The Clangers's planet is dark, therefore all planets are dark. All real planets, anyway. Captain Kirk kept visiting places with lurid red or green or pink skies, but that wasn't real, not like the planet the Clangers lived on.

At least, that's the best explanation I can come up with for the fact that when I hear "planet," I think of something grey, with craters, against a dark black sky. Oliver Postgate has much to answer for.