Here’s what you’ll learn when you read this story:

• Hinging on the ideas of Copernicus, who concluded that Earth is not as unique as we think it is, two philosophers explored what this might mean for consciousness beyond our planet.
• They suggest that consciousness might come down to substrate flexibility, or whether the same function can be carried out by entities made of different materials.
• While there is still no evidence for life (let alone consciousness) outside of Earth, it might appear in a completely unrecognizable brain.

In 1514, Polish astronomer Nicolaus Copernicus disrupted everything humanity thought about its place in the universe. Years of peering at the night sky (and a lot of math) had brought him a revelation: Earth is not the center of the universe, and our planet instead orbits the Sun.

In Copernicus’ time, this proposition was extremely scandalous, as it pointed to the idea that Earth is not necessarily special—our planet is just one of innumerable others in myriad galaxies scattered throughout an infinite universe. Some current astronomers and physicists push back against the idea that Earth isn’t special on a cosmic level, but even they acknowledge that we’re far from the only world that exists out there.

This lack of universal uniqueness can bring another question to mind: If Earth is just another planet, and beings on Earth have developed this thing we call consciousness, could it be that there are conscious species on other planets?

Eric Schwitzgebel (a philosopher from the University of California, Riverside) and Jeremy Pober (a former UC Riverside grad student who is now a philosophy postdoc at the University of Lisbon in Portugal) theorize in a recent working paper that consciousness might not be confined to the carbon-based biological substrate in which it first emerged on Earth.

Consciousness, which the duo refers to as a target phenomenon, needs a physical substrate to exist. In the case of consciousness on Earth, that substrate is whatever the conscious human or cat or octopus is made of. The authors call target phenomena or properties that can occur within multiple substrates “substrate flexible.”

There are varying degrees of substrate flexibility. An atom of a particular element isn’t very flexible at all, since only a few highly specific physical properties make it what it is. An object such as a cup is much more flexible, since a receptacle that holds liquid can be made from a plethora of materials. In this way, substrate flexibility depends on how specific the target property is.

For instance, if the target property of a cup is that it holds liquid, the cup can take numerous forms and be made out of numerous substrates. If the target property of a cup is that it holds hot liquid, however, there are far fewer substrates that your cup can be made out of. The specificity of “hot liquid” as the target property narrows the substrate flexibility of the cup.

With this in mind, trying to figure out how many substrates could sustain the target phenomenon of consciousness means asking what consciousness even means. Does it mean a brain capable of solving problems? Remembering? Dreaming?

Pober and Schwitzgebel tackle the question of which substrates could host consciousness—and thus who, or what, could potentially be conscious—using a framework that echoes some of the abstract thought processes of Copernicus. In doing so, they reached the conclusion that the universe must contain “at least a thousand different behaviorally sophisticated species” that are made of diverse substrates, and that even though some of these species are composed of substrates that would seem unexpected to us, they can actually be described as conscious.

“Given that it’s likely that functionally complex, behaviorally sophisticated entities have arisen or will arise many times in the observable universe, in diverse substrates, we argue that it would be a violation of a principle of Copernican mediocrity to hold that among these diverse entities, only we, or only we and a small proportion of others who share our substrate, are conscious,” the duo said.

While the team readily imagines the possibilities of consciousness taking hold in different substrates, they acknowledge the uncertainty attached. For one, the version of life (and, therefore, consciousness) that exists on Earth is the starting point for all theories on extraterrestrial life—since Earth is the only planet we know of that hosts life—but standards for what actually qualifies as “consciousness” may differ in different places throughout the universe. It’s hard enough to find two Earth scientists who completely agree about what consciousness is, let alone two hypothetical scientists from opposite sides of the cosmos, and the team doesn’t define consciousness themselves.

There’s also a possibility—as much as Copernicus may have not wanted to entertain it—that life-forms on Earth really are the only creatures made of a substrate that can accommodate consciousness at all. For their part, Schwitzgebel and Pober don’t claim in their paper that extraterrestrial life definitely exists. However, they still think it’s worth theorizing about how the realm of consciousness could extend beyond our planet.

“Suppose that your best guess estimate is that on average each galaxy contains a million planets where species of [Earth] level behavioral sophistication eventually evolve (even if technological civilizations rarely arise),” they said. “The observable universe would then host, over its lifetime, a quintillion qualifying planets […] some of these life forms will be strange indeed.”