For Mercury, the outcome is clear: doom. At 36 million miles from the Sun now, it is too far behind the other planets even when the starting gun goes off. After a few million years, the Sun will catch up and expand right past the planet. Mercury will literally be inside the Sun.
What happens to it then? Interestingly, the outer envelope of a red giant is almost a vacuum. The mass of the Sun is still roughly the same, but the volume increases hugely; when it becomes a red giant the Sun will have a million times the volume it does now, so its average density will drop by that amount. In reality the density in the outer layers is even less than that, because a lot of the mass of the star is stored in the core. In the end, the density is less than one one-thousandth of the density of the Earth’s atmosphere, almost a laboratory vacuum.
But there is matter there, thin as it may be. Mercury orbits the Sun once every eighty-eight days, so as far as Mercury is concerned it’ll be sweeping through stationary material. As it plows through this matter, what is essentially air resistance will slow its orbital motion in the same way a parachute slows down a skydiver. In just a few years, Mercury will slow so much that it will spiral into the center of the Sun, where the increasing density of matter will accelerate the tiny planet’s orbital decay. If it doesn’t vaporize first, Mercury will fall into the center of the Sun, where it will most certainly meet its doom.
Pfffsssssst!
Of course, if drag on the Sun’s matter slows Mercury down, the reverse is true as well: Mercury will accelerate the particles in the Sun’s outer layers. As Mercury slowly spirals into the Sun, it will speed up the Sun’s spin. It won’t be by much, just a percent or two. By the time the plunge into the heart of the star is over, the only indication that the solar system ever had a planet called Mercury will be a very slight increase in the Sun’s spin.
What of Venus? As it happens, our knowledge of how the Sun will expand into a red giant is still a bit too uncertain to know if Venus will evade getting eaten or not. Some models show it escaping, while others show it suffering the same fate as its little brother. Even if it does manage to stay outside the Sun’s greedily expanding surface, Venus is doomed. From just a few million miles away, the Sun will fill Venus’s sky. The surface of Venus is hot to start with—900 degrees Fahrenheit, thanks to its runaway greenhouse effect—but when the Sun looms so terribly over the Venusian surface, the temperature will scream upward to nearly that of the Sun itself. Venus’s crust will melt and its atmosphere will be blown away.
The Earth may fare somewhat better. Some studies show the Earth’s orbit expanding more quickly than the Sun, while others show us being consumed by the ever-growing star. Astronomers are still arguing over the details, which are important in this game of catch-me-if-you-can.83 Depending on the details of how the Sun expands and how much mass it loses, the Earth will end up being about 1.4 times farther from the Sun than it is now. The Earth is currently 93 million miles from the Sun, so when the Sun stops expanding that distance will increase to 130 million miles.84
Even if we escape being engulfed, don’t breathe a sigh of relief just yet: remember, the red-giant Sun is huge. It will fill a large fraction of the Earth’s sky, radiating down on it at a temperature of over 5,000 degrees Fahrenheit. The Earth’s surface temperature at that point will be about 2,500 degrees, hot enough to melt nearly every metal and rock on its surface. Even before the Sun swelled up the Earth was quite dead, its oceans having boiled off and the atmosphere ripped away. But during the Sun’s red-giant phase, the crust of the Earth will melt as well, and that, pretty much, will be that. While it’s not literally the end of the world, it’s certainly the end of the world as we know it.
We still have room for one more “however,” however. While the Earth will be totally stewed, it’s not the only usable planet in the solar system. Mars too will move out from the Sun, but, unfortunately, will also be too hot for life. Even Jupiter’s moons will warm up too much to sustain us (the average temperature will be something over 500 degrees, hotter than your kitchen oven when you bake cookies). Jupiter’s moon Europa is an icy body, and thought to have liquid water under the surface. When the Sun expands into a red giant, Europa might entirely vaporize.
It’s possible that no existing place in the solar system will be cool enough to support life as we know it. Even the distant moons of Uranus and Neptune will be too warm. You’d have to be about 4.5 billion miles away from the Sun to get temperatures near where they are on Earth today. Of all the places in the solar system, in six billion years only the (currently) icy bodies slowly orbiting the Sun well past Pluto’s orbit may be cool enough for us. They would melt all the way through, becoming essentially giant drops of water a hundred or so miles across, with a red, swollen Sun glaring down on them. It’s known that currently these objects are loaded with organic chemicals. When those icy bodies warm up, all sorts of interesting things could happen to those chemicals. The bodies will stay liquid for hundreds of millions of years while the Sun remains a red giant, which begs the question:
What life might evolve under those circumstances?
ASIDE: DAVID AND GOLIATH
At this point in the life of the solar system, things don’t look so good for the home team. The Sun is a swollen, distended blob, it’s eaten one planet, fried three others, vaporized a retinue of moons, and generally made things uncomfortable for almost everyone else.
But what are you gonna do?
Actually, that’s an excellent question. So far, this story has unfolded in this manner because we’ve let it. That is, if we sit back and watch, this is the way it will play out.
But it doesn’t have to be that way.
For example, it will take several hundred million years for the Sun to go from subgiant to giant. During that time, the temperature on Earth will be unbearable. And once the Sun does go all the way to giant, even Mars won’t look so good. But a billion years is a long, long time, and during that time Mars may be the place to be.
It’s smaller than the Earth, and has very little atmosphere. We can’t do much about its small size, but we can bring it air . . . by dropping bombs on it. Bombs in the form of comets.
Comets are large chunks of rock and ice, and some, in the distant outer solar system, are quite large, hundreds of miles across. They move so slowly in that far realm that it wouldn’t take much of a kick to drop a few into the inner solar system. Attaching a small rocket to one would do the trick. Letting smaller pieces hit Mars, one at a time, could easily bring enough water to fill ponds, lakes, and eventually oceans. Careful manipulation of its atmosphere, using genetically engineered plants and chemical processing, could encourage the development of breathable air. Some people estimate it might only take a century or two.
This type of practice, making a planet more Earthlike, is called terraforming. It’s a staple of science fiction, but it’s based on fact; the physics, chemistry, biology, and other fields of science involved are generally well understood. The devil’s in the details of course, but we have plenty of time to work them out. I’m guessing that in that dim future, billions of years down the road, a century or two here and there will hardly matter.
In fact, we’ll have the technology to start work like this on Mars much sooner than six billion years from now; realistically it could start early in the next century. Which raises the question: in six billion years, won’t we have terraformed all the planets? Perhaps. With an ever-burgeoning population, future humans will look across the gulf of space at all that real estate with envious eyes, and slowly and surely, as H. G. Wells once wrote, they will draw their plans against them.
Still—and stop me if you’ve heard this before—there’s no place like home. The Earth is a pretty good place, and we’ve spent a lot of time evolving here to make ourselves fit in. Is there no hope for our home planet?
Actually, yes, there is, and the solution is simple: we just need to move it farther from the Sun.
How hard can that be?
Okay, in p
ractice, pretty hard. The main problem is that the Earth is a big, massive object, so moving it takes a lot of energy.85 To move the Earth out to where the temperature will be more hospitable (around Saturn’s current orbit) takes roughly the same amount of energy that the entire Sun currently emits in a solid year. That’s the equivalent of exploding 200 quadrillion one-megaton nuclear bombs.
There might be some environmental effects from that.
There are alternatives. We could strap a few million rockets nose-down onto the Earth and fire them off, but it’s hard to know where we’d get enough fuel for them. Plus, the Earth’s rotation and revolution around the Sun would complicate things (we can assume, however, that by the time we need to do this our technology will be up to the task).
But there’s a better way, the environmental impact of which—if we’re careful—is essentially zero.
When we send probes to the outer planets, we can give them a boost in speed by “borrowing” (really, stealing) energy from the orbital motions of other planets they pass on the way. This is the so-called slingshot effect. If we want the probe to speed up, we send it on a path so that it comes in from “behind” a planet, catching up to it as the planet moves in its orbit around the Sun. As the probe passes the planet, it picks up some energy from the planet’s orbital motion, which increases the probe’s speed. The planet loses the same amount of energy, and slows down a bit in its orbit. Since a planet is typically a lot more massive than a space probe, it slows down very little, an immeasurable amount really, while the probe can gain quite a bit of speed. This means we can send probes to the outer planets without having to carry vast amounts of fuel.
Taking energy away from a planet will drop the planet ever so slightly closer to the Sun. But, if we do this in reverse—send the probe in “ahead” of the planet—then the probe loses energy, giving it up to the planet. The probe slows and drops closer to the Sun (useful for getting probes to the inner planets, such as Mercury) while the planet gains energy, moving outward from the Sun.
If we want to move the Earth farther out from the ever-increasingly sizzling Sun, this is an excellent way to do it. Instead of space probes, we can use asteroids, which are much more massive. That means the energy exchange is greater, requiring fewer slingshots. Moving asteroids isn’t all that difficult; in that case strapping a rocket onto one would work pretty well. By aiming it just so, the asteroid could give some of its orbital energy to the Earth, moving the Earth just a teeny bit outward from the Sun. Lather, rinse, repeat . . . a million times.
This scenario has been studied by the astronomers Donald Korycansky, Greg Laughlin, and Fred Adams, and they found that by using a large but typical asteroid, such a maneuver could feasibly move the Earth slowly out to a safe distance from the Sun.
Here’s how you do it. Start with a large rock about 60 miles across that is well out in the suburbs of the solar system. Change its orbit using a rocket or some other method so that it drops into the inner solar system. Aim it (here a rocket would be useful for fine-tuning) so that it passes in front of the Earth, missing us by about 6,000 miles.86 The exact amount of energy transfer depends on a lot of factors, such as the angle of the incoming rock, how close it passes, and so on, but in general a single passage of a rock this size would add about ten miles to the Earth’s orbital radius.
That’s not much, of course, but at first it doesn’t need to be much. Small steps are okay; we have plenty of time!
From this point we have two options. We could, for example, wait a few thousand years, find a second asteroid, and have another pass. But this is wasteful; for one thing there aren’t enough asteroids of this size in the solar system to do the trick. We’ll run out while the Earth is still too close to the Sun.
A second option is better: instead of simply throwing away the first asteroid, we recycle it. A little preplanning and care can save the day. Instead of letting the asteroid go away, we time the passage so that as it heads back out into deep space, it passes by either Jupiter or Saturn. This time, though, it passes behind the planet, gaining energy. Then the orbit can be adjusted again (using the onboard rocket; if it uses solar energy we even get our fuel for free) to pass by the Earth another time. If we do this, the asteroid becomes a sort of interplanetary orbital energy messenger, taking energy from Jupiter or Saturn and delivering it to Earth.
As Earth moves out, Jupiter will move in—remember, we’re stealing its energy—but Jupiter is so much more massive than the Earth (300 times, in fact) that it migrates far less than the Earth does. Moving the Earth outward far enough to keep it temperate while the Sun is in its subgiant phase (about 50 million miles or so) will require Jupiter to move only a few million miles inward (it is currently about 400 million miles from the Sun).
This will pose a problem when the Sun evolves into a red giant, however. The Earth will have to move out past where Jupiter is now. We could still steal from Jupiter’s orbital energy to do this, but once the two planets get near each other Jupiter’s gravity starts to affect the Earth directly. 87 Any encounter like that between the largest of the solar system’s planets and us is bound to have an unfortunate outcome: the most likely scenario is that the Earth gets ejected from the solar system altogether (see chapter 5).
It’s possible we could use a second set of asteroids to move Jupiter out farther from the Sun as well by stealing energy from Saturn, Uranus, and Neptune. At this point, though, the math gets pretty complicated, and results are difficult to pin down.
However, we have a few billion years to work out exactly how we’ll play musical planets. We’ve probably figured it out well enough for now. It’s a viable system, and one our descendants may very well have to employ.
THE HELIUM FLASH AND CORE HELIUM FUSION
Age of the Sun: 12.233—12.345 billion years
(Now + 7.633—7.745 billion years)
So now the calendar reads 7,633,000,000 AD (give or take a millennium), the Sun is a huge red giant, just reaching its maximum extent to a diameter of over 100 million miles, Mercury is gone, Venus may still be around but suffering, Earth is still here but possibly orbiting much farther out than it did when the Sun was middle-aged, and Pluto is a prime condo spot (complete with planet-spanning swimming pool). A time traveler from the twenty-first century would hardly recognize her neighborhood.
But we’re not done, not by quite a bit. The Sun won’t stay a red giant forever. And, as usual, the key to what’s happening lies deep in its heart.
The core of the Sun is now pure helium, and contracting. It’s degenerate, and heating up. The hydrogen around it is fusing into helium in a thin shell, adding more ash to the core. Remember too that since the core is degenerate, its pressure doesn’t change as mass is added. The temperature keeps going up, though.
At some point, something like 600 million years after beginning its transformation into a red giant, the core reaches a temperature of 100 million degrees Fahrenheit. Then all hell breaks loose. Well, to be more accurate, all hell is released, but it doesn’t break loose.
At that temperature, helium can fuse into carbon. Now, if the core were just a normal ball of gas heated to that temperature, the helium would fuse, heat would be released, and the gas would expand, adjusting its internal pressure to accommodate the extra heat—this is essentially what the core and outer layers of the Sun have been doing for billions of years, playing temperature, gravity, and pressure against one another.
But the core isn’t normal. It’s degenerate. It can’t adjust its pressure. So as the temperature increases, it cannot increase its size to compensate. Somewhere, deep in the core, the temperature reaches that critical point, and fusion of helium into carbon begins. This releases energy, which raises the core’s temperature.
This is bad. The fusion rate for helium is ridiculously sensitive to temperature. A slight increase in temperature and the fusion rate screams up, raising the temperature even more, again increasing the fusion rate. Within literally seconds this vicious circle ru
ns away, and the inside of the Sun’s helium core explodes like a bomb.
The energy release is difficult to exaggerate: it’s colossal, epic, titanic. In that one brief moment, called the helium flash, the core of the Sun releases as much energy as all the rest of the stars in the galaxy combined. It may actually release 100 billion times the Sun’s normal output, all in a few seconds.
You’d think this would tear the star apart in a supernova, but in fact, it doesn’t. It does a funny thing: because this is all happening deep inside the core, the matter itself absorbs all the released energy. This infusion of energy is enough to overcome the degeneracy of the core, which suddenly becomes normal matter once again. It’s under tremendous pressure, to be sure, but it’s no longer held in the sway of that weird quantum mechanical state. Once the degeneracy is released, the runaway fusion flash is dampened, and everything settles down into a nice steady state.
With that huge explosion safely absorbed, and the core back to being a regular old gas, helium fusion can proceed at a more leisurely pace. So now we have a core of helium fusing into carbon (there are also some minor avenues of fusion that are producing oxygen and neon as well), releasing heat. Outside this is still a thin shell of hydrogen fusion, and surrounding that, a hundred million miles deep, are the outer layers of the star.
Ironically, however, the amount of energy being generated in the core due to helium fusion is now less than was emitted when the core was degenerate and shrinking. This means less heat is being transported into those deep outer layers, which were before being supported by that extra heat. Once the core cools off, the outer layers respond by shrinking back down. On a relatively short time scale (about a million years), just as the red giant is reaching its maximum possible size, the legs are kicked out from under it. The Sun shrinks.
Death From the Skies!: These Are the Ways the World Will End... Page 21