Andrew Sullivan has a neat little post that brings up the topic of Intragalactic Ethics.
There is a link to a Josh Rothman column that points out:
There’s a long pro-terraforming tradition (especially among philosophically inclined science-fiction readers): Turning a lifeless place into an inhabitable one seems like a noble goal. Meanwhile, others argue that we have a moral, and possibly even an aesthetic, obligation to leave extraterrestrial life untouched.
My biggest question is where this moral/aesthetic obligation would come from.