I believe that there are different alien life forms in other planets.
first, we must know what is life,
second,different life condtion result in different life forms.human still can't recognize all the virus the life forms on the earth, so now we can say nothing of the life forms on other planets ,which have so different life condition compare with earth.
third, all these just theory we must prove it .
Te knowledge we owned is not enough, but we are more and more close to the it, and it will be a long and hard time