​Our Galaxy May Be Littered With Dying Earths

We all have our favorite doomsday scenario, but there's one apocalypse science tells us we can't avoid: The slow extinction of life on Earth as our sun grows hot and then flickers out. Humanity may be long gone by the time Earth transforms into a hellish wasteland, but for the morbidly curious, scientists have painted a detailed portrait of our biosphere's slide into oblivion. Still, imagining the end is one thing, but actually seeing Earth's inevitable collapse would be an entirely different experience.

Lucky for us, our galaxy is likely brimming with examples. In fact, earthlike planets with dying biospheres may outnumber healthy ones ten to one, according to a study in pre-print this week. First contact, then, may find us confronting the apocalypse.

It's predicted that in about a billion years, as our sun brightens, the Earth's surface will be radically transformed. Slowly at first, the climate will start to heat up, leading to species extinctions and ecosystems shifts. But after a hundred million years or so of gradual climate change, the apocalypse kicks into high gear: The oceans will start boiling away, the air will become saturated with water, plate tectonics will grind to a halt, and Earth's geologic carbon cycle will shut down. This last point will spell doom for plants, which require atmospheric CO2 for photosynthesis. Soon, all higher life forms will be extinct. Microbes may cling on for another billion years longer.

In broad brushstrokes, this is how scientists predict the end will go down. But for obvious reasons, testing hypotheses about the biosphere's collapse would be both dangerous and immoral. To space, then, researchers have decided to go.

"The only possible method for testing predictions about Earth's far future is to find and study extrasolar Earth analogues that are further along in their habitable evolution than the present-day Earth," the authors write.

In the study, scientists probed the likelihood of finding a dying world on which we may, one day, be able to study our own inevitable fate. Such a planet would have to have spent a decent chunk of time—say, 4.5 billion years—within the habitable zone of its star, in order for an Earth-like biosphere to evolve. But we'd also want the planet to have spent time at the inner edge of its star's habitable zone—that delicate boundary where things are a bit too toasty for life to hang on. This would ensure that the planet's biosphere is now on the fritz, offering a true representation of Earth's distant demise.

To begin the search, the researchers focused on six G-type stars—the same type as our sun—within ten parsecs of Earth. For each one, they conducted a climate modeling exercise wherein a hypothetical Earth-like planet was placed within its star's habitable zone long enough for a biosphere to develop. Running each planet's climate model out for billions of years, the researchers assessed whether the any of the worlds would, today, look like a far-future Earth.

Their results point to 61 Virginis—a star located in the constellation Virgo, some 27 light years away—as the top candidate for harboring a dying world with detectable biosignatures in our cosmic neighborhood. In the other star systems, hypothetical worlds trended toward the very late stages of biosphere collapse. Such planets may still host a few hardy microbes, but not enough to produce biosignatures we can detect from afar.

The authors findings suggest that only a small fraction of aging, G-type stars could host a dying Earth. Still, extrapolating to the entire galaxy, there may be as many as 50 million apocalyptic worlds out there—ten times more than there are planets with early-stage or Earth-like biospheres.

The notion that the first aliens we discover may be staving off their own annihilation sounds a bit depressing, but scientists have a lot learn from these dying words. They may shed light on the long-term geologic carbon cycle, how ocean evaporation impacts climate, and what happens to plate tectonics as a planet's core cools.

More saliently, they'll allow us to witness the end of the world.