Back in the 1800s, in the halcyon days of evidence-free speculation on life in the universe, the Scottish “natural theologist” Thomas Dick famously proposed that our solar system could harbor about 22 trillion beings. That figure may sound preposterous, but Dick wasn’t entirely out of step with the scientific mood of the time. There was logic to the contention that abundant life must exist elsewhere, since the alternative was that the Earth was somehow special, which was at odds with everything we’d learned from Copernicus. It was also at odds with the belief—which Dick had—in a benevolent and generous God, busy spreading its creations across the cosmos.
Even for the time, though, Dick was extreme in his thinking about where those creations might be. For example, he considered the rings of Saturn as providing a potential surface for life, along with pretty much every other place, from Saturn itself to Jupiter, Uranus, Mars, Venus, and so on. (Neptune was yet to be discovered.) Add it all up and, assuming a similar density of species to the density of humans on Earth, you arrive at a figure of about 22 trillion. Dick even noted that if the sun were also habitable, its surface alone could harbor more than 30 times that number of beings.
Today, we know there are not nearly that many multicellular entities, let alone humans, in our solar system (barring some really unfortunate oversight yet to be understood), and we have a vastly better understanding of the possibly habitable real estate. Gas giants like Jupiter and Saturn are easily ruled out for any surface-dwelling life (although Carl Sagan and Edwin Salpeter once wrote a great scientific paper in 1976 on options for atmosphere-dwelling creatures in Jupiter). And almost anywhere with a solid surface would still require substantial environmental engineering to be places where life like ours could be even halfway comfortable, whether on Mars or perhaps in repurposed, hollowed-out asteroids.
That doesn’t mean that it’s not a fun, and even informative, thought exercise to try to estimate the maximum capacity of the solar system for sustaining human populations. In 2005 for example, a paper in the Journal of the British Interplanetary Society (which, rather unfairly, sounds like a publication from an alternate reality in which humans have already spread throughout the solar system) examined the limits imposed by the availability of key biological elements like nitrogen and phosphorus in the solar system.
If just the most readily accessible stores of those elements (along with all the other necessary but more abundant elements, like carbon and oxygen) could be extracted from all the carbonaceous asteroids in the solar system (potentially more than a hundred million objects), that alone could let you “grow” about 6,000 times Earth’s biomass; that is the totality of all living organisms on the planet. So then, assuming the same biomass-to-human ratio as on Earth, the solar system could support about 50 trillion people. If you could somehow access and repurpose all of the biologically useful contents of the asteroids, this figure could go up to thousands of trillions.
Of course, those estimates assume mechanisms exist to turn all of those raw ingredients into living things and that there’s somewhere to put them all. In that context there is also an implicit assumption that the energy exists to support all of those biomasses—energy for maintaining environmental temperatures and for metabolic processes like photosynthesis or chemosynthesis, which turn energy and raw materials into food.
The largest readily available power source in the solar system is of course the sun. Earth intercepts about 173,000 trillion watts of solar power (though about 30 percent of that is reflected back into space before reaching the planetary surface). The total power consumption of humans, at present, is only about one ten-thousandth of this, but plenty of other life on the planet uses the sun’s energy, too. We could conservatively suppose that since an entire planet like the Earth supports roughly 8 billion people, the total intercepted solar power is what’s necessary to sustain the environment, the biosphere, and us.
Luckily those 173,000 trillion watts are merely one ten-billionth of the total electromagnetic output of the sun, streaming out in all directions from the solar surface. So, if one Earth can sustain 8 billion humans (a figure that is somewhere between conservative and generous, given our perilous impact on the planet thus far), then the solar system gets enough energy from the sun to, in principle, support 10 billion such worlds—and thus 80 million-trillion of us.
So that’s enough materials for biomass to go along with 50 trillion people, enough solar power for 80 million-trillion. This suggests we might run out of raw materials before we’d run out of energy. But even more worrisome is the possibility that we might run out of the physical environments for all of those humans to occupy. This kind of thought experiment naturally leads us to the ideas of Freeman Dyson’s famous one-page paper in 1960 where he outlines the concept of what has become known as a “Dyson sphere.”
This is a structure built to enclose the sun (in Dyson’s original conception, itself inspired by science fiction author Olaf Stapledon’s Star Maker), but to do so at the orbital radius of the Earth. In other words, this sphere, or aggregated swarm of structures in orbit (because a solid spherical shell might be unstable), would provide an inner surface where the solar power received per unit area is a match to what the Earth presently receives—leading to a potentially habitable environment with about 550 million times the surface area of the Earth (or nearly 2 billion times Earth’s land surface). The materials to build this structure, which could be a few meters thick, would all come from a repurposing of Jupiter’s matter, about 317 times the mass of the Earth.
But what would it look like to for 80 million-trillion humans to live on nearly 2 billion times the surface area of all of Earth’s landmasses? They would have an average population density of about 290 people per square kilometer. Superficially, that actually doesn’t sound too bad. Manila, in the Philippines, has an average population density of more than 71,000 people per square kilometer; Manhattan has about 28,000 per square kilometer. In fact, the state of Connecticut is a pretty close match to our Dyson sphere, with an average of about 288 people per square kilometer.
In other words, with some coordinated shuffling around, there could be regions of ordinary urban density and regions of open space. Which leaves us with the rather surprising conclusion that what Thomas Dick proposed back in 1838 was, if anything, a gross underestimate of what is actually possible in terms of the capacity for people in a star system. Admittedly he didn’t imagine reconfiguring the entirety of Jupiter to build a Dyson sphere, but in a bizarre fashion he was on the right track.
The biggest constraint is the availability of raw material to actually make 80 million-trillion humans, plus the attendant biomass in other living things to support those people and their environment. This number is 100,000 or so times more than the extrapolated availability of bioelements from carbonaceous asteroids. On the other hand, if we were capable of repurposing all of Jupiter to build a Dyson structure, we could surely also repurpose other worlds to build living things.
In very approximate terms, the total matter of Saturn, Uranus, and Neptune might provide about 10 billion-trillion tons of biologically useful material. The present biomass on the Earth is estimated at around 1 trillion tons, yielding the possibility of forming about 10 billion new terrestrial-size biomasses that could support a total of about 100 million-trillion humans. More than enough to occupy all of the adequately powered terrain of a star-enclosing structure or orbital swarm.
Naturally there are all kinds of caveats. Not least are the assumptions about Earth’s own capacity to sustain humans, since it’s unclear that our present population numbers are really viable for this planet. And at the root of Dyson’s concept is the assumption that simply having the right amount of incoming stellar power translates to a habitable environment. This is a gross simplification of what we know of how ecosystems function, with their constantly incoming and outgoing fluxes of energy and matter and their dynamic variations and evolutions.
But perhaps the most interesting aspect of examining our solar system this way is to realize how parochial we might be in our thinking about life’s growth and potential cosmic occupancy. Just as we once imagined that most planetary systems would look something like ours and then discovered that exoplanets offered far more diverse and alien architectures, our preconceptions about life being a light dusting around a star, confined to a few lucky planetary surfaces, could be woefully blinkered.
Maybe somewhere out there are places where life has gnawed its way through entire worlds to become the bulk of what encircles a star. Thomas Dick was excited to imagine people inhabiting a plurality of stately worlds, but that scenario might only represent a primordial state, before those worlds are remade into something more in step with life’s voracious needs.