Everything is absolutely possible, except time traveling into the past, of course. If you watch programs such as through the wormhole with morgan freeman, or through the universe with stephen hawking, you can learn a lot about this type of science. Most scientist, until recently, believed we needed to find a planet with water in order to find a planet that could possibly sustain life. We recently discovered this was an awful way of thinking. Another planet could harbor life, but not life in the way you're probably thinking. If we ever do discover life on another planet, without it discovering us first, it is likely that it'll be in the form of bacteria or microorganisms. As far as extra terrestrials go, there chance is low but it's there. The only real sign the human civilization has ever had was back in 1979 when we received a radio frequency while scanning the stars. This frequency was from over 200 light-years away, and it's exactly the type we'd expect from an extra terrestrial being. We searched the stars of that area for years, but no other frequencies were ever recorded. We, as a planet, take a lot of precaution when it comes to trying to contact an outside world. If you've ever watched any alien movies, you know the dangers this poses. The scientist of the world are strictly against us trying to communicate with alien life-forms, because if we were ever to make contact, it would be a large danger to our planet. On another note, if we were ever taken over for our resources by another civilization of alien life-forms, it would more than likely be by a nomad civilization. Theoretically, these are alien civilizations that eventually outgrew their home planet, and now travel the universe in ships, exploring every planet they pass for fuel, energy, and resources to build more ships to hold more of their kind. tl;dr? You missed some great insight on the topic at hand.