By the 1700s, there could no longer be any doubt. Earth was just one of many worlds orbiting the Sun, which forced scientists and theologians alike to ponder a tricky question. Would God really have bothered to create empty worlds?
To many thinkers, the answer was an emphatic "no," and so cosmic pluralism - the idea that every world is inhabited, often including the Sun - was born. And this was no fringe theory. Many of the preeminent astronomers of the 18th and 19th century, including Uranus discoverer Sir William Herschel, believed in it wholeheartedly, as did other legendary thinkers like John Locke and Benjamin Franklin. How could so many geniuses believe in something so silly?
To answer that question, we need to understand just what science was hundreds of years ago, at a time when it was freely intertwined with philosophy and theology, when analogies and metaphors were considered almost as good a form of evidence as data and observation. Cosmic pluralism offers us a chance to look back at how very differently people used to approach scientific inquiry...and, as a bonus, it provides some wonderfully insane quotes from otherwise brilliant scientists.
The Birth of the Plurality of Worlds
While a version of cosmic pluralism was first espoused by the ancient Greeks as a way to imagine a universe without gods, we'll be picking up the story in the Renaissance. Two great astronomic innovations had placed Renaissance and, later, Enlightenment philosophers in a difficult position. Nicolaus Copernicus demonstrated that the Earth was not the center of the universe, and the telescope provided clear proof that celestial bodies like the Moon and the planets were all worlds in much the same way Earth was, and that they potentially might be inhabitable.
These twin discoveries suggested that the Earth was not fundamentally unique in the cosmos. The theology of the period held that Earth existed as the expression of God's plan, so it was a bit puzzling why he had apparently created a bunch of other worlds that were sort of like Earth but served no clear purpose. There were two possible solutions, both of which were vigorously championed by their proponents: either Earth was still unique from all other worlds in some way more subtle than humans could comprehend, or else every other planet was inhabited as well, preferably by Christians.
As you might imagine, the latter theory makes for way more interesting reading, as scientists tied themselves in logical knots in a desperate attempt to knit together the scientific data with the demands of their philosophies. And the most powerful blunt instrument at these thinkers' disposal was the analogy.
Arguments of Analogy
For all the fierce debate on cosmic pluralism, most people agreed on one crucial point: direct observation wasn't going to prove anything. Writing in the 1850s, the Scottish physicist and astronomer Sir David Brewster considered the planet Mars, which he pointed out was always at least 50 million miles from Earth. (The closest distance between the two planets is actually more like 34 million miles, but let's not quibble too much.) Even with a telescope with a power of 1,000, Mars would still appear to be 50,000 miles away, which is much too far away to say with any certainty whether it is inhabited or not.
By this standard, Brewster argued, even the Moon would not appear close enough for anyone to say with certainty if any Moon people lived upon it. But what telescopes did provide was lots of evidence of how similar all the planets were, and from this it was possible to construct analogies as evidence for cosmic pluralism. As an example, the Brewster considered the eye, which comes in vastly different shapes and sizes in different animals but "is always adapted to the existence and properties of light." In fact, the presence of eyes demands the presence of light, or else "all this delicate optical machinery would be wasted."
This is a recurrent theme in all the arguments for cosmic pluralism - if one thing works in a certain way or has a particular feature, then surely another thing would have the same workings or features. That might sound like I'm being patronizing, but it's hard to describe the arguments involved in any more sophisticated fashion. One wonderfully weird bit of reasoning came from Johann Bode, an otherwise rightly acclaimed German astronomer who named Uranus and described its orbit with great precision, thanks in part to his work with Bode's Law. And yet he also had this to say about the existence of people on the Sun:
"Who would doubt their existence? The most wise author of the world assigns an insect lodging on a grain of sand and will certainly not permit...the great ball of the sun to be empty of creatures and still less of rational inhabitants who are ready gratefully to praise the author of life."
Basically, if you're going to argue that a grain of sand is inhabited while the Sun isn't, you're calling God an idiot. That doesn't exactly leave much room for reasonable debate. In any event, metaphors and analogies were considered acceptable arguments because not only were science, religion, and philosophy all jumbled together, but also because all the different scientific fields were basically interchangeable. The biology of the eye or the ecology of sand were directly comparable to the habitability of the Sun. If you've ever wanted evidence that people in the past viewed the world in a fundamentally different way than we do now...well, I doubt you're going to find much better evidence.
Christianity Conquers the Cosmos!
So, thanks to the power of analogy, 18th and 19th century thinkers were able to demonstrate that the entire solar system was teeming with life. But what sort of life? Were these lunatics (actual term for Moon people) and solarians strange, unknowable alien beings? Would the vastly different conditions of life on other worlds force these creatures to take on bizarre shapes simply to survive?
That was not, as it happens, the generally held view. Sir David Brewster used the mother of all run-on sentences to list every bit of evidence that other planets and celestial bodies behaved in the way our world does, and from this he reached one inescapable conclusion:
We trace throughout all the heavenly bodies the same uniformity of plan, is it possible to resist the influence an uniformity of purpose; so that if we find a number of spheres linked together by the same bond, and governed by the same laws of matter, we are entitled to conclude that the end for which one of these was constituted, must be the great general end of all,—to become a home of rational and God-glorifying creatures.
Brewster is making what's known as a teleological argument, which holds that existence has some intrinsic goal or purpose. The basic idea hangs around to this day in some of the arguments for intelligent design or the finely tuned universe, but those only go so far as to say humanity's existence is the goal. As far as Brewster was concerned, that wasn't nearly enough - what's the point of aliens if they're not going to have human religion?
The Alien Census
This argument cut to the heart of what made cosmic pluralism so appealing to 18th and 19th century thinkers. At its most basic, the theory expanded God's reach to the entire cosmos, which seemed to be more in keeping with the whole concept of omnipotence. (As you might imagine, there was plenty of disagreement on this, with some arguing a universe full of more or less identical Christians devalued the whole point of being one in the first place.)
Perhaps no one took this idea further than the Reverend Thomas Dick, a Scottish minister who was also well-versed in astronomy. Writing in 1837, he argued that there were 80 million stars in the visible universe and each had at least thirty satellites, meaning there were 2.4 billion planets in the cosmos...all inhabited. Even his view of the solar system was gloriously unwieldy, as Michael J. Crowe describes in The Extraterrestrial Life Debate, 1750-1900:
"Basing his calculation on the population density of England being 280 persons per square mile, and neglecting the possibility of oceans elsewhere, Dick constructs a table in which he assigns every planet and planetoid in the solar system, except Vesta, a higher population than the Earth."
Just how outlandish were Thomas Dick's calculations? He had determined that nearly nine billion people lived on Mercury, 53 billion on Venus, four billion on the Moon, 15 billion on Mars, almost seven trillion on Jupiter, five and a half trillion on Saturn, and another trillion on Uranus. But perhaps my favorite figure is his estimate for Saturn's rings, which for some reason he pegged at exactly 8,141,963,826,080. Altogether he placed the population of the solar system at just under 22 trillion, and even then he was quick to point out that the Sun could accommodate 31 times that number.
Of course, there's one planet whose population Dick didn't calculate using this method, and that was Earth. After all, if you were to take his whole "280 people per square mile covering the entire planet" method and apply it to our planet, you get a total population of 55,143,172,000 people, all Christian (and possibly English as well). In 1837, the planet's population was just about a billion, and only a minority of these were Christian. Indeed, as far as Dick and others were concerned, the rest of the universe was this teeming mass of people unanimous in their religious faith, whereas the Earth was this weird exception with theological disagreements and varying population densities and oceans.
And now, a word from Sir William Herschel
I should stress that, much as it's fun to examine all these weird arguments, the people making them were not idiots. Most of them were extremely bright, well-educated people who were expert in both science and theology, and they got into trouble because they directed their intellects towards tying together two fields that were fast becoming incompatible. Cosmic pluralism is the last gasp of a world in which science and philosophy existed in the same sphere, even as they were pushing in almost diametrically opposed directions. That tends to make people look silly in retrospect.
Even Sir William Herschel, who could make a very strong claim to being the greatest astronomer of his era, was not immune. The man who discovered both Uranus and infrared radiation was firmly convinced of the plurality of worlds. He looked at the Moon and observed that it was quite large like Earth, had valleys and mountains like Earth, had a large orbital partner just like Earth (of course, this partner was Earth), and all the other celestial bodies would appear in the Moon's sky just like they did on Earth. All that, as far as Herschel was concerned, was more than enough to say that the Moon should be inhabited like Earth.
To read Herschel is to be thrown into a weird world where surprisingly modern astronomical understanding freely mixes with completely bonkers ideas. He notes that the Moon probably has little or no atmosphere and probably little or no water, and yet he somehow sees this as support for its habitability:
My answer to this will be, that that very difference which is now objected, will rather strengthen the force of my argument than lessen its value: we find, even on our globe, that there is the most striking difference in the situation of the creatures that live on it. While man walks on the ground, the birds fly in the air, and fishes swim in the water; we can certainly not object to the conveniences afforded by the moon, if those that are to inhabit its regions are fitted to their conditions, as well as we on this globe are to ours. An absolute, or total sameness, seems rather to denote imperfections, such as nature never exposes to our view; and, on this account, I believe the analogies that have been mentioned are fully sufficient to establish the high probability of the moon's being inhabited like the Earth.
You can admit it - William Herschel just kind of blew your mind with that logic, right? But even then, he's not done. He uses this apparently now established high probability of lunar habitability to prove the Sun is also teeming with life. He argues that someone living on the Moon might look at the Earth and just see it as the larger object around which his world revolves.
But, of course, we know the Earth is inhabited, and would mock the lunatic for his ignorance. So then, who are we to look at the Sun and presume it is just the larger object around which we revolve? And so, according to Herschel, "we need not hesitate to admit that the sun is richly stored with inhabitants." He then turns around and reels off a bunch of well-supported scientific observations about how far-distant stars must be objects like our Sun...and then he goes right back to how all these suns too must be inhabited.
Cosmic Pluralism Today
For all its popularity and widespread acceptance well into the mid-19th century, cosmic pluralism was always an untenable compromise between science and theology, and improved understanding of astronomy and biology made it clear that the Moon, the Sun, and the planets around us were definitely not inhabited, and certainly not by intelligent, God-fearing aliens.
By the dawn of the 20th century, it was back to being just another fringe theory. Indeed, a century's worth of scientific progress arguably brought astronomers to the precise opposite conclusion in the form of the Rare Earth hypothesis. This idea holds that the conditions of life are so unlikely that we might be on one of the very, very few planets in the universe capable of supporting life.
And yet, on some level, cosmic pluralism has been making a comeback of late, albeit without its former theological aspects. While Earth remains the only known home of life in the universe, intelligent or otherwise, there's growing scientific support for the idea that other bodies in our solar system, such as Mars or Saturn's moon Titan, either are home to very basic life or were at some point in their long histories.
But why limit ourselves to just the solar system? Thanks to the Kepler mission, we've now estimated that there could be as many as two billion Earth-like planets in our galaxy alone, and that might be a gross underestimate.