We've barely begun the planet detection process and with incredibly limited instruments are finding planets basically whereever they can exist within the limitations of our instruments. Simulations show that "earths" can easily exist in stable orbits. It would be astounding for there to be some kind of cosmic exception to earth size worlds in habitable zones as moons or planets. As soon as we get sensitive enough instruments in place we will find more planets than stars, and if only .0001% are "habitable" that will still be billions and billions as Carl S used to say and millions in our own milky way galaxy.