We don’t know whether Earth is the only inhabited planet. It is possible that there is life elsewhere in the universe. Whether such life-forms have spiritual, rational souls subject to the need for salvation would be an entirely different question that the Church then would have to ponder.
Assuming that Earth is the only inhabited planet, one can speculate about why God chose to create a massive universe and populate only a single small planet. Perhaps the difference in size between the universe and our own planet is a symbol of the difference between God and his creation, between the infinite and the finite.