The Comfort Zone The 'comfort zone' is a spherical region of space around the sun. It is the region that receives an amount of heat from the sun sufficient to make average temperatures just right for life. And what is 'just right', you might ask? Why, it is simply the temperature range that permits water to be a liquid most of the time. Life processes require a liquid of some sort. In order for life to develop on a planet, a liquid must widely cover that planet. This requires that the average temperature on the planet be above the freezing point of the liquid, and below its boiling point. In our solar system, the liquid is water, and the region where temperatures are just right is shown above (not to scale). Earth finds itself right in the middle of the zone. Lucky, or what!?! Luck had nothing to do with it! There are three planets that are close to this zone. Venus is just on the inner edge, and so is a little too hot. Mars is on the outer edge, and so is a little too cool. Both of these planets, however, could have supported liquid water much of the time, if other conditions had also favoured it. But they didn't. Venus developed a thick atmosphere, which helped cause temperatures to soar well past the boiling point of water. Mars was too small to hold onto an atmosphere for very long; when its atmosphere disappeared, so did the water. Earth, however, found itself, three billion years ago, exactly in the right spot to enable water to be a liquid. And earth had  lots  of water... oceans of it! Being at the right place to keep temperatures pleasant, and water liquid, earth became the planet in our solar system where life could develop. And of course it did! Water Resources