Why the Solar System is not going into a spin

The Solar System, with its eight planets, and Pluto.

And yet they turn! Inspired by the legendary phrase of Galileo for whom it is indeed the Earth that revolves around the Sun and not the contrary, this modified tirade could summarize the new discovery of a trio of astronomers from the Paris Observatory-PSL published in Physical Review X, May 3. Federico Mogavero, Nam H. Hoang and Jacques Laskar finally understood why our eight planets and our Sun did not turn into a gigantic billiard table, with bodies colliding with each other. In other words, why this beautiful arrangement seems so stable for at least three billion years, despite the gravitational interactions between all these “balls”.

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This question arises, in fact, since Newton, who was the first to describe the forces acting between these bodies and predict the elliptical trajectories of the planets around the Sun. He thinks that the disturbances between planets exist, but that God, from time to time, restores a little order to avoid the big upheaval.

At the beginning of the XXe century, the mathematician Henri Poincaré demonstrates that, as soon as more than three bodies are involved, the System is, in fact, chaotic, that is to say that trajectory forecasts become impossible; some diverging dramatically from the elliptical orbits. But, for a long time, astronomers reured themselves, believing that the time after which this chaos would appear would be very long. Error ! In 1989, Jacques Laskar demonstrates that divergences manifest themselves after… only 5 million years. He even specifies that any trajectory prediction beyond 60 million years is impossible. Conversely, tracing the positions of the planets more than 60 million years back is also illusory…

Objects staggering at the edge of a cliff

In 2009, the astronomer calculated that the smallest “ball” in billiardsMercury, would have 1% risk of colliding with Venus in the next 5 billion years, the estimated lifespan of our star. The fault of Mercury entering into synchronization or resonance with the movement of the giant planet Jupiter, which leads to the transfer of energy from the large to the small, until it deviates from its orbit.

“The question that remained was why this probability of collision was so low, given the duration of 5 million years previously calculated”, recalls Jacques Laskar, who has therefore just proposed an explanation with his colleagues. To this end, a million hours of calculation and extended time horizons of up to 100 billion years, over thousands of trajectories, were necessary.

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