We live on the edge of a knife, never quite knowing when a true existential threat to life on Earth will come—something large enough to shake the foundations of our species and possibly wipe all us all off our planet. But that doesn’t mean we can’t prepare ourselves.
While the following estimates are just guesses about what could kill off all Earthly life for good, they’re based on some real science. For one thing, we have Earth’s geological record, which allows us to see traces of things like impacts (using craters) and deadly explosions (using ice cores and tree rings). We also have astronomical records; we have observed the night sky long enough to build up a catalog of how often these kinds of life-threatening events happen, and how far away they are on average.
Combining geological records of past catastrophes with the latest in astronomical research, here’s our ranking of the seven greatest cosmic threats to Earthly life.
No. 1 — Extreme Solar Events
Average time between disasters: 100 years
In 1859, the sun rocked our planet with a deadly electromagnetic storm. Telegraph operators were shocked—literally—when the storm passed, and the Northern Lights could be seen as far south as Miami. Known today as the Carrington event, it was caused by the most powerful solar outburst ever recorded.
The sun has these kinds of outbursts, known as coronal mass ejections, all the time. And careful analysis of tree-ring data has revealed that ejections 1,000 times more powerful than the Carrington event occurred tens of thousands of years ago—but ancient people didn’t really care or notice, because these storms cause the greatest damage to our electronics. Based on past solar observations, a Carrington-level storm will almost certainly happen within the century, and at best it will be a mild inconvenience as electronics briefly flicker. At worst, our civilization will shut down.
No. 2 — Small Asteroids
Average time between disasters: 1,000 years
Every few years, an asteroid the size of a bus slams into our atmosphere at a speed of up to 70,000 miles per hour. When it detonates, it releases energy equivalent to a medium-scale nuclear bomb. Thankfully, most of these interlopers detonate over open ocean and high up in the atmosphere. But eventually our luck will run out, and a metal-rich asteroid, capable of surviving its screaming descent through our atmosphere, will aim itself squarely at a densely populated portion of the globe. Our only hope will be to develop an early-warning and deflection system so we can launch a spacecraft at the incoming asteroid in hopes of changing its course.
No. 3 — Natural, Long-Term Climate Change
Average time between disasters: 100,000 years
By far the greatest threat facing humanity is human-caused climate change. While Earth has been warmer in the past, the climate has never changed so quickly and to such a large degree. We have built our civilization with the expectations of a stable climate: with cities planted near ocean shores, with reliable seasonal variations to grow our crops, and without the threat of random violent weather events.
The good news about human-caused climate change is that we can do something about it. The bad news is that Earth itself is perfectly capable of making long-term shifts in the climate whether we like it or not. Changes to the sun’s output or a rebalancing of Earth’s atmospheric gases can both cause a climate shift; e.g., the hypothesis that Earth spent a billion years encased in ice, and another billion years without any polar ice caps at all. These shifts would happen much more slowly than human-caused change, but that just makes them so much harder to fight. We’ll have to adapt to the new sea levels, the new seasons, and the new weather patterns as we go.
No. 4 — Large Asteroids
Average time between disasters: 1–10 million years
Just ask the dinosaurs how fun it is to be hit by an asteroid a few miles across. Yes, while dinosaurs are still around—they’re now called birds—a major impact 65 million years ago destroyed every single land species larger than roughly 100 kilograms, including all but a few lineages of the dinosaurs. The impact was so powerful that it shook Earth like a bell, triggering volcanic eruptions around the globe. It kicked up a giant plume of dust that enveloped Earth in a millennia-long fallout winter.
Thankfully, these planet-scale disasters are much rarer than the smaller asteroid impacts that only cause local devastation. They seem to crop up every few million years or so, and the last one was about… ten million years ago. Whatever detection and deflection system we develop for small-scale asteroids better be well prepared, because it’s inevitable that we’ll be putting it to the ultimate test one of these days.
👀 Check out NASA’s asteroid deflection test
No. 5 — Gamma-ray Bursts
Average time between disasters: 10 million years
Astronomers have scanned the skies and found no immediate threat of a star exploding in our cosmic faces; the nearest stars that are likely to pop off are simply too far away to be a hazard. But there are bigger, deadlier cosmic threats that have a much larger range—and are much more difficult to detect. These are the gamma-ray bursts, or GRBs.
These happen when an exploding star triggers the formation of a black hole in its core. The material of the dead stars swirls around the newly formed black hole, compressing and heating up as it does. Complex forces trigger the launching of massive jets of gamma-ray radiation, the most powerful kind of radiation. GRBs are among the most energetic events to occur in the entire universe and have been implicated in previous extinction episodes on Earth. When the gamma rays blast over Earth, they strip away our protective ozone layer, exposing the surface to the full onslaught of the sun’s ultraviolet radiation. GRBs can originate from stars much farther away from Earth and still be deadly, meaning that we need to get better at mapping as many stars in the galaxy as we possibly can.
No. 6 — Supernovas and Kilonovas
Average time between disasters: 100 million years
While exploding stars aren’t a threat right now, that doesn’t mean that things won’t change in the future. Stars are constantly in motion, swirling in orbits around the galactic center, but also moving here and there like fish swimming in a school. Our nearest neighbor stars won’t stay this way forever. Over astronomical timescales, a large star, big enough to potentially pose a hazard, may wander into our galactic neighborhood. These large stars die through supernova explosions, which can quite literally rip away our entire atmosphere.
There is another kind of explosion, however, that’s even sneakier. It’s called a kilonova, and it’s what happens when two neutron stars—the leftover cores of giant stars—collide. These are much rarer than supernovas, but the neutron stars are far smaller, meaning they’re harder to detect—so they can sneak in nice and close to Earth before exploding without warning. To the best of our knowledge, Earth has not been struck by a deadly supernova or kilonova event in the past... but the future is wide open.
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No. 7 — Hostile Aliens
Average time between disasters: 1 billion years
To date, astronomers have not identified any evidence whatsoever of alien life, intelligent or otherwise. We have received no artificial radio signals. We have seen no signs of mega-engineering projects. As far as we can tell, we’re utterly alone. But it’s a big universe out there, with hundreds of billions of stars and trillions of planets in the Milky Way galaxy alone. As we continue to explore our galaxy, we may yet find signs of life somewhere. We may even finally meet another intelligent species. But even if we do, they will be very far away from us. A distance of even a few thousand light years will be enough to preclude any form of physical contact, since we just can’t traverse those kinds of distances.
Still, the aliens may or may not be a threat. They may not even recognize us as alive and intelligent. So we’ll be thankful for those enormous gulfs of interstellar nothingness. It’s a big galaxy out there, with plenty of room to share.
Paul M. Sutter is a science educator and a theoretical cosmologist at the Institute for Advanced Computational Science at Stony Brook University and the author of How to Die in Space: A Journey Through Dangerous Astrophysical Phenomena and Your Place in the Universe: Understanding Our Big, Messy Existence. Sutter is also the host of various science programs, and he’s on social media. Check out his Ask a Spaceman podcast and his YouTube page.
