If you define an "alien" as anything that, by our earthly definitions, is alive, then it stands to reason that they do in fact exist. However, the first definitive discovery of extraterrestrial life won't be anything that's like us; the first discoveries will be more on the order of bacteria or viruses. Already around the moons of Jupiter there's evidence that liquid water exists -- and that is the first and critical determining factor to finding anything that has life processes like our own. Mars may also have water in liquid state under its surface; for that matter, also the Moon has had ice discovered in deep crevices, and maybe there's water further down.
As far as the Vulcans or Little Green Men showing up on terra firma, that's not likely to happen anytime soon, if ever, with our current understanding of science and physics. If, in fact, (and not science fiction) we're able to leave the Solar System easily, and visit other Earth-like planets, then there's a good chance we may even find life developed on par or above humanity. See links for the Drake Equation -