The pure definition of an "Alien species" is one not native to our planet.
I mean that Aliens have been on Earth for a while, not saying they were born here lol. They can come and go as they please, they have the technology.
Basically the conditions that were met for the human race to become sentient is are very rare and it's also very unlikely that the same living conditions happened elsewhere in the universe.
I remember seeing a TV program that said there are other planets in the universe that have a sun and can retain life. Keep in mind that nature adapts to it's surroundings, so even if the planet in question has a totally different atmosphere of Earth, doesn't mean that it can't contain it's own life species.
To think that Earth is the only planet with life out of a zillion different galaxies all through space is quite unreasonable. We just don't have the technology to reach them and explore them, but other species in the universe might