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       xkcd - A Webcomic - Solar System Model (2709)
        
       Image:
 (PNG) solar_system_model.png
        
       The Earth is, on average, located in the habitable zone, but at
       any given time it has a certain probability of being outside it,
       which is why life exists on Earth but is mortal.
        
       Comics reproduced from www.xkcd.com under license.
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