Just how big is the universe I wonder - and how many others are actually out there? A quick internet search or an email to Professor Brian Cox would probably be a good place to start in answering such a question. All I know at the moment is that the Doctor Who universe recently collided with the Star Trek universe.... and the results will be available on CD & download next month!
In the meantime, Chase Masterson, Leeta in Star Trek: Deep Space 9, has written a fascinating article for the Star Trek web site about the two series and her forthcoming role as Vienna Salvatori in Dr Who: The Shadow Heart.
Here's the link: Chase Masterson Meets Doctor Who