“Mom, look,” I said, holding up nine sheets of construction paper that I had Scotch-taped together. On each page, I had drawn a made-up planet and listed its salient characteristics: tree-shaped algae, oceans of melted Play-Doh, quicksand surrounded by ice floes. A friendly alien inhabitant waved from the surface of each planet. I explained to my mother how each extraterrestrial’s body was uniquely suited to the conditions of its world (in my less-than-humble 7-year-old opinion).
Coincidentally, that same year, 1992, astronomers discovered the first planets outside the solar system. Alexander Wolszczan and Dale Frail found two giant, atmosphere-less worlds orbiting the pulsar B1257+12. The dead star, as dense as an atomic nucleus, bathed them in zombie radiation. Although those sorry planets are not friendly to life, they provided the first proof that our solar system is not the only solar system.
I knew nothing of this discovery, but for second-grade me, the existence of other worlds—and the species surely swinging from their algae trees, whatever those are—seemed as obvious a scientific conclusion as “people are happier on weekends.”
Twenty-two years later, we know of 1,852 more planets. Extrasolar solar systems dangle throughout the universe like archival and mismatched Christmas ornaments. Astronomers now estimate that our galaxy contains at least as many planets as stars—about 100 billion of them. We regularly find so many planets that we now talk about them in bouquets: 1,091 new candidates! Wait, here are 715 more! The idea that our solar system’s existence is unique feels hopelessly outdated, like thinking Earth has a sharp edge off which you can sail your ship.
Now that we know planets are normal, we are obsessed with finding out if Earth is normal, too. We can hardly bring ourselves to think of other solar systems except in terms of how they compare to our own. Do they have multiple planets, like ours does? Do they orbit in a plane? Are their Jupiter-size planets far away from the star, too? Reports of individual planets mostly appear when those planets are tantalizingly close to Earth’s size, Earth’s mass, or Earth’s “Goldilocks” distance from the sun, which allows the presence of liquid water. We want the universe to be a funhouse mirror, reflecting back a slightly warped—but recognizable—version of ourselves.
Throughout most of human history, the opposite has been true. Humans used to run screaming from evidence that we were not special, not the center of the universe. We believed all other celestial objects orbited Earth. When Galileo proved that Earth orbits the sun, the rotating glass spheres we thought held the stars over our heads shattered, and they sliced into our minds. After a few centuries of suppression by the Catholic Church and some burnings at the stake, we eventually adjusted to the idea. But then Friedrich Bessel dethroned the sun, too, proving in the mid-1800s that it’s simply a close-up version of the constellations’ countless constituents.
The insults continued to mount. The sun is an unremarkable star. And our solar system doesn’t live in the galaxy’s cultural center, but in some godforsaken suburb of a spiral arm. Our Milky Way isn’t the whole universe but just one of at least 100 billion other galaxies, spread across tens of billions of light-years. Each decentralizing discovery disturbed Earthlings.
Every telescope named after a dead male astronomer, from Planck to Hubble to Chandra, tells the same story over and over with different details: Earth is just a speck in a no-comment cosmic neighborhood. (Add to that the indignity of discovering that humans were not formed by God’s hand in the Garden of Eden but are the products of billions of years of aimless evolution by natural selection.)
We have become so used to our cosmic marginalization that we’ve reached a philosophical turning point: What disturbs us now is to think Earth is somehow special.
Modern astronomy actually relies on our mediocrity. There’s even a principle for it, conveniently known as Mediocrity Principle. It holds that nothing about our place in or experience of the universe is special; the cosmos looks and behaves the same no matter where we point our telescopes.
According to the Mediocrity Principle, solar systems like ours abound. And now that NASA announces exoplanets in Costco-size variety packs, our collection of known planets contains more and more small orbs that might have solid surfaces and liquid H2O—in other words, worlds more and more like our own, in a habitable zone where life as we know it could exist.
But astronomers are obsessed with finding one that really is an “Earth-like planet” (aka “Earth 2.0,” “Earth analog,” or “Earth twin”). And the popularity of stories about the next almost-but-not-quite-Earth-like planet indicates that the public is invested, too.
Here’s why: Without a twin, we are special. We might be alone, and we are thrust back toward the philosophical center—a worldview that is so last century.
We tick off ever-more-Earth-like boxes as we get closer and closer without obtaining a cigar:
- the first planet around a sun-like star (51 Peg b, found in 1995);
- the first planet that is not a gas giant like Jupiter (CoRoT-7b, 2009);
- the first not-a-gas-giant in the habitable zone (Kepler-22b, 2011);
- the first Earth-size rocky planet (Kepler-78b, 2013);
- and, most recently, the first Earth-size rocky planet in the habitable zone (Kepler-186f, 2014).
These are the discoveries that scientists and the press pluck from the grab bag of planets and put on individual display. The rest can stay statistics.
We seek Earth analogs because maybe someday we can rocket toward them with Matthew McConaughey, to start a new civilization after we turn actual-Earth into a hellscape with no icecaps. We seek them because Manifest Destiny, because curiosity, because science. Because the familiar comforts us and because we’re self-centered. But mostly, we crawl toward Earth’s twin—and its potential inhabitants—so we can breathe an oxygenated-atmosphere sigh of relief: We don’t have to crush the lens through which we view the universe.
The zeitgeist has caught up with scientific philosophy. Earthlings have made a huge cultural shift, from wanting to be the center of the universe to wanting, so badly, not to be anomalous and alone. Exoplanet astronomer Sarah Ballard of the University of Washington sums it up with a fortune cookie she once received: “ ‘Optimists believe we live in the best of worlds, and pessimists fear this is true.’ But the opposite is true for astrobiologists.” The time has come when the opposite also rings true for the rest of us.
In the past century, our conception of the universe has grown larger and stranger—full of dark matter, dark energy, and who knows what other dark things we don’t yet understand. The discovery of Earth 2.0—and perhaps its inhabitants—will transform a mostly invisible, ever-expanding, largely empty, and numbingly cold universe back into a cosmos we can connect with and draw on construction paper, rather than one at which we mostly marvel.
