National Geographic News is running an interview with space scientists Seth Shostak and Bruce Betts on whether the extraterrestrial worlds in the Star Wars films are realistic. In many respects, it turns out to be more a question of biology and (gasp!) extraterrestrial anthropology than geology or physics.
For example, Shostak says:
What we might complain about is that so many of the galactic sentients [intelligent life-forms] seem determined to live on planets. Truly advanced life is likely to build its own habitats, and escape the limited area and resources of a planet. In Star Wars, it seems that only Monsieur Vader has figured this out, building his own artificial habitat, appealingly monikered the Death Star, even though it's not a star at all. But "Death Space Habitat" doesn't have as nice a ring to it.
Aside from the spoiler questions (how do all those planets have the same oxygen level, anyway?), Bett and Shostak consider the makeup of a galactic empire:
[I]t all seems unlikely, because the various inhabitants, many of which are biological, will have evolved at different times. Consequently, the top species will be many millennia ahead of the number two species, in terms of evolution, and millions and billions of years ahead of your average intelligent species. They won't want to share drinks with them in a Mos Eisley cantina.
But we need them there, to show us how Ardipithecus walked!
Much interesting consideration of the ecological makeup of different planets and whether they would be likely to occur.
But why, oh why did they have to go here:
NG News: What about Naboo, for example, the home planet of Queen Amidala? It's an idyllic world (see picture) populated by peaceful humans and an indigenous species of intelligent amphibians, the Gungans.
Shostak: We have rather few examples of two or more intelligent species simultaneously sharing a planet, but it has happened. The Neanderthals coexisted with Homo sapiens for millennia. So maybe it's possible to share, as long as neither species has the technology to obliterate, enslave, or merely cook and eat each other.
Jar-Jar a Neandertal? In the words of Darth Vader, "Nooooooooooooo!"