Extraterrestrials! They are an idea from science whose impact on popular culture has been huge. Think of those big movie franchises that involve beings from other worlds: Marvel, Star Wars, Star Trek and more.
Extraterrestrials have not always been popular. Aristotle, the great Greek philosopher from before Christ, reasoned that Earth is the only world that exists. No other worlds. No extraterrestrials.
Aristotle influenced many Greco-Roman, Jewish, Christian and Islamic thinkers. However, in 1277, the bishop of Paris officially condemned Aristotle’s idea that Earth could be the only world. An omnipotent God, the bishop said, could make any number of worlds.
Different medieval thinkers, most of them Catholic clergy, got creative imagining God’s other worlds and their extraterrestrial inhabitants. One cardinal, Nicolas of Cusa, wrote about how it would be harder to converse with extraterrestrials than with dogs. After all, we have things in common with dogs, like mouths and voices. We might have nothing in common with extraterrestrials.
The extraterrestrials idea truly caught fire after 1543, when Copernicus proposed that Earth circles the sun, and Mars, Jupiter, etc. do, too. Aristotle had said the sun circled Earth, while Mars and Jupiter were wandering lights in the sky that had less in common with Earth than a flame had with a rock. Under Copernicus’s revolutionary idea, those planets had something in common with Earth: circling the sun.
The invention of the telescope and developments in physics soon showed that planets have moons, rotate and have gravity, like Earth. Astronomers concluded that planets were other “Earths.” And, that they must have intelligent life. Why would Earth be unique among worlds? Scientists at that time thought that life arose spontaneously from matter (another idea from Aristotle), so why wouldn’t life be everywhere?
Scientists figured stars were other suns, with inhabited planets circling them, too. A prominent early 19th-century science writer, Thomas Dick, estimated 22 trillion intelligent beings in our solar system alone; 62 septillion in the universe overall.
At that century’s end, some astronomers claimed to have detected evidence of an advanced civilization on Mars. The New York Times and the Wall Street Journal published that big news. Edgar Rice Burroughs and H. G. Wells wrote novels about Martians. Extraterrestrials became big in popular culture.
But since then, scientists have learned that life does not spontaneously generate from non-living matter. Our telescopes have improved. Our robotic probes have traversed the solar system. Our radio telescopes have scanned for interstellar radio chatter for six decades. Over the past 30 years, our ability to detect advanced civilizations, even in other galaxies, has grown dramatically. But we have detected nothing.
Meanwhile, we have learned that stars and planetary systems are wildly diverse; few solar systems look like ours. We have learned that there is no intelligent life, and maybe no life at all, on Mars or any other solar system planet. We have no idea how life arises. We know only that Earth has it.
Earth is unique in our solar system. The likelihood is growing that Earth is pretty unusual overall. The extraterrestrial-filled universes of Marvel, Star Wars and Star Trek might have seemed plausible back in the 60’s and 70’s when they were created; they don’t now. Just as in the days of Copernicus, we stand in the middle of a great revolution in our understanding of the universe.
Chris Graney lives in Louisville and is an adjunct scholar with the Vatican’s astronomical observatory.