Christiaan Huygens, the 17th-century astronomer and discoverer of the moon Titan, in his book Cosmotheoros, argues for the habitability of all the planets. His idea, as far as I can understand it, is that God would not have created the planets pointlessly, but that such worlds, akin to the Earth in size and substance, must exist in order to house life: "should we allow the Planets nothing but vast Deserts, lifeless and inanimate Stocks and Stones, and deprive them of all those Creatures that more plainly speak their Divine Architect, we should sink them below the Earth in Beauty and Dignity; a thing that no Reason will permit...".
And, this being so, Huygens argues that each planet will have its own airs and waters, suitable to its size and distance from the sun, from which the life of each world can find sustenance; and that the life will be different from ours in detail, but of the same basic kind, with shapes suitable to their mode of locomotion, which will in turn be constrained by the geography of each planet; and that among them will be "their Inhabitants: not Men perhaps like ours, but some Creatures or other endued with Reason. For all this Furniture and Beauty the Planets are stock'd with seem to have been made in vain, without any design or end, unless there were some in them that might at the same time enjoy the Fruits, and adore the wise Creator of them."
And he goes on to ascribe to his 'Planetarians' reason, and the five senses, and arts, and sciences, and geometry and astronomy and writing, and an ordered society; and by turns he argues that they must have a form not utterly different from the human. All very plausibly, sensibly, and Platonically argued; and, alas, for the most part very far from the truth.
I think this works - maybe combine it with autopope's idea of an emerging deity derived from humanity as the Creator. The original book is here. Any better suggestions?