That is a very scary thought. The last thing Steve thinks his planet needs is an outside threat. They have enough issues trying to keep the ones currently living there in order.

"So you don't age?" Steve can do the math, twelve bodies over two thousand years means the Doctor can live in a body much longer than a human but if he's only been this old once or twice... He's not trying to be nosey, but simply understand. Steve is nothing if not very, very curious.

Steve's curiosity brings him to another question. "So beside the body swapping and how old you live, is there anything else that makes you different from humans? I assume for the time and space travel you need the TARDIS, or is that not the case?"