One of the safer bets when it comes to Doctor Who predictions is that, at some point, humanity is really going to crack this space travel thing as a viable prospect for those that havent got significant government backing and air force training, and collectively as a species well start spreading throughout the cosmos. Earth wont be our sole home any more, as we colonise the other planets in our solar system and, if faster than light travel is ever invented, even further afield than that. Again, thats the optimism of Doctor Who coming through. After the whole first contact, space spiders thing, the scope of humanity expands a whole bunch. In the episode the Tenth Planet, analysis of Cybermen ships that crashed back in the sixties reveal the tech needed for proper space exploration, by 2058 colonisation had begun, and by The Waters Of Mars (set around 2100) interstellar travel had us scattered all over the shop. As soon as we get that technology, thatll almost certainly happen; other ideas, like humans beginning to identify as Martians and Venusians instead of Earthlings as we spend longer living on those planets, is also pretty plausible.