My own interest in exoplanetary research was recently reignited while writing my book "Tending the Epicurean Garden" as a modern intro to Epicurean philosophy. While discussing the importance of naturalist cosmology, I came across the ancient atomist "DOCTRINE OF INNUMERABLE WORLDS", which was first given testimony in Epicurus' Letter to Herodotus. It's fascinating to think that there were men with modern concepts of science living 2,300 years ago, and further back to Democritus, who probably also believed in this.
The doctrine was based on the observation that there didn't seem to be an observable boundary or limit to the universe and to the amount of particles in every direction. Here is the relevant quote from the Letter to Herodotus, Section 3:
"First, an infinite number of worlds exists in the universe, some of which worlds are like and some of which are unlike our own. We conclude this because the ultimate particles are infinite in number, as was proved already. No matter how far they move out into space, it is not possible that the number of particles has been used up in the formation of any number of worlds. Thus there is no obstacle to the existence of an infinite number of worlds, and we conclude that there are innumerable worlds in the universe, including those, like our own, which contain living beings."
Lucretius, in his "De Rerum Natura" (On the Nature of Things) later elaborated the doctrine in more detail. This doctrine eventually inspired Giordano Bruno, and led to his execution by the Church. Now it's being vindicated in exoplanetary research!