Re: How critical is the distance of the earth to the sun for life to exist?

      I also remember being told that if Earth were just a bit closer to 
or farther from the sun then life could not exist.  Actually, there is a 
range of distances within which the Earth could orbit the sun and still 
support life.  I vaguely recall hearing about this on a television 
program, but I have been unable to find any reference to this "habital 
zone," as it was called, anywhere else.  
     According to Cesare Emiliani, a geologist at the University of 
Miami, "given a planet like the Earth as a substrate, it is virtually 
impossible to prevent the evolution of live."  We can take this idea and 
expand it a bit further.  What does the Earth provide that allows life to 
form?  Could other planets provide these same essentials but in different 
ways?
     Jonathan I. Lunine, a scientist at the Lunar and Planetary Laboratory 
in Arizona, writes that life needs a source of carbon and water.  He does 
mention that oxygen is not necessary, because we know that cells can use 
an alternative compound such as carbon dioxide (as in plants) for cellular 
respiration.  Emiliani adds that we need a source of energy, and for 
primitive life that was UV light from the sun, lightning, and volcanic 
heat.
     Recent evidence shows that these conditions might exist elsewhere in 
the solar system.  Right now scientists are looking at Europa (a moon of 
Jupiter), Titan (a moon of Saturn), and Mars.  Europa shows evidence of 
liquid water under its icy surface - it probably has a molten core that 
provides enough heat to keep water liquid.  Titan has an atmosphere but 
probably only frozen water on its surface; however, meteor impacts may 
melt the ice and allow brief periods for liquid water to exist.  Mars 
shows evidence that liquid water once flowed on its surface and that 
perhaps a thick atmosphere once covered the planet.
     So we can see that energy sources can come from places other than the 
sun.  A planet's interior may provide thermal energy that can help life 
carry out its chemical reactions.  Even if a planet were too far away to 
receive enough sunlight for life to exist, over time the greenhouse effect 
(given a thick enough atmosphere) would warm temperatures enough for 
liquid water to exist (as is theorized with Mars).  And we know that 
carbon is fairly common in our solar system - a lot of it trapped in the 
form of carbon dioxide, carbon monoxide, or methane.
     But if we turn our attention back to Earth, evidence shows that the 
early sun, when life was just starting out 3.8 billion years ago, was much 
fainter than the sun we know today.  So the Earth didn't receive nearly as 
much solar energy as it does now, yet life still evolved.  Lunine writes 
that at that time the sun's luminosity was 75% of what it is today, and 
that it was 25% dimmer.  This means that past temperatures were 
significantly lower than today.  He goes on to say that at 75% of present 
luminosity, the mean temperature of the Earth would have been 268 K 
(that's -5.15 *C, below the freezing point of water!) had it not been for 
the greenhouse effect.  So an atmosphere capable of trapping heat is 
important for planets too far from the sun to support liquid water.
     All this evidence can lead us to only one conclusion.  The distance 
of a planet (or moon) from the sun may or may not plays a role in the 
formation of life.  We Must consider other factors.

Hope this helps.

I consulted the following books:

"Earth: Evolution of a Habitable World" by Jonathan I. Lunine.

"Planet Earth: Cosmology, Geology, and the Evolution of Live and 
Environment" by Cesare Emiliani