The history of astronomy has been a history of receding horizons. The invention of the telescope took us beyond our naked-eye capabilities, to millions (and later billions) of stars within our own Milky Way. The application of photography and multi-wavelength astronomy to telescopes brought us beyond our own galaxy, to the distant “island Universes” populating all the space we can access. Yet, for all we know about the distant Universe, there still may be undiscovered worlds in our own Solar System. Why is that?