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.