How Stable Is the Solar System?
Chaos once ruled the planets. Might it do so again?
Ask any favour, Helios promised, and I will grant it. He soon regretted his rash oath: for Helios was tasked with driving the Sun across the sky, and Phaethon - his son - demanded the right to do the same. In vain he sought to recant his promise; even Zeus, he argued, would not dare traverse the turmoil of the rushing sky.
Yet Phaethon was determined, and eventually Helios relented. At the hour of dawn he led his headstrong son to the golden chariot of Vulcan, placed the reins of the four horses in his hands and bade him farewell. Phaethon departed in joy; but things soon took a turn for the worse.
The horses, sensing a different master, ran wild; careening between the stars. In fear Phaeton dropped his reins and watched in horror as the Earth swung wildly below him. For a moment the Sun drew too close, burning cities and mountains and drying the great deserts of Africa.
Watching the devastation, Zeus felt compelled to intervene. From the highest peak of heaven he summoned the power of the Gods and hurled a spear of lightning towards Phaethon. The chariot was destroyed; the horses scattered across the skies. Phaethon plunged Earthwards, leaving a trail of smoke in his wake.
Thus the Greeks explained the lands they found around their own: the charred deserts of Africa and Arabia; the frozen Poles. Yet the legend of Phaethon hints at a darker narrative: of an earlier instability in the heavens themselves; of a time when the Sun was much closer and hotter, when chaos ruled and devastation followed.
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Today that idea looks fantastical. The celestial bodies - the Sun and Moon, the planets and stars - move in regular, well-defined orbits. The prospects of the Sun coming closer, or the Moon moving further away, belong to the realm of fiction. Yet the evidence suggests this stability is somewhat of an illusion.
Perhaps the earliest sign of this came in the Victorian era, when scholars discovered the asteroid belt. This, a zone of fragmented rock lying between Mars and Jupiter, forms a kind of ring around our star. For decades the belt puzzled astronomers. Why, they wondered, had it not coalesced into a planet; thus creating another rocky world like Mars or the Earth?
Some hypothesised that it had. Referencing the ancient legends, they imagined a lost planet had indeed once stood there, a world they named Phaethon. For years they argued about what had happened to it. Did a tremendous collision rip it apart, or had Jupiter, in a fit of rage, shredded it and fed upon the remains?
Whatever it was, the loss of Phaethon hinted at a turbulent past hiding under the Solar System’s modern respectability. And indeed, before long researchers found other clues pointing to the same conclusion. Perhaps the most obvious are deep scars on the Moon: craters lingering from a sudden and intense bombardment of asteroids.
Observations of other star systems, too, showed ours to be an outlier. The giant planets of our system sit too far out; in others they drift inwards, leaving chaos in their wake. Had our solar system been more like the rest, the Earth should long ago have been destroyed, sent plunging into the Sun by the force of Jupiter.
In an effort to explain these mysteries, a team at the Observatoire de la Côte d’Azur, an institute in Nice, France, built a computer model. It was programmed to simulate the orbits of the planets over hundreds of millions of years, tracking their movements through the Solar System.
By running the model over and over again with different inputs - the initial sizes, positions and speeds of the planets – the astronomers could simulate thousands of possible histories of the Solar System. Not all of those models, of course, produce a solar system that looks like ours does now: in some the planets end up in a different order, or scattered in wild orbits.
The team thus filtered the results to select only those that produced an outcome that resembles the modern solar system. That allowed them to trace the likely past movements of the planets.
The findings came as something of a surprise. The Solar System, they suggest, used to be much smaller. The outer gas giants probably formed much closer to the Sun than they are now. Beyond them, stretching far around the planets, once lay a vast disk of rock and ice; a cloud of asteroids and comets far larger than anything that remains today.
Over millennia, the gravity of the gas giants slowly pulled this rock and ice inwards. Each encounter between the planets and the cloud pulled the gas giants outwards; a process that gradually expanded the solar system. This long shift took place over hundreds of millions of years until – quite suddenly – Jupiter and Saturn fell into a special arrangement known as a resonance.
The resonance occurred when Saturn’s orbit was precisely twice that of Jupiter’s: that’s to say, every time Saturn orbited the Sun, Jupiter would have done so twice. That created a pattern of repeating gravitational encounters, a phenomenon that rapidly unsettled the Solar System and threw it into chaos.
Saturn was probably thrown outwards, hurled towards Uranus and Neptune. Its sudden arrival in the outer solar system disrupted those two planets as well: both hurtled outwards, smashing through the debris surrounding the Solar System. In the tumult the two planets may even have switched order, with Neptune thrown far into the frozen darkness.
Amid this chaos, asteroids and comets would have surged through the Solar System, raining down on the inner planets. The model predicts a sudden and catastrophic burst of asteroid impacts on the Earth and Moon: an event that surely melted continents and boiled oceans. Life – which we know had already emerged – faced a fight for its very survival.
The asteroid belt, far from being a smashed planet, probably emerged from this chaotic period. Not all the debris that fell upon the inner solar system ended up hitting the planets. Most of it likely remains, shepherded by Jupiter into an orderly ring around the Sun. It lingers there today – a reminder of a long ago time when life itself was imperilled by the gas giants.
Intriguingly, some simulations suggested a slightly different series of events. Had one of the ice giants – perhaps Neptune – come close to Saturn, it too could have triggered a sudden bout of chaos. But in this scenario the ice giant rarely survives the encounter: it is flung outwards at such speeds that it leaves the Solar System forever, abandoned in the void.
Since Neptune is still with us, researchers initially dismissed these results. But what if the ice giant was not Neptune? What if our Solar System once contained an extra planet, another frozen gas giant? And what if it was this world, and not Neptune, that was lost in a burst in chaos and instability? Models suggest the possibility is unlikely – but not impossible. Earth may indeed have a long lost sibling out there somewhere; a planet drifting alone in the vastness of space.
If such chaos once devastated our Solar System, could it ever happen again? To find out, astronomers have tried running the same kind of simulations forward: looking at what might happen, instead of divining what already has.
In principle such calculations should be easy. The motion of the Earth – and that of the other planets, moons, asteroids and comets that comprise the Solar System – is simple to compute. The Sun is the overwhelming giant of the system, making up 99.8% of its mass. Its gravity dominates the motion of the planets, herding them into predictable orbits governed by simple laws of physics.
That system is fairly stable – as its continued existence after four billion years attests. But there are other, more subtle, influences acting on the planets. First is the tug exerted by the planets themselves. Jupiter – the largest planet – makes its presence felt most strongly, but all the planets pull, to some extent, on all the others.
Second is the influence of the assorted debris – the comets, asteroids and other rocks – that litter the solar system. This, of course, is small – but over millions and billions of years even the smallest effects can add up.
The third influence comes from outside. Over long periods of time the stars move, drawing closer and further from our Sun. Right now the closest known star lies roughly four light years away – meaning its influence on the planets is miniscule. But every now and then one makes a far closer approach: in roughly a million years Gliese 710, a small orange dwarf, will pass with a sixth of a light year of Earth.
Such close encounters have the potential to disturb the orderly flow of the planets – or, at the very least, the outer cloud of comets and asteroids that still surrounds the solar system. Analysis of passing comets suggests that many of them were indeed pushed inwards by such encounters. Over time these small effects may disturb the planets slightly, altering their course by just enough to trigger chaos a billion years from now.
There is also the question of what we don’t know. To accurately predict the movements of planets over long time periods, we need to measure the positions, speeds and masses of everything in the solar system to extraordinary precision. That, simply put, is far beyond our capabilities. We would need to know the mass of leaves growing in the summer, the weight of ice melting in the Arctic, the precise distribution of rock and magma in the heart of each planet – more, in short, than we can ever know.
This is an example of the butterfly effect: the idea that small shifts can result in enormous and unpredictable changes over time. Such systems are unpredictable even in theory; the initial conditions can never be measured accurately enough to foretell the future.
Even if we could, there is another stumbling block: the famous three-body problem. For two objects - say a planet and a star - moving under gravity, physics can quite easily produce an equation that will tell exactly where both will be at any given time.
But if you add a third object, the situation suddenly becomes more complicated. That simple equation no longer applies - and, in most cases, no other can be created. Instead physicists must painstakingly simulate the exact motion of each object over time.
In other words, if you want to know where the three bodies will be in several million years time, you must first calculate where they will be at every moment between then and now. There are no simple shortcuts here: the laws of physics dictate that the task requires immense computation.
Our solar system is no exception to this rule. Its eight major planets and one star have no simple equation guiding their motion. Add in the asteroids, comets, passing stars and the leaves of summer, and the result is an unpredictable mess: we neither know precisely where we are, nor where we are going.
The only solution is to make a guess about the current state, run a long simulation forward and see what happens. And then do that again, making a slightly different assumption about our position. And then again, for as many subtly different starting points as you can manage. The end result is not so much a definite answer, but rather a set of probabilities. A map of the likely outcomes; a sense of what may come to pass.
So what does this map look like? First of all, and rather reassuringly, it appears quite likely the Solar System will survive, more or less intact, for several billion years to come. Then, as the Sun exhausts its fuel and begins to die, it will expand, enveloping the inner planets and slowly consuming them. The outer planets will linger on, circling the dying star as it fades away for countless years longer.
Yet, there are hints of possible instability. The most likely scenario involves Mercury, the small innermost planet of the Solar System. It happens to lie close to a resonance with Jupiter – the same kind of event that caused chaos billions of years ago. In roughly one percent of models, this resonance gradually pulls Mercury into a wider and wider orbit, until – some three billion years from now – disaster strikes.
In some scenarios Mercury smashes into Venus or plunges into the heart of the Sun, creating spectacular explosions, but leaving the Earth mostly intact. Yet in the worst cases, the disruption sends Mars hurtling towards Earth, either skimming past or directly onto a collision course. In either scenario, life as we know it would face certain destruction.
Luckily, we’d have plenty of warning. The fate of the Solar System can be predicted with decent accuracy for millions of years ahead. If Mercury ever looks likely to cause chaos, we’ll know about it well in advance. And that, hopefully, would give us time to do something about it.
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