Speculation about extraterrestrials is not all that new. There was a vibrant debate in 17th-century Europe about the existence of life on other planets.
This was the consequence of the transition from a Ptolemaic view, in which Earth was at the centre of the universe and everything revolved around it, to a Copernican view in which the Sun was at the centre and our planet, along with all the others, revolved around it.
It followed that if we were now more like other planets and moons close to us that revolved around the Sun, then they were more like Earth. And if other planets were like Earth, then they most likely also had inhabitants.
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