I never encountered the term alien planet but exoplanets are planets outside our solar system. In a way all planets are alien to one another. Alien to us is anything extraterrestrial in this context.
Yeah that was my first thought when reading the headline: "yet another habitable planet found" is getting old, now we need to mention it's "alien" (generally associated with green men, or more broadly, life not originating from earth; not with a planet) to get attention.
