This question is lifted from author Sahar Sabati's informative Ask an Author series, which ran from 2015-2016 and included responses and reflections from English-speaking authors from around the globe. Since I'm often asked these questions by readers, I have republished my answers here with Sahar's kind permission.
A scientist by training, yes, I believe in the possibility of life existing on other planets. If all the life forms on Earth have crawled out of a primordial soup, then why couldn’t that have happened somewhere else, and in some other form? Professor Stephen Hawking, one of the world’s sharpest minds, is of the same opinion. In fact, Hawking has warned that trying to make contact with aliens could be inviting disaster, since if those aliens are dependent on the same resources as us, they might be looking to usurp those resources. A scary thought! Although, who is to say that those alien forms are not already here, occupying our world in some altered or supernatural state? There are plenty of unexplained phenomena, which could be attributed to this kind of otherness. But as authors, whatever our genre, be it hard science fiction or paranormal romance, it our responsibility to make our readers believe, through worlds and characters which suspend disbelief, which draw you in, captivating and engaging you. So, for us, whether or not you believe beforehand is irrelevant, because—if we’ve done our job well—you will believe by the time you close the book.