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Here’s a funny question: How will the solar system die?
OK, so maybe it’s not that fun. But the science is at least interesting.
Most of the time when you read articles on this topic the answer you get is that the Sun will deplete its nuclear fuel, transform into a red giant, engulf Mercury, Venus, and Earth, blow its outer layers, and then in a white dwarf, cooling for eternity until it turns black and freezes almost to absolute zero.
Be careful, all of this is true, but that’s not really what happens to the solar system, just the Sun and the first three planets (including one in which we have a direct interest). But there are other things there as well, including Mars and four giant planets. They also matter. Heck, Jupiter on its own has more mass than everything else in the solar system (except the Sun, duh) combined, so its fate is very important.
What happens to everyone?
A recently published article examines exactly that. In general, and in the short term, the movements of the planets around the Sun are predictable. They follow the equations first established by Isaac Newton in the 17th century, and we still use these equations in the same way today.
However, in the long run, this will not work. If you have more than two bodies orbiting each other, the system usually gets chaotic after a long enough time. I don’t mean that stuff is flying everywhere; it is in the mathematical sense of chaos theory; that is, it is not possible to predict precisely where the planets will be at some point in the distant future because you cannot precisely measure their positions and movements now. Any random error, no matter how small, propagates through the equations, increases over time, and ultimately changes the configuration of the solar system in unpredictable ways.
To avoid this, you can compensate somewhat by including the uncertainties in your math and then running the equations multiple times, changing those values a little each time. The result is a bunch of different setups after a while, but then you can look at them statistically. For example, in how many simulations have Jupiter and Saturn interacted in such a way that Saturn was kicked out of the solar system? You can’t tell which SIM card is the right one, but you can get an idea of what will happen this way.
In the new journal, they went even further. On the one hand, they included the Sun losing mass as it transforms into a red giant. This is important, because in doing so, its gravity weakens and the orbits of the planets expand – they found that the planets from Mars to Neptune have their orbits grow by a factor of about 1.85 as the Sun loses. about half of its mass in the 7 billion years.
Additionally, they also included the chances of the stars in the galaxy getting close enough to the Sun to have an effect. The stars are small and far apart – the closest star to the Sun is over 40 trillion kilometers away – so encounters like this are rare.
But not non-existent. And if you run a simulation far enough into the future, a star sweeping through the solar system becomes inevitable. The scientists therefore performed their simulations in two parts. The first was in the sun losing its mass, and the second was the long period after. They included semi-random stellar encounters, using the actual galactic environment (number of stars per cubic light year and their motions) to simulate this.
They found that in phase 1 (before the sun swells) the planets are too close to the sun for this to have a significant effect. The stars would have to pass much closer even to strip Neptune, and an encounter like this occurs on thousand billion years time scale. Extremely unlikely.
But once the Sun is a white dwarf and the planets are further away, the odds increase. The Sun’s gravity is weaker, the planets are farther apart, and a chance stellar encounter has an easier time stripping the planets, throwing them into interstellar space.
They ran ten full simulations in this setup. It’s not a lot (usually in situations like this hundreds or even thousands are executed), but they got similar results whenever they felt confident in their conclusions.
Basically, they found that a star is likely to pass within a radius of about 75 billion kilometers every 10 billion years or so. It’s close enough to have an effect, and more encounters add up. In some sims, the outer planets were destabilized after about 45 billion years.
In all sims, Jupiter, Saturn, Uranus and Neptune are ejected after at most a trillion years. Not surprisingly, Jupiter is usually the last survivor; it is the closest, the most massive and the most difficult to expel.
On average, the first planet is lost after 30 billion years and the last after about 100 billion. Moreover, once the first planet is ejected, the system is destabilized enough that the next two follow within 5 billion years. The latter planets tend to linger for 50 billion longer, as there is no other planet left in the system to interact with and gravitationally push it.
I will note one important thing that they left out in their simulations: Mars. They note that this may be the last planet to survive, as it is the closest to the Sun and needs a very close stellar encounter to reject it. So if you are looking for a very Long-term real estate investment, the fourth rock of the sun – once it becomes the first and only rock – is the way to go.
In the article, they note that they do not include stellar encounters with binary stars, which are more effective at stinging the solar system, so the results they find are likely upper bounds on the duration of system life.
Additionally, the Milky Way will collide with the Andromeda galaxy in 4.6 billion years, while the Sun is still a relatively normal star, and they also ignored that. Encounters will likely happen more often when the number of stars in the resulting merged galaxy is twice as many as we currently do. Also, the collision will shake things up a lot, so it can all be moot anyway. The Sun can fall into the core of the galaxy where stars are plentiful and encounters are common, or be thrown into suburbs where encounters are rare. And all of this is long before the average stellar encounter affects their sims.
It is therefore clear that there is still work to be done here. But it’s a great step to get it all figured out.
I sometimes wonder, why am I so fascinated by this subject? I mean, I literally wrote an entire book about it. It’s more than just the morbid fascination with something like a horror movie, I think.
We consider the solar system to be immutable, but it is over a human lifetime. More than long periods, that changes a lot, and it is an upheaval of our complacency.
But more than that, there is a strange attraction to the idea of deep time, not just millions or even billions but trillion years, even eras, that make those numbers seem like a single tick of the clock. It’s a window into something most of us had never really considered before. What if we let the time go by really long? What happens then?
Well, the stars are running out. The planets are projected into space. Galaxies collide. A little happens, in fact.
The Universe is almost 14 billion years old, and we think it’s been a long time. But really, it’s just getting started.
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