture determines key parameters in the future proto-planet, such as mass, tempera-
    ture, and distance from the sun, this opens up a whole new can of worms. It means that not only must the
    initial material and proportions be identical to that of our solar system, but that the qualitative distribution
    of that material within the initial cloud of gas must be within certain specifications as well. This reduces
    the chances for an Earth-like planet considerably. Indeed, despite countless computer simulations,
    Gladman confesses that we know very little about how planetesimals form and how to derive the
    aforementioned parameters. [9]


    6.0   Therefore, we will not discover whether there is alien life through statistics

    To summarize, if we cannot answer the question of how our Sun and Earth are constituted, we cannot
    answer which factors were absolutely necessary for life. And if we do not know which factors are
    necessary for life or the environmental tolerances, it is trivial to speculate about life in other regions of the
    galaxy on the basis of statistics. If life requires an absolute clone of our solar system, then life is probably
    not a very common occurrence anywhere. If life does not require such stringent specifications, we need
    to understand nevertheless which parameters are crucial and what the range of tolerances are for each.  
    Until we have this information, all we are doing is speculating and spending money looking for inexistent
    aliens. If we judge solely by the results of such groups as SETI,  which have been searching for
    extraterrestrial signals for over 40 years, it is clear at least that human-level intelligent life is not a very
    common phenomenon.

    Despite all this ranting, the issue of whether life exists or not in other planets is not an issue of
    knowledge. Life exists (or not) in another planet of the Universe whether we know of it or not. Whether
    you hope (as opposed to believe) that life exists elsewhere is a matter that comes down to whether you
    are a pessimist or an optimist. Certainly, a mathematician cannot prove that life exists elsewhere by
    crunching numbers. If we are the only planet teaming with life in the entire universe, statistics makes the
    mathematician's guess no better than yours.


    7.0   Do humans necessarily develop in planets that are environmental twins of Earth?

    A different issue is whether intelligence inevitably arises on worlds that are environmentally virtual twins
    of the Earth. The initial conditions and tolerances are now no longer a matter of speculation, but are
    presented as a set of assumptions. The proponent is not going to theorize about vital parameters. The
    proponent is introducing them as part of his statement of the facts. What if we started the process on
    Earth all over again? Would we end up with Man?

    Intuitively, one would think that the closer the tolerances are to those of Earth, the more likely that not
    only life, but similar life developed, including human-level intelligent life.

    Some theorists have trouble with this train of thought. They argue that had environmental conditions
    been identical to what they were throughout the history of life on Earth, events would have likely taken a
    different turn at any time and Man would have remained a promise. The formation of specific types of life-
    forms, so the reasoning goes, is not a deterministic process, but relies exclusively on free will, on the day
    to day actions of countless living beings. Anything could have gone wrong along the way and the
    chances of us being here today are reduced to almost nil.

    Let’s look at this argument in more detail to see if it carries any weight.

Fig. 1   Vital specifications: the habitable zone
The figure synthesizes some of the parameters necessary for life. Here I depict: planetary mass
and density (i.e., the size of the Earth), distance from the Sun, magnetic field, tilt, deviation from
the ecliptic, and the size of the Sun. You can readily plug in a couple of others (atmosphere, types
and ratios of gases that comprise it, water, carbon, volcanic activity, etc.). You should conclude
that life is very sensitive to physical parameters, very fragile. The question is: Does quantity
(number of stars) guarantee quality (life)? Can statistics tell you whether the right combinations
for life have developed elsewhere?

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