I’d wager a guess that we are, as a species, rather fond of our home planet (our wanton carbon emissions notwithstanding). But the ugly truth is that the Earth is doomed. Someday, the Sun will enter a stage that will make life impossible on the Earth’s surface and eventually reduce the planet to nothing more than a sad, lonely chunk of iron and nickel.
The good news is that if we really put our minds to it—and don’t worry, we’ll have hundreds of millions of years to plan—we can keep our home world hospitable, even long after our Sun goes haywire.
A waking nightmare
The Sun is slowly but inexorably getting brighter, hotter, and larger with time. Billions of years ago, when collections of molecules first began to dance together and call themselves alive, the Sun was roughly 20 percent dimmer than it is today. Even the dinosaurs knew a weaker, smaller star. And while the Sun is only halfway through the main hydrogen-burning phase of its life, with 4-billion-and-change years before it begins its death throes, the peculiar combination of temperature and brightness that make life possible on this little world of ours will erode in only a few hundred million years. A blink of an eye, astronomically speaking.
The Sun sows the seeds of its own demise through the basic physics of its existence. At this very moment, our star is chewing through something like 600 million metric tons of hydrogen every single second, slamming those atoms together in a nuclear inferno that reaches a temperature of over 27 million degrees Fahrenheit. Of that 600 million metric tons, 4 million are converted into energy— enough to illuminate the entire Solar System.
That fusion reaction is not perfectly clean, however. There is a leftover byproduct, an ash created by the nuclear fires: helium. That helium has nowhere to go, as the deep convection cycles that constantly churn material within the Sun don’t reach into the core where the helium is formed. So the helium sits there, inert, lifeless, useless—clogging up the machine.
AdvertisementAt its present age, the Sun does not have high enough temperatures and pressures at its core to fuse helium. So, the helium gets in the way, increasing the overall mass of the core without giving it anything else to fuse. Thankfully, the Sun is easily able to compensate for this, and that compensation comes about through a bit of physics known as hydrostatic equilibrium.
The Sun exists in constant balance, living on the edge of a nuclear knife. On one side are the energies released by the fusion process, which, if left uncontrolled, could threaten to explode—or at the very least, expand—the Sun. Countering that is the immense gravitational weight of the star itself, pressing inward with all the might that 1,027 tons of hydrogen and helium can muster. If that force were to go unchecked, the Sun’s own gravity would crush our star into a black hole no bigger than a mid-sized city.
So what happens when an unstoppable force meets an irresistible pressure? Graceful balance—and a star that can live for billions of years. If, for some reason, the nuclear inferno randomly ratchets up in temperature, that will heat up the rest of the star and inflate its outer layers, easing the gravitational pressure and slowing down the nuclear reactions. And if the Sun were to randomly contract, more material would force itself into the core, where it would participate in the heady nuclear dance, and the release of energy that results would conspire to reinflate the star to normal proportions.
But the presence of helium ash, that nuclear trash, upsets that balance by displacing hydrogen that would otherwise fuse. The Sun can’t help but pull inward on itself—gravity is uncompromising and uncaring. And when it does, it forces the nuclear reactions of the core to increase in ferocity, raising its temperature, which in turn forces the surface of the Sun to swell and brighten.
Slowly, slowly, slowly, as helium continues to build up in the core of the Sun (or any other star of similar mass), it expands and brightens in response. It’s difficult to predict exactly when this brightening will result in calamity for our planet—that depends on a complex interplay of radiation, atmosphere, and ocean. But the general estimate is that we have roughly 500 million years left before life will become all but impossible.
The warming Sun will increase the Earth’s surface temperature. With higher temperatures, the oceans will evaporate. Since water vapor is an excellent greenhouse gas, more of it in the atmosphere will lead to even greater surface temperatures. Higher temperatures will force the oceans to evaporate even more, setting off a runaway cycle that will quickly see all of the Earth’s abundant surface water floating in our atmosphere.
Without water to lubricate tectonic activity, our plates will grind to a halt. Without tectonic activity to pull carbon from the atmosphere, our air will become chokingly thick. Within a hundred million years, we will become a twin of Venus, which experienced a similar fate billions of years ago—two worlds dead at the hands of their own stellar parent.
Planetary adjustment
The "habitable zone" is the region around a star where the temperatures can—in principle, at least—support liquid water on the surface of a planet. Close to the star, the temperatures are too high, and barring any exotic atmospheric contortions, water will be forced into its vapor form. Outside the range, it’s just too cold.
The Earth currently sits roughly in the middle of the Sun’s habitable zone, with Venus just on the inner edge and Mars almost outside of it. As the Sun ages, however, its increasing brightness will shift the zone ever further out into the Solar System.
If we want the Earth to survive the coming epochs, we’ll have to move it.
Moving an entire planet is no easy feat, as you might imagine. But thankfully, for once, we have the balance of astronomical timescales on our side. We don’t have to move the Earth today; we have hundreds of millions of years to plan our shift. And to do the trick, we can employ that same persistent force that keeps the planets in orbit around the Sun in the first place: gravity.
Our first task is to find a source of energy. Raising the Earth's orbit will require an enormous amount of energy, and the physics here is as clean as it is cruel: that energy has to come from somewhere. Thankfully, we can use Jupiter. Because it's 318 times more massive than the Earth, its simple motion through the heavens provides it with a stupendous amount of kinetic energy. Surely it won’t mind if we borrow some for ourselves.
AdvertisementActually getting energy from Jupiter to the Earth will take a little bit of orbital chicanery. To help visualize what we’ll have to do, imagine standing on a rolling platform on some railroad tracks with a train barreling toward you. You can’t get off the tracks (because then this metaphor wouldn’t be any fun), so your only chance at survival is to go at least as fast as the train. Of course, if you simply let the train crash into you, you’ll then match its speed, but probably not in the way you would hope.
Instead, you reach into your pocket and pull out a trusty bouncy ball. Let’s imagine (again, to get this metaphor to work) that this is a perfect and indestructible bouncy ball. You launch the bouncy ball at the oncoming train. It bounces off the train. You catch it and start to roll forward, just a little bit. Repeating the exercise, you find through simple conservation of momentum that you’re able to steal some of the train’s energy and give it to yourself and your rolling platform. The train barely notices—it’s a train, after all—but you certainly do. If you get enough back-and-force done quickly enough, you’d find yourself smoothly moving down the tracks, avoiding disaster.
To return to our situation with the Earth and Jupiter, the metaphor works in the sense that we certainly want to avoid having Jupiter crash into our planet. But it breaks down because interplanetary bouncy balls aren’t exactly an option. So instead, we’ll have to resort to asteroids. We can send them on long orbits that loop around Jupiter, using their gravitational interactions to speed up the asteroid in exchange for a slight slowing of the giant planet’s motion. We can then return the asteroid to Earth, looping it in the opposite direction, slowing it down and giving us a boost.
The difference from a single pass will be barely measurable, let alone noticeable. It’s not like random floating space rocks can carry that much kinetic energy with them. But we just have to set it on repeat, looping over and over for hundreds of millions of years, nudging the Earth into ever higher orbits to escape the increasing ferocity of the Sun. If our descendants can manage it, it will keep our planet in the safe band of the habitable zone.
Stellar adjustment
If planetary rearrangement isn’t your bag, but you still have the capabilities to accomplish mega-engineering projects, I have another solution for you.
The main problem with the Sun is that helium is a natural byproduct of the fusion process that powers our star. The rate of hydrogen fusion is determined by the Sun's overall mass; bigger stars burn faster, and smaller stars burn slower. So if we want to limit the amount of helium production, we need to slow the fusion reactions. The most straightforward way to do that would be to decrease the Sun's overall mass.
Thankfully, the Sun is already doing that for us, just not fast enough. The surface of the Sun constantly emits a never-ending stream of tiny, charged particles, creating what we call the solar wind. In raw human-scale numbers, the amount of mass the Sun loses through the solar wind is incredible, roughly 1–2 million metric tons per second. All that fury adds up to one single Earth-mass every 150 million years.
We’re gonna need to bump that up a bit.
One way to do this is to simply heat up the Sun's surface, through lasers, particle beams, strong magnetic fields, or whatever mechanism our descendants choose. Heating up the surface would increase the amount of solar wind production, which would increase the rate of solar mass loss. But high-energy particles whizzing out of the Sun is generally counterproductive when it comes to keeping the Earth habitable, so the next challenge is to funnel those particles somewhere safe.
One way to do that is to create a series of particle accelerator stations in orbit around the Sun’s equator. They would constantly exchange charged particles, creating a ring of current as the Sun’s belt. That ring of current would create a toroidal (or, for the more Homeric physicists, donut-shaped) magnetic field, which would funnel the beefed up solar wind into polar outflows, out along the axis of rotation of the Sun and safely away from any planets.
That toroidal magnetic field could also be used to squeeze the star in a method known—and I’m not kidding about this—as the “huff-n-puff” technique. First you shut the stations down, allowing them to fall inward toward the Sun. Then you switch them on, allowing the magnetic field to halt and then reverse their descent. The close-in magnetic field squeezes on the equator of the Sun, forcing particles to eject out of the poles.
AdvertisementIf our descendants are really industrious, they can capture the escaped solar wind and use it for other purposes, like systems of fusion reactors to power the whole enterprise. And if they’re really creative, they can just point the solar wind outflows in one direction, using them as a Sun-powered rocket to nudge our entire Solar System to new places within the Milky Way or even out of the galaxy entirely.
Of course, this “starlifting” technique makes the Sun less luminous; with less mass, the fusion reactions operate at a quieter pace, which lowers the intensity and size of our star. This would shift the habitable zone inward. We wouldn’t notice at first because our actions would counteract the natural tendency for the habitable zone to move outward. But eventually, after the Sun had lost more than 10 to 20 percent of its mass (the math is a bit imprecise because it depends on how long this siphoning procedure takes), we would be forced to migrate the Earth inward to maintain the sweet spot.
But we’d be left with a smaller star, and smaller stars live blissfully long lives. The smallest red dwarfs, with masses barely bigger than a tenth of the mass of the Sun, can burn for trillions of years. But they also tend to be temperamental. With their smaller mass, they’re more susceptible to raging fits of starbursts that can see their luminosities sporadically double. If our far-future descendants decide to embark on this path of modifying the Sun to increase its longevity, they will certainly have their work cut out for them to protect the fragile Earth.
But no matter what, if humanity is to survive all these billions of years, we will likely be an interplanetary, if not interstellar, species. There won’t be much need to rescue the Earth in this manner. Perhaps our far-future descendants will still put a plan into motion as an act of reverence to preserve the world that gave rise to them. Perhaps it will be out of necessity, as no other world will ever be as suitable for life as Earth. Perhaps it will be an art project, a chance to create beauty and wonder on an interplanetary scale, before the fires of fusion extinguish within the core of the Sun and it breathes its last, the final chapter of a story that contains the billions of years of life in this Solar System coming to a close.
