Talk:Planet X/@comment-25172205-20150412002539/@comment-25285745-20150412162703

The astronauts would have probably known if Planet X had carbon dioxide and oxygen, since their mission was to specifically land on and explore that planet. Besides, the Xiliens are at least partially cybernetic (you can see weird metal bolts on the body of one of them in one scene), so they may be enhanced beyond the need to breathe oxygen. They might also be so radically different from terrestrial life that they don't require oxygen in the first place.