Warp escreveu:
> 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.)

carbon-based life.

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