Giordano Bruno was (probably) the first European who proposed the possibility that not only was the universe infinite, but stars were not just points of light in the sky; they could be suns with their own planets, and that some of those planets might even host life.
The Catholic Church had him tried for Heresy and had him burned at the stake and his contemporaries though he was completely insane. He had some kooky ideas, but he was absolutely right about the size of the universe and stars being suns with their own planets.