But such diversity and beauty of things is available on the other planets like our Earthly realm, surely they wouldn't lack a spectator! As the elegance and artful workmanship of the animals, and the colors and scents of the flowers seem to be made available for human admiration and delight, will there not be beings on these planets who [could] enjoy so many and so pleasing spectacles? ["Quod si igitur similis quaedam rerum varietas ac pulchritudo in caeteris planetis atque in Terra hac nostra viget, nunquid spectatore carebunt! an non ut animalium elegantia et artificiosa fabrica, florum colores atque odores ad hominum admirationem aut voluptatem comparata videntur, ita et in istis existent aliqui qui tantis spectaculis tamque jucundis fruantur."--Christiaan Huygens (ca 1689).
In the first edition of Newton's Principia (1686), God is mentioned only once, deep into the technical details of Book 3: ''Therefore God placed the planet at different distances from the sun so that each one might, according to the degree of density, enjoy a greater or smaller amount of heat from the sun'' (Book 3, proposition 8, corollary 5; quoted from the Whitman/Cohen translation, p. 814). This claim was removed in subsequent editions of the Principia, and Newton moved his natural theology -- without this particular argument -- to the new conclusion of the Principia, his General Scholium.
At least one person discerned the significance of Newton's remark in Book 3 of the Principia. As Bernard Cohen argues in his (1690) Discourse on Gravity, Huygens comments on proposition 8 that it showed what kind of gravity ''the inhabitants of Jupiter and Saturn would feel,'' (quoted from Cohen's long introduction to the translation of the Principia p. 219). We know that Huygens had empirical reasons to doubt Newton's argument for the inverse-square law as a universal quality of matter, but Huygens certainly accepted an inverse-square rule for celestial gravity that governed the planets.
As an aside, despite the fact that the argument was removed after the first editions, Kant, who knew his Huygens, also saw Newton's point: ''Newton, who established the density of some planets by calculation, thought that the cause of this relationship set according to the distance was to be found in the appropriateness of God's choice and in the fundamental motives of His final purpose, since the planets closer to the sun must endure more solar heat and those further away are to manage with a lower level of heat,'' (UNH, Part 2, section 2, 284–85 (271)).
But what clearly captured Huygens's imagination is that with Newton's Principia and his own results, the physical constraints that governed life on other planets were becoming available to informed speculation. He would set out his extended thoughts on this matter in his posthumously published Cosmotheoros (1698), which was immensely popular in the eighteenth century. But that there was such life, outstripped the available evidence to Huygens and Newton. While assimilating Newton's Principia, and offering his own Cartesian response (the Discourse) to it, Huygens's found an argument for the existence of such life that I quoted at the epigraph at the top of this post.
In the manuscript from which I quote, Huygens follows Descartes in claiming that humans are ignorant of God's particular providence for humans (p. 544). But Huygens does allow some general providence. In particular, it seems he attributes to God (or Nature) a kind of (anti-anthropocentric) aesthetic maximization and conservation principle: no natural beauty is wasted and requires spectators to enjoy it. So, because the night-skies on other planets are lovely there must be extraterrestrials to enjoy it. Huygens's God would not waste the opportunity to have sentient beings enjoy the aesthetic pleasure. Nature does nothing in vain, after all. Crucially, in Huygens's manuscript pleasure is treated as an intrinsic good—this celestial pleasure exists not just to venerate God's creation, but is there to be enjoyed. In fact, pleasure is the highest gift of God [Voluptas autem summum optimumque est Dei donum]."
Huygens, who knew his Spinoza, came to see the weakness of the aesthetic-maximization-conservation argument in the move from manuscript to Cosmotheoros:
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. But this alone would be no prevailing Argument with me to allow them such Creatures. For what if we should say, that God made them for no other design, but that he himself might see (not as we do 'tis true; but that he that made the Eye sees, who can doubt?) and delight himself in the contemplation of them? For was not Man himself, and all that the whole World contains, made upon this very account? That which makes me of this opinion, that those Worlds are not without such a Creature endued with Reason, is, that otherwise our Earth would have too much the advantage of them, in being the only part of the Universe that could boast of such a Creature so far above, not only Plants and Trees, but all Animals whatsoever—Book 1, "Rational Animals in the Planets"
Huygens's supplemental argument relies on a kind of uniformity of nature (and terrestrial humility) principle and need not concern us here. The original, rejected aesthetic-maximation-conservation argument is still present, but explicitly coupled with adoration of God. Some other time I would want to explore to what degree we can salvage Huygens's principle if we firm up the requirement that makes spectators and beauty co-existing by making beauty spectator-relative.
Here I close by returning to Newton, who, we know, firmly embraced the idea that nature does nothing in vain (see the first Rule of Reasoning). For, in the General Scholium, in proposing a variety of Design Arguments, Newton calls attention to the significance of that particular ''diversity of created things, each in its [proper] place and time.' And Newton emphasizes that the beauty of our solar system is mimicked by countless other solar beautiful systems, too far apart to be of interest to us (see here for text). So, perhaps inspired by Huygens, Newton posited God's design as generating aesthetic appreciation of spectatorial extraterrestrials,--a lovely, pleasing thought.