Quoting GoaFan77, reply 2... or edit the English.string file to change the list of names planets are randomly assigned.
I'm not so sure about the latter Goa, there appears to be a 24 character limit for planet names that are defined in the string file, so I think harpo may be correct. I recently spotted this when testing the B5 planet names for the Sins of the Younger Races mod - the 'y' of 'MacArthur Midrange Colony' gets clipped off (using Dip 1.37).
Hmm, I actually hadn't tried it with planets, but with capitalship names you can have names longer than you can put in game (some of the default TEC names are actually well over the limit). Actually I think 24 might be more than you can manually type as well.