Let's define alien.
Definition number one: unfamiliar. By that description alone, a good 99% of life on this planet is alien. Breathing water, living nestled in thermal vents, stalking prey on the veldt, growing out of the Earth and eating sunlight, without eyes, without legs, with extra legs, color-blind, carapaced, marsupial, with exoskeletons, with jelly for brains, microbial, in a test tube, growing from spores. Not to mention the extremophiles, those nutty organisms that thrive in hellish environments like boiling acid, liquid asphalt, radioactive waste, and under extreme pressure.
I'…