I think the question shouldn't be about whether aliens exist or not, but rather how well they are done. There's nothing I dislike more than humanoid aliens who might as well be humans by another name. Humanoid aliens can be done well if their culture isn't some human clone or a generic warrior race. Otherwise you might as well make them human and avoid the silly explanation for their parallel growth. In fact the best aliens have little resemblance to humanity in any way.
Another thing to point out is that playing humanity doesn't have to limit physical form or culture. Cybernetics, and bio-tech can result in people who wouldn't even be recognizable as humans. If the setting has technology for uploading one's mind to other bodies or machines then the possibilities become even greater. Why terraform a planet when you can have the colonists occupy bodies built to its environment?
Hopefully this helps your roleplay even if this was posted weeks ago.