This question has been asked many times, so I definitely recommend checking out some of those that users have already posted. From a statistical standpoint, Aliens do exist. Look up into the night sky and try to count the stars. It's not even a fraction of the amount of stars/galaxies that are actually out there. Our star has one habitable planet and, as long as those same specifications are met elsewhere, it is logical life would start there as well. The numbers are the proof.