Benjamin Franklin maintained that every star is a sun, and every sun nourishes a “chorus of worlds” just like ours. Ethan Allen, the self-taught leader of the Green Mountain Boys, insisted that the inhabitants of these other earths included intelligent beings just like us. David Rittenhouse, the famous Philadelphia inventor and astronomer, made it official in a 1775 lecture that was reprinted for the benefit of the Second Continental Congress. “The doctrine of the plurality of worlds,” he said, “is inseparable from the principles of astronomy.”...More at the link re the roots of these theories in Greco-Roman antiquity.
If these peace-loving aliens were a threat to anything, it was to theology. John Adams put his finger on the problem as a young man in a diary entry from 1756. Given the near-certainty of alien life, he reasoned, Evangelical Christians must either condemn our extraterrestrial brothers to everlasting perdition or suppose that Jesus shows up on an endless number of planets in ever-changing alien incarnations. Thomas Paine later made the same point in print, rather more caustically: “The person who is irreverently called the Son of God, and sometimes God himself, would have nothing else to do than to travel from world to world, in an endless succession of death, with scarcely a momentary interval of life.”