Darren New <dne### [at] sanrrcom> wrote:
> http://www.youtube.com/watch?v=fgg2tpUVbXQ

  The video posits that life in other planets is likely due to the humongous
amounts of stars and planets out there. On the other hand, the opposite
hypothesis is scientifically sound as well:

http://en.wikipedia.org/wiki/Rare_Earth_hypothesis

  In short, the hypothesis states that life requires such ideal conditions
that most planets, and even most galaxies simply don't provide them.

  The volume inside a galaxy in which any kind of life could possibly form
is quite small. If a star is too close to the core of the galaxy, radiation
will be too high, as well as the density of stars and other objects (which
greatly increases the likelihood of catastrophic events terminating any
possible life-producing conditions). Too far away from the core, and the
metallicity of star systems will be too low to produce any planets which
could produce life.

  The shape and other features of the galaxy matter too. Some shapes make
it less likely for habitable conditions to form (because the above problems
will appear throughout the entire galaxy, among other things). This would
mean that only a small portion of all galaxies could support life of any
kind. It also means that most if not all early galaxies in the history of
the universe were inhospitable, and thus it's not very probable for life
to have been formed any time soon after the big bang.

  Even inside the "habitable zone" the conditions must be very ideal.
A double star (which is relatively common) will be detrimental to any life.
Even single stars will require other conditions to be life-friendly (such
as having giant gass planets at an appropriate distance which vacuum the
stellar system from dangerous, life-terminating objects). The star itself
must be of a certain type: Too big or too small, too hot or too cold, or
too much radiation, and it will be unfriendly to life.

  A planet must have certain conditions to produce life. Too close to the
star, and it won't have any water or other liquids (which is most probably
a prerequisite for life). Too far, and the water will be frozen. Also, the
orbit of the planet must be almost circular for this same reason.

  A moon which causes tides, and plate tectonics also help in the process
of life forming. Even if a planet is otherwise ideal, if it lacks those,
it will be much less likely for life to form.

  There are a big bunch of other conditions which must also be just right,
or else life will be much less likely. The video calls the Sun "unremarkable",
but in one thing it's extremely remarkable: It's probably one of the very
few stars in the galaxy, if not the entire universe, which has all the
necessary conditions for life.

  A common concept is that the universe must be full of life, but that's
not necessarily so. Life might be extremely rare. Intelligent life even
rarer. (The conditions necessary to produce intelligent life make this
whole situation even more complicated.)

-- 
                                                          - Warp

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