The Cosmic Tug of war

The night sky seems peaceful and orderly, but in actuality, stars are racing through the galaxy at speeds of many thousands of kilometers per hour, not bound by static formations but changing neighborhoods constantly. Fortunately, space is significantly large, so the Milky Way Galaxy stars are unlikely to hit the Earth. Unfortunately, they do not have to hit anything to make us have a horrible time on Earth, and there are already stars rushing towards us in the future.

To realize how dangerous stars are to us, we should first understand gravity. Gravity attracts every piece of matter to every other piece of matter within the universe. You are attracted by an atom million light-years away, and vice versa. Luckily, this force gets weaker over distance, and it also depends on how massive something is. Things that are close and very massive are more attractive, winning the cosmic tug of war.

This way, massive things define how smaller items behave around them. The Sun makes up 99.75 percent of all the mass within the Solar System, so it shapes everything else’s behavior and orbits. Billions of years ago, after the Sun was born, the solar system was a chaotic and dangerous place because of the forming of the planets from countless little pieces that collided frequently, 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 large sphere of comets orbiting slowly in cold storage. We don’t want this balance to be disturbed. If another star came too near us, its gravity would pull on everything in the solar system like a spoiled toddler messing up the pleasant order of the planets, asteroids, and comets. This is not some imaginary danger.

Some 70 thousand 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. Still, it could take two million years until those visitors from the Oort cloud arrive into the inner solar system. Still, there is a much bigger problem on the horizon: Gliese 710, a red dwarf with about half the Sun’s mass, is currently headed towards the solar system. It will pass through the Oort cloud in a couple of million years and become the brightest star in the night sky.

A close flyby like this might unfold over many thousands of years, disrupting the orbits of millions of objects in the Oort cloud considerably. If we are unlucky, it will trigger a whole new planetary bombardment period, just like the early solar system. The night sky may well be stuffed with comets and asteroids drizzling on the inner solar system.

The larger ones could cause dinosaur level mass extinctions, but it could get much worse. A galaxy is an intense place, and stars get near one another 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 scenario will result in the worst situation for the Earth.

The Worst case scenario

The chance of another star colliding with the Sun is astronomically unlikely, but that is not what we are worried about. If another star were to pass by about as close as the Earth is from the Sun, it could quickly eject our planet from the solar system. The chances of such a scenario are estimated to be around one in 100 thousand in the next five billion years – small, but not absurdly so. You may not know, but these things happen in the galaxy very often. What would happen if our planet gets kicked out of its orbit because a star enters the solar system?

As it enters our solar system, a small dot, somewhat orange in color, will start to appear in the sky that grows bigger and bigger 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 full of an eerie red glow. After some 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 last winter of humanity would begin. The polar ice caps begin to grow and spread while plants wither and die. Forests will freeze, and animals die in droves as the Earth passes through the orbit of Mars. The average surface temperature has plummeted to close to negative 50 degrees Celsius.

From space, the Earth begins to look like an icy moon. The blue-green surface will become the pale grey-white of death. As global infrastructure breaks down, people huddle together indoors, burning what they can for warmth because the temperature continues to drop, counting the time until they will be out of food that 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 negative 150 degrees Celsius, lower than the coldest ever recorded temperatures in Antarctica. By now, almost everyone will be dead. Without the energy from sunlight to evaporate water, clouds don’t form, and also the water cycle stops. The polar ice caps eventually touch the equator, and therefore the oceans become covered with a thick layer of ice. As more and more of its heat leaks out, more water freezes onto the underside of the ice sheet, the concentration of salt within the deep ocean grows, poisoning most animals that survived there.

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 nuclear reaction of elements within the Earth’s core. As the Earth reaches the orbit of Pluto and the Kuiper belt, the Sun continues to be 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 degrees Celsius above absolute zero below the frozen surface of the atmosphere’s gases. Unfortunately, a weird spectacle enjoyed by nobody unfolds as the atmosphere turns into nitrogen and then oxygen snow.

Over some years, it is deposited into an icy 10-meter thick sheet everywhere on the planet’s surface, with only a small whisper of gas remaining. The frozen corpses of flora and fauna are buried beneath them as Earth leaves the solar system and 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 would notice it thousands of years in advance, there is not a lot we could do to stop a star from coming in, but we could prepare. Most people would perish, but some million could survive in huge, artificial complexes powered by geothermal and atomic energy, possibly even fusion if we can learn to use the ice around us for power. Here humanity might survive for many 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 on the surface of Earth. At some point, we might commit to exploring for another home.

If our planet is lucky enough to pass by another star with a habitable planet, we could try and make a fresh start there. The space flight, interestingly enough, would become very easy without the atmosphere in the way. So it is not unthinkable that the last survivors would go away, leaving the Earth behind, and take a look at a new planet around a brand new star. Maybe in the future, thousands of years later, humanity’s descendants 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.

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Science Fiction
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