In astronomy, the "habitable zone" refers to regions in space around stars that allow for optimal conditions either for the emergence of native life via abiogenesis or for Terrestrial life to live in. These zones are usually a region of space described between two circles or spheres in which a planet, asteroid belt, or space station may orbit through. The margin of space between these two circles or spheres may vary in thickness and distance from the host star depending on the size, type, and age of the star, as well as the size and type of planet. This is also commonly known as the Goldilocks zone because it has conditions which are "just right" for life.
- If the Earth were just a few miles further away from the sun, it would be too cold for life to exist, and if the earth were just a few miles closer to the sun, it would be too hot for life to exist. Surely you can see the intelligent design in that!
". . . imagine a puddle waking up one morning and thinking, `This is an interesting world I find myself in, an interesting hole I find myself in, fits me rather neatly, doesn't it? In fact it fits me staggeringly well, must have been made to have me in it!'" - Douglas Adams (As quoted by Richard Dawkins)
The fallacy is applying life as-we-know-it to a theoretical world with significantly different environmental characteristics. Life on earth is well adapted to the conditions on Earth. If those conditions had differed, even slightly, life as-we-know-it would not be possible. The key phrase is "as-we-know-it". Compare Antarctica to the Sahara. These are some of the most extreme environments on earth, and yet we discover life forms in both of them. Bacteria and micro-organisms thrive in even more diverse environments - in sulfur ponds and surrounding volcanic vents, for example.