In short, yep, I believe there are aliens, somewhere out there. Maybe they're not intelligent in the way we define intelligence, but they're there.
For all we know, the other planets in our little Solar System may be inahbited. We just don't know it. How do you know the aliens on, say, Venus need water or withstandable heat or oxygen to survive? Maybe they're just so infinitely small that our minds can't even comprehend them, let alone detect them. Maybe there are still people living on Mars, they're just very well hidden. Maybe there's another form of intelligent, sentient life in Earth, apart from humans, but we just don't know it. Can we know for sure? Probably not.