When taking into account the vastness of all that exist, it is probable alien life exist outside our own planet earth. Though when you take into account the infinity of time you must accept that we could be the first occurrence of life, the last or any and all between. Its possible life existed somewhere in the universe 1 billion years before us and already went extinct. Its possible there are 30 other planets in the universe where life came to be right now independent of each other planet. Its possible life wont occur again after earth for another 2 billion years. When taking into account the infinity of time one can not just assume that because of the vastness of the universe that there is likely other alien life out there. You can only say it is possible.
I think there is life in other planets, also that they already tried to contact us or even got here, what I don't believe is that they would do it so ocassionally 'cause that could really create humans to go berserk and if they're testing with humans then I think the project could be a failure?
I think, on a scientific/logical scale, it is highly improbable that the life forms on earth are the only life forms within the known universe.
With that said. I believe in a constantly expanding, endless universe - so yes. Some form of "alien" species must be out there somewhere!