Allah (SWT) knows what He created and we do not know. We believe that the creation and the order is in the hands of Allah.(Al-A'raaf, 54).
We can not reject the possibility of creatures in other planets as we have no evidence suggesting impossibility of creatures in any part of the universe. No doubt our knowledge as ordinary human beings is limited but the question itself (Whether there are aliens on other planets or not) does not have direct effect on our religious duties and our day to day life. That is why we need not to focus on such issues more that believing that Allah creates whatsoever He wants and selects.(Al-Qassass; 68).