In answer to your title question, we don’t know whether Earth is the only inhabited planet. It is possible that science may someday determine that there is life elsewhere in the universe. Whether such life-forms have spiritual, rational souls subject to the need for salvation would be entirely different questions and may well be something that a Church council would have to determine should the need to answer such questions arise.
Assuming that Earth is the only inhabited planet, it is only possible to speculate on why God chose to create a massive universe and populate only a single small planet. Perhaps the differential in size between the universe and our own planet is meant to be a symbol of the differential between God himself and his creation, between the infinite and the finite. Perhaps there are other reasons, but we may never discern all of them this side of heaven.
