NASA has an Office of Planetary Protection. Its purpose is not to protect Earth from invasion by some green skinned monsters, but rather to protect the rest of the Universe from us. With the recent evidence of liquid water on Mars , the possibility of life on the red planet has become a bit more possible, and protecting potential martians from an alien threat is a real challenge.
While Earth is the only planet for which the presence of life is certain, we know that living things can be extraordinarily hardy. We’ve seen organisms thrive in areas of extreme temperature and acid environments. Tardigrades (also known as water bears) have survived the cold vacuum of space, and bacteria have been found within nuclear waste. So it’s not unreasonable to imagine a terrestrial organism hitching a ride on a spacecraft. If that spacecraft comes into contact with a potentially habitable environment they could become an invasive species on another world.
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