I would appreciate any thoughts on what makes a planet habitable for Humans.
I am making asumptions that life and a similar atmosphere evolve given a range
of physical aspects of the planet.  The question is what physical aspects
simply disallow earth like conditions.

eg Temperature range of 280K to 315K (where temp is purely dependant on dist
     from the sun and the suns temperature..)
   Atmospheric presure ? - I know nothing of human tolerance
   Planetary Mass ? - again gravity at surface is important, how much
     can human bodies take day after day.  Also how does the mass effect
     atmosphere.  I thinking of planets between .3 and 3 times mass of the
     earth.  I suppose density should be important as well.

Climate etc does not concern me, nor does axial tilt etc etc.  Just the above
three factors and how they relate to one another.

Jonathan
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