Ronald Bailey ponders our responsibility in exploring other planets:
[O]ne chief reason [to avoid human contamination of new planets] is to prevent inadvertent contamination by Earth microbes from being mistaken as evidence for the existence of extraterrestrial life. But do we have an ethical obligation to prevent harm that might be caused by Terran life to extraterrestrial life? Even more broadly, do we have the right to change the environments of other worlds even if they do not contain any living organisms?
Josh Rothman weighs the arguments:
About these questions, moral philosophers disagree. 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.
The Dish 