Budget constraints mean the new series of Doctor Who is largely earthbound, says writer Russell T Davies.
Freema Agyeman and David Tennant star in Doctor Who
In the most recent series of Doctor Who, only two programmes were set on other worlds.
"People will say, 'Why doesn't he visit alien planets more often?'" he said. "But that's because they are expensive. They're hugely expensive."
Davies also told Doctor Who magazine that these episodes gained the lowest viewing figures of the series.
"The programmes that do show alien planets are not prime-time programmes," he said.
"Star Trek and Stargate are subscription-based programmes for a dedicated audience."
The writer added that he would not be using forests and quarries as stand-ins for alien landscapes, as was often the case in classic Doctor Who episodes.
"The mockery we would get walking into a forest and saying that we're on the planet Zagfon!
"If you think we had one or two bad reviews in the second series, they would become like a machine gun the moment we started doing that."
Davies, who also acts as executive producer of the Doctor Who programmes, admitted the series had become London-centric, despite being produced by BBC Wales.
The series frequently uses London as a backdrop
"For your ordinary viewer, it's the default setting," he said.
"You've got big vistas and things, and it's good for the image of the programme."
However, the writer said he would "love to shoot on the streets of Manchester".
A third series of Davies' Doctor Who is due to air in spring 2007, following a Christmas special this year.
It will see actress Freema Agyeman, 27, taking over from Billie Piper as the Doctor's assistant.
Two spin-off series, Torchwood and The Sarah Jane Adventures, are currently in production.