The odds of finding life on another planet hinge on the answers to two big questions. First, how often does life arise? Second, once it does arise, how likely is it to persist without being completely wiped out? The first question is extremely difficult, especially since we have exactly one example of a life-spawning planet. But the second question is easier to answer—at least for Earth—and a trio of astrophysicists have given it a shot.
In a paper delightfully titled “The Resilience of Life to Astrophysical Events,” David Sloan and Rafael Alves Batista, both from the University of Oxford, and Avi Loeb, from Harvard University, estimated the odds that a space-borne catastrophe like an incoming asteroid would completely sterilize the Earth. Reassuringly, they think those odds are astronomically low—about one in 10 million for every billion years. “The conclusion we come to is that life, once it starts anywhere, is hard to eradicate,” Sloan says. In other words: Life finds a way (even if you bludgeon it with a giant space rock or an exploding star).
To be clear, the three researchers aren’t concerned with the fate of humans—a fragile, fleshy species that, in Loeb’s words, can be “killed by climate change or affected by bad politics.” Instead, the trio wanted to know what it would take to wipe out all life on the planet. And to do that, they focused on the world’s hardiest animals—the tardigrades.
Tardigrades, also known as water bears, are microscopic, eight-legged, water-living animals. They’re oddly endearing with their shuffling gaits, rotund bodies, and puckered mouths. They’re also the epitome of resilience. In stressful environments, they can expel all the water in their bodies and enter a shriveled, dormant state. In this form, as I’ve written before, they don’t need food or water. They can endure temperatures close to absolute zero and as high as 151 degrees Celsius for a few minutes. They can withstand the intense pressures of the deepest ocean, doses of radiation that would kill other animals, and baths of toxic solvents. They are, to date, the only animals that have been exposed to the naked vacuum of space and lived. If you want to know what it would take to sterilize the world, you need to know what it would take to kill tardigrades.
(Ironically, a finger will do the job: One scientist once told me that tardigrades are vulnerable “to mechanical damage,” which means that you can squish ’em. But assuming that there’s no vengeful tardigrade god with plans for a globe-trotting smiting spree, what else would it take?)
Sloan, Batista, and Loeb consider three possible killers: a colliding asteroid, like the one that killed most of the dinosaurs; a nearby star going supernova; and gamma-ray bursts—high-energy explosions unleashed by some distant stellar calamity.
In the case of the asteroid, anything that was directly punched by the rock would obviously be in bad shape, but the collision would also throw up so much dust that it could block out the sun and instigate a global freeze. The supernova and the gamma-ray bursts would both kill in similar ways. They’d pummel the planet with intense radiation. They’d destroy the ozone layer, allowing yet more radiation to reach the surface. They’d convert nitrogen and oxygen in the atmosphere into nitrous oxide, which would create acid rain and a sun-blocking smog.
All of these scenarios would be very bad news for humans and for anything else on land. But species that live underground or in the deep oceans—including many tardigrades—would be largely shielded by soil or water. They already survive in the absence of sunlight. You couldn’t acidify the oceans enough to kill them. You could irradiate them, but these animals are exceptionally resistant to radiation, which would also get weakened by any overlying water. Sloan, Batista, and Loeb calculated that the amount of radiation it would take to kill a tardigrade on the ocean floor would also cause the oceans to boil—a far more pressing issue. The extended heat would kill them before the radiation did.
So sterilizing the world, er, boils down to heat. Could an astronomical cataclysm impart enough energy onto our blue marble to raise the temperature of its water beyond 100 degrees Celsius?
An asteroid could do it if it weighed more than 1.7 quadrillion metric tons, making it somewhere between 10 and 1,000 times heavier than the rock that smote the dinosaurs. In our solar system, there are only 19 known asteroids that are big enough (along with a few dwarf planets like Pluto), and none of their orbits coincide with Earth’s. Beyond our solar system, the three researchers calculate that the odds of being struck with one of these life-ending monsters within a billion-year timespan is just one in 100,000.
Supernovas and gamma-ray bursts both release ocean-boiling amounts of energy, but that energy dissipates with distance. To annihilate all tardigrades, a supernova would need to happen at most 0.13 light-years away. Our closest star, Proxima Centauri, is already 4.25 light-years away and is too small to go supernova; even if it did, it would raise the ocean’s temperatures by a paltry 0.1 degrees Celsius.
All of these calculations are based on the Earth, but we’re not alone in our safe distance. Batista created a simulation that accounts for the density of stars throughout our galaxy, and the fact that those in the center are more closely packed. But even there, only 1 percent of planets would eventually be sterilized by an exploding star. Gamma-ray bursts are also unlikely annihilators. Based on how often they happen, the odds that they would occur within the sterilization radius of a planet are just one in 10 billion for every billion years. (Again, we’re talking about what it would take to kill tardigrades. “The death sphere gets much bigger when you consider an animal like us,” says Sloan. “God, I just invented the phrase ‘death sphere’. That’s scary.”)
Eventually, the sun will expand and potentially engulf the Earth in fire, before collapsing and triggering a permanent freeze. “The end of the sun would terminate life on Earth, but before then, it’s unlikely that astrophysical phenomena would do it,” says Loeb. “Within the lifetime of the sun, tardigrades will likely survive.” And tardigrades are only the hardiest animals around. If they can survive, microbes probably will, too.
The Earth has experienced at least five major mass extinctions in its history, where the majority of living species disappeared forever, but something always survived these brushes with oblivion. And even in the face of far greater calamities, Sloan, Batista, and Loeb think that life would still persist in some form.
Clark Chapman, from the Southwest Research Institute, endorses their conclusion. “While human civilization, as it exists today, is at some very small risk,” he says, “it is virtually certain that Earth cannot be sterilized for billions of years.” Still, he notes that things may have been different in the solar system’s past, “when bodies vastly larger than today’s asteroids and comets collided with planets that might have already had life and sterilized them.” The Earth, for example, was possibly hit by a Mars-sized object in its early history, creating the Moon.
Besides, Chapman says, nobody knows if evolution on other planets would tick along in the same way as it has on Earth. Would it spawn tardigrade-like organisms that tolerate extreme conditions? Would those organisms find shelter in safe spaces like deep oceans? “It’s highly speculative to guess about the evolution and survivability of life on extra-solar worlds," he says.
Sloan admits that he and his colleagues made a lot of assumptions in their work. But when scientists think about life beyond our planet, they cannot help but use life on our planet as a starting point. And Sloan now feels more optimistic about the search for such life.
In the last few decades, astronomers have discovered thousands of other planets that lie beyond our solar system. Some of these exoplanets are Earth-sized, and some orbit the habitable zones of their respective stars—the Goldilocks region where they get enough heat, but not too much. Whether they—or closer neighbors like Mars—are home to life is an open question. But “if life did get started elsewhere in the galaxy,” says Sloan, “we should expect it to still be there.”