The important concept here is the frost line. That is, in the accretion disk consisting mostly of dust that the planets formed out of, the distance from the new

The important concept here is the frost line. That is, in the accretion disk consisting mostly of dust that the planets formed out of, the distance from the newborn sun where water ice could exist in direct sunlight without sublimating and being blown away. This line is somewhere between the asteroid belt and the current orbit of Jupiter. Water inside this line has only arrived here by stray comets, nearly all of the water of the solar system is outside it.