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In a few million years, we’re going to have to leave Earth if we want to survive. But how long can we actually outrun extinction before the universe becomes uninhabitable to us?

Host: Reid Reimers

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[ ♪ Intro ].

Modern humans evolved a couple hundred thousand years ago. And between ice ages, volcanoes, climate change, earthquakes, droughts, floods, and Reddit being down sometimes, we’ve seen some stuff.

But on cosmic timescales, two or three hundred thousand years is just a drop in the bucket. The longer we’re around, the more likely we are to witness something truly Earth-shattering, whether metaphorically or literally. And someday, if we’re still here, we’ll have to face things like Earth melting as it’s engulfed by the Sun.

Or the Sun going out. Or galaxies colliding. One way or the other, we’re going to have to leave home if we want to survive.

And even if we had fancy starships, how long could we actually outrun extinction before the universe became uninhabitable to us? It’s such a big question that we’ve made a special, extra-long episode about it. Because the answer is a surprisingly long time.

Now, to be clear, we probably won’t have to leave home any time soon. So there’s plenty of time to get our spaceships ready. Like, we might hear a lot about climate change these days, but no matter how destructive and disruptive it is, researchers don’t think that it will threaten our entire species.

There are plenty of variables to consider, but overall, the odds are pretty good that we’ll get through it one way or another. The same goes for one of the Internet’s favorite doomsday scenarios: a supervolcano. Even though that would be rough, it probably couldn’t take us all out; there are just too many people spread throughout the world.

But we will have to leave Earth someday. Because even if we dodge a huge asteroid impact or the takeover of artificial intelligence, our planet will eventually stop being habitable, thanks to the Sun. The Sun brightens as it ages.

And generally, that will make Earth warmer over the next few hundred million years, which will cause all kinds of problems. For example, rocks tend to erode more easily at higher temperatures, so in the future, more new, rocky surfaces will be exposed to the atmosphere than today. Exposed rock can react with carbon dioxide, so the more exposed rock there is, the less CO2 will be left in the atmosphere.

That might seem like a good thing, given how worried we are today about climate change. But remember that plants need CO2 like we need oxygen. And according to some estimates, in about 800 million years, there actually won’t be enough of the gas left in the atmosphere for photosynthesis.

If those estimates are right, that would be the end of all life on Earth bigger than a bacterium, and they’d go extinct about a billion increasingly warm years later. Future people might prevent this by building something to block the extra sunlight, or they could use repeated asteroid flybys to gravitationally inch Earth’s orbit away from the Sun. But they might also decide that Earth just isn’t worth it anymore, and could leave.

The most obvious option would be to go to Mars, since it’s closest. We’d still have to give the planet an atmosphere and a liveable temperature, but with a dash of terraforming, we’d be fine, although Mars still won’t be a permanent solution. Five or six billion years from now, the Sun will finally start running out of hydrogen fuel in its core.

After that, it will fuse helium, and it will start growing. First, it will engulf Mercury, and then Venus. And 7.6 billion years from now, its outer layers will probably pass through Earth’s orbit, too, likely destroying almost everything we’ve ever built.

The people on Mars wouldn’t be much better off, either. Mars will never fall into the Sun, but it will become too hot to be comfortable for life. And with the Sun so much closer, solar radiation will rip apart any Martian atmosphere our descendants try to create.

But we wouldn’t be out of options yet. Because while the Sun balloons, the outer solar system may have it pretty good. Jupiter’s moon Europa and Saturn’s moon Enceladus are both ice-covered today, but they’ll heat up as the Sun expands and will eventually become water worlds.

And, briefly, we could live there. But again, that wouldn’t last forever. In fact, we might be on the move after as little as a hundred thousand years, once more, because of the Sun.

Around that time, the Sun will have expanded so much that its outer layers will finally escape into space, leaving behind a tiny white dwarf star surrounded by a big, cloudy nebula. Comparatively speaking, white dwarfs don’t emanate much energy, so the newly-minted one at the center of the solar system will leave the remaining planets and moons as cold as interstellar space. And without a good heat source, there’s no usable energy.

So any civilization that’s still around will have to move on. By this point, though, we’ll probably just fold our Dyson spheres into overhead compartments and head off to colonize some other planetary system. Because, hey, if we haven’t figured out interstellar travel in seven billion years, well, we’ll have a problem.

But let’s assume we figure it out, and don’t spend the next forever binging Netflix. Today, the closest star is Proxima Centauri, which is about four light-years away. It’s a red dwarf star, which means it will likely outlive the Sun, and it already has a potentially-habitable planet.

But since red dwarfs also emit some seriously dangerous flares every now and then, it might not be our best choice. Besides, current estimates suggest that there are billions of planets out there that could support water, so we have plenty of other options. Which is good, because, today, there’s really no way of knowing where Proxima Centauri will be that far in the future.

Stellar orbits in the Milky Way naturally get rearranged over billions of years, so Proxima Centauri might not be our closest neighbor anymore. Also, and maybe more importantly, the Milky Way won’t exactly be a thing in seven billion years. It will probably collide with the Andromeda Galaxy even before the Sun expands into the inner solar system.

Or, at the very least, they’ll pass close enough to each other to cause some chaos. Thankfully, there’s almost no chance of stars actually smashing into each other, since stars are just too spread out. But the collision will make long-term wayfinding tricky.

Stars will be thrown all over the place while the new galaxy, known as Milkomeda, takes shape over billions of years. Yes, that is the real proposed nickname, and no, I don’t support it. So maybe the next star we’ll go to hasn’t been discovered yet, or born.

And, even if we do have the technology to leave the solar system, we’ll have to watch out for the dangers of interstellar space. For example, we’ll have to keep an eye out for supernovas, since colliding gas clouds will produce lots of enormous, short-lived stars that explode at the end of their lives. And radiation from those explosions could easily wipe us out mid-journey.

But if we do learn to safely travel to new stars, that would allow humans to keep thriving for a long time. We’re talking trillions of years. Because even when our new home star goes out, we could hop over to another system.

As a bonus, the formation of Milkomeda will also make a good number of smaller, longer-lived stars like our Sun, so we’ll have a lot of options. But even though trillions of years is an unimaginably long time, it’s not forever. Eventually, all the Sun-like stars in the universe will wink out, one after another, and there won’t be enough big clouds of hydrogen-rich gas left to replace them.

But even then, that won’t be the end of humanity. Our corner of space will get darker and cooler, since it will be lit mainly by tiny, long-lasting red dwarfs. But we could still survive around them, like our ancestors huddled around their flickering campfires.

We would have to find a way to live with the violent solar flares, though. Or if nothing else, we would have to find a way to harness the red dwarf’s energy without being in range of those flares. But assuming we figure that out, we’ll be able to stay alive long into the future.

Eventually, though, even those stars will die, along with any remnants like white dwarfs. And finally, the universe will be black. At this point, our only decent source of energy will be black holes, since there will still be plenty of those floating around, with gas falling into them all the time.

If our descendants are surviving on homemade, nuclear fusion power, which seems likely, considering all the stars will be gone, they’ll need that gas to keep making energy. Of course, with black holes gobbling up that gas and the expansion of the universe spreading it out more and more diffusely, it will be hard for them to find enough fuel. There will always be the odd proton or dead planet gliding through the vastness of empty space that we could somehow harness resources or energy from.

But just about everything else is eventually going into a black hole. Fortunately, it will take a while for black holes to vacuum the universe, and that’s putting it lightly. For you fans of metric prefixes, astronomers estimate that it will be about a billion yottayears before there’s pretty much nothing out there but black holes.

That’s a billion trillion trillion years, or a one with 33 zeros after it. And I think we can agree: That’s a long time. Many physicists who are trying to combine everything we know about the universe into a single elegant idea think that, coincidentally, protons will decay after about this much time, too.

And that would definitely be the end of all life. But if they’re wrong, physicists and mathematicians have found that even in that dark, black hole-filled universe completely devoid of raw materials, life could still persist. They’ve discovered that if you dive close to a spinning black hole and drop something in it just the right way, you can steal energy from the black hole itself.

The exact mechanism for how it works is pretty complicated, and at the end of the day, no one is totally sure if you could power a civilization this way. But for now, we think it’s still an option. And we have time to figure out the details.

Then again, black holes aren’t forever, either. In 1974, Stephen Hawking published his discovery that black holes very, very slowly radiate energy in a process now known as Hawking radiation. Hawking radiation is incredibly weak for any reasonably large black hole, so it’s not like we could use it to heat our space colonies.

But the important part is that it causes black holes to slowly shrink. And after almost a googol years, that’s a 1 with one hundred zeros after it, even the biggest black holes in the universe will evaporate due to Hawking radiation. After that, it’s hard to imagine any kind of life flourishing.

There won’t be many atoms left, and any energy will be so uniform that we won’t really be able to do anything with it. This is known as the heat death of the universe, and it’s how today’s best cosmological models predict the universe will finally end. Even here, though, there may be the tiniest glimmer of hope.

In 1979, the physicist Freeman Dyson, the same guy who made Dyson spheres famous, imagined an organism that worked kind of like a computer:. When it’s on, it uses energy to think or do something, and when it’s sleeping it does essentially nothing. Dyson was able to show that by sleeping for longer and longer between each pair of thoughts, the organism could live an infinite amount of time, but only ever use a tiny bit of energy.

So with a big enough battery, this thing could last forever. And that brings us to our final thought:. By this point in time, humans might not technically be humans anymore, thanks to evolution.

In fact, we might have changed so much that nobody will remember that time we had ten fingers and breakable bones and disease. Realistically, that day could come much sooner than trillions of years from now. So maybe we could become, or even choose to become, this weird species that waits a quadrillion years between thoughts.

Maybe we’ll decide that’s worth immortality. Or, maybe we’ll just decide we’ve lived long enough. But, hey, we have a long time to sort out those decisions.

In the meantime, the best thing we can do is keep taking care of the planet we have, so we can make the most of the next 800 million years. Because, ultimately, scientists can tell us a lot about what the universe will be like in the far-off future. But as for what humans will do until then… well, that’s up to us.

Thanks for watching this special episode of SciShow Space! If you’d like to see us do longer episodes on other topics, let us know in the comments! And as always, if you want to keep up with our latest episodes, you can go to and subscribe. [ ♪ Outro ].