We often think of the end of Earth as a matter of it getting engulfed by the sun during its
red giant phase.
But that’s not the only possible end for earth, there are three other ways that Earth
might either come to an end, or survive a fiery death.
First and foremost is the uncertainty surrounding the red giant phase itself.
There are uncertainties just how much the sun will expand, and while most models suggest
that earth will get engulfed, there’s a possibility that it won’t, in which case
earth might survive first as a molten cinder world orbiting very close to the surface of
the red giant sun, or as a frozen former cinder world forever orbiting a dead white dwarf
sun.
The next possibility is us.
It is physically possible to migrate a planet over a long period of time to follow the sun’s
expanding habitable zone.
This will need to happen far earlier than the red giant phase due to the increasing
luminosity of the sun, starting in a few hundred million years.
The migration would be accomplished by many repeated passes of a gravitational source
such as a series of asteroids to nudge earth’s orbit further out to compensate, and eventually
during the red giant phase itself into at least near the orbit of Jupiter if not further
depending again on the uncertainties about the sun.
But that only buys a short time because once the sun is finished with the red giant phase,
it becomes a white dwarf and the solar system as a result will dramatically cool.
There earth runs into problems as there may be no way to avoid a surface freeze, though
earth at that stage will still maintain some geothermal energy, one estimate on how long
it will take the earth’s core to cool put it on the order of 91 billion years from now,
long after the sun becomes a white dwarf.
Here it seems more likely that by that time in the far future, we’d have colonized other
star systems nearby, even if they don’t have suitable planets.
There are always habitats like O’Neill cylinders and it’s been suggested that space offers
certain advantages over planets.
All you need is raw materials, and you can get that from asteroids, and put all of your
manufacturing and agriculture on habitats.
That’s actually a somewhat neglected potential solution to the Fermi Paradox, that all alien
civilizations opt for space instead of their home planets and that we don’t really have
a great chance of detecting a group of rotating habitats at great interstellar distances,
especially if they’re not not emitting strongly radio, or do not pass in front of their star
from our perspective.
Whether we take advantage of passing stars to set up planetary colonies, or simply habitats
if there aren’t suitable worlds to terraform if raw materials are available or transported
from our dying star system, or we outright hollow out an asteroid and turn it into a
generational ship that we can use to cross to nearby star system, that may be simpler
than migrating earth while knowing that there will be hard end no matter what when the sun
reaches its white dwarf stage.
And there are two other problems with migrating earth around the solar system.
The first is the risk that you might miscalculate an encounter with an asteroid or comet, which
would need to be about 100 kilometers wide to be effective.
That’s almost certainly larger than the asteroid that killed the dinosaurs, so if
one of the gravity assist asteroids were to accidentally hit earth, then we’d almost
certainly go extinct.
But that’s not the only risk.
The second is even worse.
With some much gravitational maneuvering going on, there is a chance we could flat out lose
the moon.
That’s actually one of the worst things that could happen to us, because not only
would Earth’s climate be heavily disrupted, probably fatally so for us, it would also
severely affect the earth’s rotation and eventually its orientation.
Perhaps something more doable using this technique however might be in the cards far earlier
than the end of the solar system as we know it.
This would be to migrate Mars.
Here there is no substantial moon to lose, and second it doesn’t really matter if an
uninhabited Mars accidently gets hit by an asteroid.
If it’s only sparsely populated, it might be relatively straightforward to evacuate
that population as soon as we know we miscalculated on the asteroids.
This would give the benefit of a more habitable Mars, but at the same time terraforming seems
easier, but it’s actually seeming less easy than was once thought.
This stems from recent work on determining just how much carbon dioxide Mars has locked
up in its polar caps and such.
The easiest way to terraform Mars would be to melt the carbon dioxide and water ice and
get a greenhouse effect going.
Unfortunately, we now know that Mars simply doesn’t have enough carbon dioxide to do
it.
The means that any terraforming we do is going to require materials off planet at Mars,such
as huge amounts of CFCs or other heavy greenhouse gasses, and that may be forever infeasible.
Still, it’s possible that Mars may have subsurface deposits of carbon dioxide we’re
not aware of that might help make up for it, but there are still further problems.
One is that we’re not sure how much nitrogen Mars has, and for anything earthlike we’re
going to need it.
But we don’t see much.
But there’s also the option of intentionally bombarding Mars with comets that may allow
for long term terraforming, though clearly it wouldn’t be safe to live there for a
long period of time until after the bombardment was finished.
But there is another type of situation earth could find itself in sometime before the end
of the solar system that could change the equation, and our own equation, very seriously.
This goes back to stellar encounters which I talked about in a recent video, where the
motion of the stars brings stellar encounters relatively often across geologic time offering
a perfect opportunity during one of these encounters to colonize another star system
without having to travel vast distances across interstellar space.
But, there is also a way for this to go badly.
During a stellar encounter, there isn’t much of a chance of any of the planets being
affected because the encounters, while they happen, the last one was just 70,000 years
ago, they tend to be distant out in the oort cloud, and have produced only negligible effects
on the orbits of the planets.
But in a certain situation this may not be the case, and if a large enough star passed
close enough and slowly enough and repeatedly perturbed Earth, there is a small, but not
that small of a chance that Earth could achieve the sun’s escape velocity and get ejected
long before the sun’s red giant phase.
Indeed, it could conceivably happen sometime within the next few millions years if the
right circumstances are present.
Now there are currently no known future stellar encounters that could do this, but the further
out you try to project here the harder it is to know exactly what will happen in stellar
encounters, or even which ones will occur.
So we’re safe for now, but in the far future it could happen, though probably won’t.
But the chances are less than 1 in 100,000, so this very likely does occasionally happen
to an earth-like, and perhaps even inhabited planet out there somewhere in space.
So what would happen if earth got ejected from the solar system by a passing star?
It would become a rogue planet right before our very eyes, and there very likely would
be nothing we could do about it, even through trying to gravitationally migrate it.
There are several possibilities here, it’s possible that earth could get really unlucky
and fall into one of the gas giants, but if it did not it would get ejected from the solar
system.
If we were still here, this wouldn’t really be a fast process, but rather a terrifying
long term one with all sorts of climate related changes and instability.
But society would go on for a time and we could prepare some core surviving form of
civilization to survive indefinitely using geothermal energy.
This would not be a large population, the days of Earth sustaining billions of people
would be over, since one way or another we’re dependent on the sun.
But a core group could remain even as earth’s surface freezes solid and a permanent state
of night takes over.
But as long as geothermal energy exists on this world life could continue as a kind of
interstellar analogue of an ice shell moon, with hot spots and oceanic vents providing
an environment for at least microbial life to hang on, with human technology allowing
a surface presence of us, but living more like a true space colony than a species native
to this world.
At the same time though, there is a saving grace.
Time.
Any stellar encounter is going to be known about long before it occurs, which means you
can plan ahead for it.
Preparing Earth, and moving most of the population to space habitats could provide an effective
mitigation strategy.
Those left behind may not have a planet to live, but would benefit from both ample solar
energy, and the option to migrate the habitats outward much more easily than migrating a
planet as the sun’s luminosity increases.
That’s actually quite an interesting scenario in itself, where you have hundreds of cylinders,
each with thousands or more people living on them, slowly but surely migrating outward
over time, but with a bold group of volunteers electing to remain on spaceship earth when
it really does become a spaceship and head out into space as a rogue planet.
But there’s a further dimension.
Even if we’re long gone by the time an ejection of earth could happen, the story may not necessarily
end with Earth forever wandering the galaxy.
There is a non-zero chance, very low, like 1 in 300 billion, that after a long period
of wandering rogue planet earth could get captured by an entirely different star system.
If by some lucky chance life survived, even microbial, it might flourish again, adapt,
and live far longer than it would have in its solar system of origin.
The odds are terrible for this scenario, earth will most probably be destroyed by the sun,
but it’s fun to think about an earth living a new life as a rogue planet or even a member
of another star system entirely, and if it’s a red dwarf, it could be in that situation
orbiting a living star for potentially trillions of years.
What an odd state of affairs that would be, but I leave you one last thought.
The odds here are terrible, just to get ejected by a passing star is approaching one in between
15 and 100 thousand, and to get captured by another star the odds are even worse, but
the odds aren’t really so bad if you look at the whole of the observable universe.
We know earth-like planets exist, we also know life can exist in the universe, but we
don’t know how likely it is, but amongst the one billion trillion stars present in
the observable universe, then this sort of ejection scenario has probably happened to
someone, sometime, or at least a microbe planet.
At least the ejection part.
Getting captured is less likely, that has to happen under very specific special circumstances.
In our situation, it would be very unlikely for a rogue planet to manage to get captured
in a solar orbit around the sun, not impossible with Jupiter involved, but it’s an interesting
take on the dynamics possible in the universe.
Panspermia indeed, only this time not a rock, but an entire ecosystem could get transferred
somewhere else in the galaxy.