They most definitely exist. First of all, you see Uranus in a telescope, a rather low powered one at that. Secondly (and most importantly), these planets are discovered by measuring their gravitational effects on the planet nearby, in this case Saturn. Because of the pull of Uranus, Saturn's orbit "wobbles" slightly due to the perturbation, and thus a model for another planet is made. The same happened with Neptune, though its effects were on Uranus. I wouldn't worry about it, I'm sure they're out there.