There is an equation which you can derive which relates the temperature of a dust grain (or any object, really) to the distance it is from a star:
In this equation T* is the temperature of the star, R* is the radius of the star and Tg is the temperature of the dust grain.
The distance that the earth is away from the sun is called an astronomical unit or A.U. so earth is 1 A.U. from the sun.
If you put the temperature of water freezing and the temperature of water boiling in the above equation you find that from our Sun, liquid water is possible from 0.51 A.U. to 0.95 A.U.
If you are surprised that liquid water is not possible here on Earth there is, of course, an explanation and it is the greenhouse effect. The earth also has some stored heat in its core. So the earth’s average temperature just happens to fall exactly in the realm of liquid water.
This range — 0.51 A.U. – 0.95 A.U. — is extremely small in astronomical terms. At roughly 0.5 A.U. water boils. At right around the orbit of the earth, water freezes. Aren’t we lucky to have ended up right here in just the right place?
Of course, those that didn’t aren’t around to blog about it.