We’re interested in how exo-civilizations develop on their planets. Given that more than 10 billion trillion planets likely exist in the cosmos, unless nature is perversely biased against civilizations like ours, we’re not the first one to appear. That means each exo-civilization that evolved from its planet’s biosphere had a history: a story of emergence, rising capacities, and then maybe a slow fade or rapid collapse. And just as most species that have ever lived on Earth are now extinct, so too most civilizations that emerged (if they emerged) may have long since ended. So we’re exploring what may have happened to others to gain insights into what might happen to us.

Of course, we have no direct evidence relating to any exo-civilizations or their histories. What we do have, however, are the laws of planets. Our robot emissaries have already visited most of the worlds in the solar system. We’ve set up weather stations on Mars, watched the runaway greenhouse effect on Venus, and seen rain cascade across methane lakes on Titan. From these worlds we learned the generic physics and chemistry that make up what’s called climate. We can use these laws to predict the global response of any planet to something like an asteroid impact or perhaps the emergence of an energy-hungry industrial civilization.

To launch our science of exo-civilizations we started with those laws of planets, building the right equations to capture the intertwined evolution of a planet and its young civilization. But planetary laws of physics and chemistry only tell part of the story. If we want to know the possible fates of other civilizations on other worlds, we had to bring some biology to bear too.

Science fiction has given us enduring images of alien races. Not surprisingly, most of them look a lot like us but with different kinds of foreheads or ears, or a different number of fingers on their hands. In developing our first cut at a science of exo-civilizations, my collaborators and I weren’t interested in what aliens might look like or what kind of sex they have. To do our job we had to avoid the specifics of both their individual biology and their sociology because science provides us little to work with on those fronts. There was, however, one place where biology was up to the task.

Population biology was a radical new field back in the early 20th century. Rather than just collecting statistics to describe animal populations, a few ambitious researchers like Alfred Lotka wanted to create basic mathematical models of things like predators and prey to predict the evolution of their linked populations. Predators (like wolves) eat prey (like bunnies) so they can make more wolf babies, thereby increasing the wolf population. Bunnies do a fine job of reproducing on their own, but if too many are eaten, their population numbers suffer. Today, population biologists, ecologists, and their compatriots use mathematical models to study everything from the spread of disease to the propagation of invasive species. The approach has even found its way to the study of human civilizations, including their collapse in places like Easter Island.