Well in biology we learn that most living things require a few "building blocks" to sustain life. We classify things as alive that use these BBs and things that dont use these BBs are classified otherwise, like viruses. But youve asked an excellent question and thats the beautiful thing about science, its not permanent. An example would be a new rocket that is theoretically impossible, since it goes against a theory widely accepted in science. its an impossible microwave thruster Source http://inhabitat.com/nasa-confirms-the-impossible-propellant-free-microwave-thruster-for-spacecraft-works/ I assume if alien life is found and it is somehow made of something like dark matter NASA and science will be as shocked as they are about the thuster above And on that note....where the hell is this plasma rocket thats supposed to get us to mars in a monthish?
From what I understand now scientists are starting to expand and looking for places with liquids(any liquid not just liquid water) and active volcanoes. Its no longer about "earthlike" in the entirety. We know life needs some type of liquid to become more complex and volcanoes indicate there is heat which is also required for life to form. Just because we evolved with this atmosphere and set of chemicals does not mean other life forms did. We could be the odd balls and lufe actually forms more often in environments we couldnt hope to ever visit.