To a certain extent, yeah. Not the stereotypical green aliens with big eyes and a giant brain, but most likely some microscopic organism or a bug, considering an alien is just anything extraterrestrial.
Aliens as we know it probably don't rely on water as animals on Earth do, which is why I think scientists should try expanding their searches to non-aquatic planets. It's also possible that something may exist at the 'seafloor' of one of those aquatic planets.
This doesn't really relate to the question, but I think it would be awesome if there was an organism that lived in the core of another planet...or even here or Earth. It would be totally unexpected and never sought out, which I think would be pretty cool.