Alien Life might be non-existent?

Though I'm not saying I believe in aliens think about all the other things scientists have said are impossible. In the 1400s if someone would have suggested the concept of the light bulb, scientists would believe that, that would be impossible. How do we know if as our technology advances, that we won't find another planet very similar to earth, that is ale to sustain life. Looking up into the night sky, its hard to believe that scientists know everything about it. Maybe after more research something may surprise them....