Aliens, everywhere: Why scientists once assumed every planet was inhabited

On April 17, news burst upon the world — thanks to observations from the James Webb space telescope — that a planet 124 light-years away might contain telltale chemical traces of life. The planet, named K2-18b, appears to house dimethyl sulphide or dimethyl disulphide. These are both gases which, at least on Earth, are produced only by living things.

Whether this should make us excited about the prospect of distant lifeforms — basking on K2-18b in the ruby light of its red dwarf star — is a matter of debate. The evidence is shaky. What’s more, history shows we have a tendency for wishful thinking in this department. It’s best to be cautious.

Yet it is undeniable that momentum is building in the field of astrobiology, which studies life throughout the Universe and the ways we might detect it. Within decades, NASA plans to launch an ambitious new space telescope, the Habitable Worlds Observatory, in hopes of answering questions about whether life flourishes elsewhere.

But the very fact that we are surprised and excited about tentative sightings of organic gases on a planet 124 light-years away is a massive departure from what people thought only a few generations ago. That we now think of “life elsewhere” as an open question — one requiring caution — is a dramatic shift away from what’s been the default assumption for many centuries now.

Ever since people, roughly four centuries ago, began accepting that there are other solar systems besides ours, they immediately assumed that their planets simply must be occupied. Historically, scientists have tended to presume our cosmos is not only full of life but, quite literally, teeming with it. 

Why? Because they thought that if this wasn’t the case, then our Universe would contain a lot of wasted space. The following is the story of how, historically speaking, humans have tended to assume that every single surface in the cosmos — including suns — must house reasoning animals, going about their lives in much the same way we go about ours. 

Ultimately, it proves how far we’ve come, in our generations-long inquiry into the Universe, to even pose the matter of “life elsewhere” as a question — one whose terms need careful defining and refining. In particular, we’ve come a long way from the early speculators who gleefully conducted cosmic population censuses, outputting eye-watering numbers by assuming each and every solid object in the cosmos is bursting with life.

A crowded Universe

In the early 1500s, the Polish polymath Nicolaus Copernicus initiated the string of discoveries that eventually snowballed into our contemporary understanding of Earth’s place: as but one planet among uncountably many, within an unfathomably large Universe.

But, only decades beforehand, some voices were already conjecturing in this direction. In 1440, the German theologian Nicolaus Cusanus argued that there must be countless other stars and planets. All of them, he thought, must be inhabited. He even argued that suns themselves must be peopled. “By solar beings,” Cusanus envisioned, “bright and enlightened intellectual denizens”.

Cusanus wasn’t alone. The Italian philosopher Giordano Bruno was soon making similar claims. In 1584, he reasoned that just as we would deem it “ill” if our own planet were not full of life, “it would be no less ill if the whole of space were not filled.”

It took centuries of inquiry for humans to fully realize that the things we find valuable are not the same as the things that are independently real. It’s understandable: We are all victims of wishful thinking. But back then, it was extremely common to assume that everything that exists must exist for a purpose. Accordingly, it seemed a gratuitous waste for outer space not to be chock-full of living, sensing beings. As Bruno put it, we wouldn’t like the “nonexistence” of our living world, so why extend the ignominy of inexistence to our living neighbours?

Bruno got in trouble for his views — he was burned at the stake — but across the ensuing century, similar beliefs spread, became mainstream, and eventually became unanimous. Even John Milton nods to other populated worlds in 1667’s Paradise Lost. (Satan passes by some of them on an interstellar jaunt, “but who dwelt happy there / He stayd not to enquire.”)

The conviction that every surface in the Universe must be inhabited even led the English astronomer Edmond Halley to reason, in 1692, that the Earth itself must be hollow and filled with nested concentric spheres within, like a titanic Matryoshka doll. Why? Because this would maximize the real estate inside our globe for living, thinking, and appreciating beings.

Another scientist, the French diplomat Benoît de Maillet, theorised in the 1720s that even outer space’s void is teeming with microorganisms. He claimed “seeds” of living creatures circulate throughout space, agglomerating around planetary bodies like “filings of iron” around magnets. This also led him to reason that all planets share the same species.

Three decades later, in 1759, Adam Smith — the founder of modern economics — assured that the cosmos maintains “at all times the greatest possible quantity of happiness” for all “inhabitants of the Universe.” 

It’s convenient that our word “economy” derives from the Ancient Greek for “house,” given that, in Smith’s eyes, our cosmic home is a maximally economical one. No space is wasted. Smith considered it unthinkable that “all the unknown regions of infinite and incomprehensible space may be filled with nothing.” There must be happy consumers, everywhere.

The late 1700s and early 1800s saw the development of the first truly large-scale reflecting telescopes, revealing ever-greater galactic magnitudes and volumes. In 1844, the Scottish geologist Robert Chalmers rhapsodized that nature’s “aim” simply must be “to fill up every vacant piece of space with some sentient being to be a vehicle of enjoyment.” Anything else would seem a cosmic waste, given the magnitudes being discovered.

Cosmic censuses

It was only a matter of time until someone conducted a cosmic census. That is, an attempt to gauge just how much enjoyment our cosmic neighborhood might house. A Gross Domestic Product for the stars, if you will. This fell to the British theologian and astronomer Thomas Dick, who provided the first population estimates for the bustling Universe he believed he was living in.

In 1838, Dick started out modestly. That is, focusing on just our Solar System. Generously, however, he included the surface areas of Saturn’s rings, alongside the known satellites of other planets, assuming all are inhabited. Betraying his conviction that his own nation was the navel of the known Universe, Dick proudly extrapolated from England’s contemporary population density — of “280 inhabitants per square mile” — to output a total circumstellar population of “21,891,974,404,480.” (Rounding numbers wasn’t yet a scientific convention.)

Two years later, Dick used similar reasoning to guestimate the total population for what was then the visible Universe. He put this at an ample “sixty quadrillions,” or, more precisely, “60,573,000,000,000,000,000,000.” Dick, a church minster, was confident in his claim that “there is but one religion throughout the Universe”: conveniently, his own.

As late as the 1880s and 1890s, some scientists were still maintaining that the Sun itself is inhabited. This was in spite of the invention, decades before, of spectroscopy, which enabled the determination of the Sun’s temperature and chemical composition. Still, some writers maintained that “there may be creatures on the Sun which thrive upon incandescent hydrogen.” 

In 1896, a German by the name of Carl Goetze published a book whose title translates as The Sun Is Inhabited. In it, he rejected the idea that the Sun is a superheated, gaseous body, instead insisting it has life on its surface. Naturally, he assumed it would be identical to Earth’s. Goetze pictured mammoths and dinosaurs lumbering in swamps throughout the sun’s clement polar regions. He was also convinced there were “humans” living there.

Goetze’s ideas — outlandish even for the time — didn’t catch on. But even mainstream scientists, like the Anglo-German biochemist William Thierry Preyer, could at the time remain toying with claims that suns themselves may represent “glowing organisms whose breath may perhaps be shining vapor, whose blood may be flowing metal, and whose food may perhaps be meteorites.”

The beginnings of astrobiology

This assumption that life is cosmically omnipresent reached its peak throughout the late 1800s. But it began falling apart throughout the early decades of the 1900s. This was due to the discovery of radioactivity, confirming outer space is full of noxious radiations and emissions, alongside building awareness of the stringency of the conditions required for life to survive.

The influential English cosmologist James Jeans summed it up in a talk given at London’s University College in 1926. Recent breakthroughs in physics, he pronounced, had made clear that “the physical conditions under which life is possible form only a tiny fraction of the range of physical conditions which prevail in the Universe as a whole.”

Nonetheless, old habits die hard. In 1953, the American astronomer Kenneth Heuer published a beautifully illustrated book called Men of Other Worlds. In the final chapter, Heuer, directly inspired by Dick, repeated the task of taking the Solar System’s census. Taking into account the discovery of new moons since Dick’s time, from Io to Triton, Heuer updated the total surface area of the Solar System to “47,230,298,000,” before outputting its total population as “535,092,960,000.”

Heuer’s overall smaller estimate was based on his marginally more sensible idea of extrapolating the total population of the Earth as a whole, rather than just that of England.

Given how far knowledge had come about our neighboring planets by the 1950s, Heuer was probably not being entirely serious. Men of Other Worlds is self-consciously archaic, constantly gesturing back to earlier speculators and dreamers. The foreword advertises it as a work for anyone who likes their “science spiced with fantasy.” Aside from speculating on asteroids populated by “living minerals,” “singing stones,” and “talking rocks,” Heuer also claimed Jupiter’s “Red Spot” might be a supermassive iceberg “floating in an ocean of permanent gases.” There, perhaps, is more fantasy here than science.

Kenneth Heuer’s 1953 update of Dick’s population census for the Solar System (including newly discovered moons).

Nonetheless, the fact that the fantasy remains alluring tells us something important. Historically speaking, it seems clear that people desperately want there to be life elsewhere. That’s why we should be abundantly cautious when it comes to claims we’ve found its signatures, no matter how well-founded they are.

After all, the founding of the modern field of astrobiology — as a serious science — rests in the historic realization that life isn’t simply everywhere there is a surface. First, we might need to specify what types of surfaces might accommodate living matter, while also questioning what life even is in the first place.

We’ve come a long way since Thomas Dick’s cosmic census, conducted just under two centuries ago. However, by looking outward and discovering that the Universe isn’t as bustling as we first assumed, we must now learn to look inward, coming to appreciate just how fragile a foothold Earth might be in the cosmic perspective. This way, we might acknowledge the consequent responsibility we have for our miraculous, wonderful life.

Earlier this year, the British space scientist Margaret Ebunoluwa Aderin-Pocock claimed it is “a human conceit to think we’re alone.” Perhaps it is a conceit, perhaps not, but it’s certainly humbler than assuming every single cosmic surface is thronging with things like us.