The 19th century was still trying to wrap its head around the theory of evolution when a group of scientists decided to up the ante and declare that the universe was teeming with “ultra-terrestrial” life.

Evolution was occurring on myriad planets, they said, and the Earth was just one tiny outpost of creation. “Is it not evident that this world has been thrown without any distinction into the planetary cluster, and that it is not better adapted than the others to be the exclusive seat of life and intelligence?” asked the famed French astronomer Camille Flammarion.


The debate over the “plurality of worlds” unfolded throughout the 1800s, in books and articles written by scientific luminaries whose imaginations had been stirred by a convergence of recent discoveries. It was known that our solar system was itself revolving within the Milky Way, along with other stars believed to be orbited by planets. Measurements of stellar distances hinted that the universe, if not infinite, was still pretty damn big.

The French astronomer Pierre-Simon Laplace had conceived a credible (though, ultimately flawed) model for the formation of solar systems, which, he said, emerged from the centrifugal forces of contracting, spinning nebulae. Meanwhile, geologists had begun measuring the age of the Earth in terms of millions, not thousands, of years. And, even before Darwin published On the Origin of Species, many naturalists were coming to terms with the basic tenets of evolution: life changed over time and adapted to its surrounding habitat.


"Plurality of Worlds" by Leonhard Euler

The advocates of the “plurality of worlds” hypothesis—let’s just call them “pluralists,” for short—saw these discoveries as evidence for a dynamic process of creation, whereby new planets were constantly forming to serve as vessels for life.

And, they declared, nothing in this theory contradicted religious teachings. Creation, they said, was not an act of God, but the consequence of natural laws—established by God—which did not require His constant intervention. And, just as God had created laws, such as gravity, which governed the physical realm, so too had the “Divine Author” drafted “beautiful regulations” to guide the development of organic life. “For it may be asked, if He, as appears, has chosen to employ inferior organisms as a generative medium for the production of higher ones, even including ourselves, what right have we, his humble creatures, to find fault?” wrote Robert Chambers, a Scottish geologist and natural philosopher, in his controversial, though hugely popular 1844 book, Vestiges of Natural Creation.


What’s more, Chambers reasoned, just as no part of the universe could claim refuge from the laws of gravity, it stood to reason that all worlds were also subject to the laws of life, in the form of evolution:

The fact of the cosmical arrangements being an effect of natural law, is a powerful argument for the organic arrangements being so likewise, for how can we suppose that the august Being who brought all these countless worlds into form by the simple establishment of a natural principle flowing from his mind, was to interfere personally and specially on every occasion when a new shell-fish or reptile was to be ushered into existence on one of these worlds? Surely the idea is too ridiculous to be for a moment entertained.


Skeptics, however, questioned the plausibility of life emerging on planets orbiting outside of Earth’s “temperate zone,” which placed our planet neither too close nor too far from the sun. If only one world in our solar system could sustain life, how could anyone convincingly argue for life across the cosmos?

The pluralists responded with explanations that—seen from the vantage point of today—revealed how much was still to be learned in the fields of astronomy and physics. Scottish mathematician James Mitchell suggested that the moons of distant, cold worlds could act as mirrors, capturing and reflecting the sun’s rays onto the surface. Since the number of moons per planet seemed to increase in proportion to their distance from the sun, he saw this as evidence of divine providence.

Thomas Collyns Simon, a British science writer, argued that the sun’s rays didn’t dissipate as they passed through the vacuum of space. All planets, he concluded, received the same amount of light and heat; surface temperature was determined by the composition and density of the atmospheres. British astronomer Richard Proctor (who produced one of the earliest maps of Mars in 1867) theorized that larger planets, like Jupiter, which took longer to cool down in the aftermath of their fiery, molten creation, sustained life by emitting their own heat.


But, these deus ex machina ideas aside, the pluralists’ unwavering faith in the existence of inhabited worlds stemmed primarily from their observations regarding the diversity of terrestrial life and its imperative to adapt to even the most extreme environments. “If one could judge of a purpose (according to our way of thinking) in all that is going on around us, our Earth might teach us to regard the support of life as Nature’s great purpose,” observed Proctor.

Likewise, Flammarion wrote:

[Life’s] force is so powerful that no element appears capable of struggling advantageously against it, and tending to spread itself in every place, nothing can stop its action. From the high regions of the air, where the winds carry the germs, to the oceanic depths, where they undergo a pressure equal to several hundred atmospheres, and where the most complete night extends its eternal sovereignty; from the burning climate of the equator and the hot sources of volcanic regions to the icy regions and the solid seas of the polar circle, life extends its empire like an immense network, surrounding the whole Earth, amusing itself with all obstacles, and passive over all abysses, so that there is not in the world any district which can pretend to be beyond its absolute sovereignty.


Earth’s moon, though, still presented a dilemma. Thorough examination by telescope had left virtually no doubt that it was lifeless. Yet, it existed in the same temperate zone as the Earth. So, where was everybody?

Proctor’s blunt response was that if life has a beginning, it must also have an end. There were limits to nature’s power of adaptation. Life had not been able to take root on Earth until the planet had passed through the initial period of its fiery formation, and life on Earth would eventually end as our planet entered a period of decrepitude, or our sun diminished and cooled. “The life-supporting era of a planet is short compared with the duration of the planet’s existence,” he concluded. Therefore, “it is antecedently improbable that any planet selected at random—whether planet of our own system, or planet attending on another sun than ours—is at this present time the abode of life.”


The moon, he believed, had once supported life, but had long ago passed into its period of decrepitude. Similarly, he argued:

Mars was a world like our own, filled with various forms of life. Doubtless, these forms changed as the conditions around them changed, advancing or retrograding as the conditions were favorable or the reverse, perhaps developing into forms corresponding to the various races of men in possession of reasoning powers….[but] theoretical considerations render it extremely probable that Mars has long since passed the life-bearing stage.

Proctor saw a universe where planets were undergoing a vast period of preparation to become suitable vessels for life, even as other planets were dying. Within our own solar system, he believed that Jupiter exhibited the same characteristics as Earth’s early evolution and was the most likely candidate to carry on creation:

We see that [Jupiter’s] whole surface is enwrapped in cloud layers of enormous depth, and undergoing changes which imply an intense activity (or, in other words, an intense heat) throughout his whole mass, We recognize in the planet’s appearance the signs of as near an approach to the conditions of the Earth when as yet the greater part of her mass was vaporous… Then, doubtless, will follow a period (far longer than the life-sustaining portion of the Earth’s existence) during which Jupiter will in his turn be the abode of life.


Image by Jon Lomberg

Perhaps the most compelling argument offered by the pluralists—or, at the very least, the one that seemed to provoke the most theological angst among their critics—was the question, if there is no life elsewhere in the universe, then why had God created so many stars and planets? As Mitchell wrote:

Is it possible to believe that these orbs, so numerous, so vast, so widely scattered in the boundless expanse of space, should have been created for the sake of our Earth, which is scarcely a grain of sand in comparison? The idea is absurd and cannot be admitted. If not made, then, for the sake of our earth, there can be little reason to doubt, since the Creator does nothing in vain, and all Nature is full of his goodness, that they were formed for the sake of their own inhabitants.


For his part, Flammarion bluntly advised humanity to get over itself:

How little founded is the sentiment which animates us when we fancy that the universe is created for us, poor beings lost on a world, and that if we should disappear from the scene, this vast universe would be marred, like an assemblage of inert bodies, and deprived of light!


William Whewell summarized the dilemma with the question, “Can the Earth be thus the center of the moral and religious universe, when it has been shown to have no claim to be the center of the physical universe?” Whewell—a polymath, theologian and philosopher, who invented the term “scientist”—rejected Darwin’s theories and also emerged as the most eloquent critic of the “plurality of worlds” thesis. For starters, he believed that the number of worlds in the universe had been grossly overstated. He pointed, for instance, to the large number of dual stars, arguing that “a system of planets revolving around or among a pair of stars, which are, at the same time, revolving about one another” was “impossible to arrange in a stable manner.

As to why God would create such a vast universe, if not to populate it with life, Whewell argued that the physical laws conceived by the Divine Author routinely “produce many other effects, in which we can see no purpose.” The laws of “aggregation and cohesion,” for example, were requisite to give humanity a “firm Earth stand on.” Those same laws, he noted, were behind the force of crystallization, which created precious gems that we admire for their symmetry and beauty. The stars and planets, he concluded, were likewise “adjuncts of the general laws” that God had willed into existence explicitly for the creation of the Earth and the sun.


Image via NASA

A century and a half later, the debate over extraterrestrial life is more rooted in cosmology than theology. Yet, the essential question hasn’t changed: How unique is the Earth? Although we’ve confirmed the existence of exoplanets, the more we learn about the origins of our solar system, the more we appreciate how a rare confluence of events—notably, the formation of our asteroid belt—could be the deciding factor in whether a planet is suitable for the evolution of life.

On the other hand, we’ve also learned that the expanse of Flammarion’s “empire of life” on Earth is vaster than he ever imagined. The discovery of ecosystems thriving in the most extreme environments—such as those clustered around hydrothermal vents at ocean depths devoid of light—supports the idea that life can exist anywhere that we find energy, water and organic matter. Proctor had been way off base when he speculated that Jupiter was a viable candidate for life—but the ocean beneath Jupiter’s moon, Europa, holds possibilities


On one point at least, there is now, as there was 150 years ago, general agreement: in a universe of wonders, life is the most extraordinary.

Mark Strauss is an editor and writer in Washington, D.C.