The night sky seems peaceful and orderly. But in reality, stars are careening through the galaxy at
speeds of hundreds of thousands of kilometers per hour. Not bound by static formations but
changing neighborhoods constantly. Fortunately space is big, and so the stars of the Milky Way
are very unlikely to hit us. Unfortunately, they don’t have to hit anything to make us
have a really bad time on earth. And there are already stars starting to get very close.
To understand how dangerous stars are to us, we need to talk about gravity.
Gravity attracts every piece of matter to every other piece of matter in the universe.
You are attracted by an atom a million light years away and vice versa.
Luckily, this force gets weaker over distance and it also depends on how massive something is.
So things that are close and are very massive are more attractive, winning
the cosmic tug-of-war. This way, massive things define how smaller things behave around them.
The sun makes up 99.75% of all the mass in the solar system and so it shapes the behaviour and
orbits of everything else in it. Billions of years ago, after the sun was born the solar
system was a chaotic and dangerous place as the planets were formed from countless
little pieces that collided constantly. But over the eons, a stable balance emerged.
Today most planets and asteroids have settled into safe and predictable orbits.
We have the inner and outer planets, the asteroid and kuiper belt.
And at the edge, the Oort cloud, a giant sphere of comets orbiting slowly in cold storage.
We really don’t want this balance to be disturbed.
If another star came too close to us, it’s gravity would pull on everything in the solar system
like a spoiled toddler, messing up the pleasant order of the planets and asteroids and comets.
This isn’t some imaginary danger. Some 70,000 years ago,
a red dwarf, brown dwarf binary system, passed through the Oort cloud and messed things up.
It might even have sent a deadly onslaught of asteroids our way. But, it could take two
million years until those visitors from the Oort cloud arrive in the inner solar system.
But there’s a much bigger problem on the horizon: Gliese 710, a red dwarf with about
half the mass of the sun, is currently headed towards the solar system. In about a million
years it’ll pass through the Oort cloud and become the brightest star in the night sky.
A close flyby like this would unfold over hundreds of thousands of years,
disrupting the orbits of millions of objects in the Oort cloud considerably. If we are unlucky,
it will trigger a new period of planetary bombardment, similar to the early solar system.
The night sky could be filled with comets and asteroids raining down on the inner solar system.
The larger ones could cause Dinosaur level mass extinctions and would be bad for the stock market.
But it could get much worse. The galaxy is an intense place and stars get close to each
other regularly. So it is possible that a star could come much closer and not just
pass us but fly directly through the inner solar system. This would be very bad in the extreme.
The chance of another star colliding with the sun is astronomically unlikely,
but that isn’t what we’re worried about. If another star were to pass by about as
close as the earth is from the sun, it could easily eject the earth from the solar system.
The odds of such an event are estimated to be around 1/100,000
in the next five billion years. Small, but not absurdly so. As we discussed in another video,
there seem to be billions of rogue planets, doing their own thing in the galaxy and this is one way
to make them. So if this were to happen with an average red dwarf, what would happen on earth?
Kicking Earth out of the Solar System
As the star enters the solar system a small orangish dot appears in the sky that grows bigger
and brighter for months, eventually becoming visible during the day. It would get bigger and
much brighter than the moon. Too bright to look at directly. The night sky would be filled with
an eerie red glow. After a few months it would start shrinking again. But so would the sun.
Over a few years, the sun slowly grows smaller in the sky, and with it warmth and light start to
dissipate. All around the world, as the days turn dark, the final winter of humanity would begin.
The polar ice caps begin to grow and spread while plants shrivel and die. Forrests freeze and
animals die in droves. As the earth passes the orbit of Mars the average surface temperature
has plummeted to near -50 C. From space, earth begins to look like an icy moon,
the blue-green surface becoming the pale grey-white of death.
As global infrastructure breaks down, people huddle together indoors, burning what they
can for warmth as the temperature continues to drop, counting the days until they’ll be out of
food which no longer grows. Everybody living at the surface is living on borrowed time.
By the time earth reaches Jupiter’s orbit surface temperatures sink to -150 C, lower than the
coldest ever recorded temperatures in Antarctica. Needless to say by now almost everyone is dead.
Without the energy from sunlight to evaporate water, clouds don’t form and the water cycle
stops. The polar ice caps eventually touch at the equator, and the oceans become covered in a thick
layer of ice. As more and more of its heat leaks out more water freezes onto the bottom of the ice
sheet; the concentration of salt in the deep ocean grows, poisoning most animals that survived here.
Although around hydrothermal vents communities of extremophiles might adapt even to these
circumstances. Deep below the surface some bacteria would not notice much of any of this,
as they are still kept warm by the radioactive decay of elements in the earth’s core.
As the earth reaches the orbit of Pluto and the Kuiper belt the sun is still the
brightest star in the sky, but it is one among many, with stars now visible during the day.
The temperature is now barely 40 C above absolute zero,
below the freezing temperature of the gasses in the atmosphere.
A weird spectacle, enjoyed by no one unfortunately, unfolds as the atmosphere
turns into Nitrogen and then oxygen snow. Over a few years it is deposited into an icy 10 meter
thick sheet all over the planet’s surface, with only a thin whisper of gas remaining.
The frozen corpses of flora and fauna are buried beneath them.
As earth leaves the solar system it becomes a rogue planet.
Traveling alone through the dark. Lifeless and in solitude.
But weirdly enough, there is hope.
Humanity would not be surprised by this potential extinction event. We’d notice it
thousands of years in advance. There is not a lot we could do to stop a star. But we could prepare.
Most of us would perish. But a few million could survive in huge artificial complexes,
powered by geothermal and nuclear energy, possibly even fusion if we can learn to
use the ice around us for power. Here humanity might survive for hundreds of thousands of years.
At some point we would become used to our circumstances and new generations would watch
documentaries in disbelief, about the time we had our own star and could walk the surface of earth.
And at some point we might decide to look for another home. If the earth were lucky enough
to pass by another star with a habitable planet, we could try to make a fresh start. Spaceflight,
oddly enough, would become very easy without the atmosphere in the way. So it’s not unthinkable
that the last survivors would leave earth behind and try again on a new planet, around a new star.
Maybe one day, thousands of years later, the descendants of humanity will tell legends
about earth’s ancient past. Stories of our lost home. Of a mysterious
icy planet, floating alone and empty through the dark of space.