When it comes to planets grouped together, three's a crowd

Three’s a problem

One of the more poignant moments in the movie Star Wars, is when Luke Skywalker gazes wistfully into the sunset on the planet Tattoine.

What makes the moment more striking is Tattoine orbits a double star, so there are two stars embedded in the sunset. This piece of movie magic is dramatic but not scientifically realistic. Orbiting two stars as close as it appeared in that sequence, would render the planet uninhabitable.

When Isaac Newton came up with his idea of gravity, he ran into a problem. He could calculate precisely what is happening with two bodies orbiting around one another, but when he tried to calculate what would happen with three similarly sized bodies orbiting one another, he got stuck.

Since then, other mathematicians, including Euler and Lagrange had a go at the problem and got stuck too. The issue became known as the "Three Body Problem".

Now we have computers, we can use a different approach. We can just set up three bodies, assign them speeds and directions, and then see what their mutual gravitational attraction makes them do.

What we see is complete chaos. Nothing seems to follow the same orbital path twice. One of the bodies may be thrown out of the system altogether. In addition, changing the velocity of one of the bodies, even slightly, produces utterly different results. This chaotic behaviour is unlikely to produce worlds with life, because as the distances between bodies vary, temperatures will fluctuate wildly, so you would be deep-frozen or roasted, and if your world is thrown out of the system altogether, it is probably all over.

Scientists found that increasing the number of bodies makes the situation worse. That issue is now known as the “N-Body Problem,” where n is any number that is equal to or bigger than three.

That raises an intriguing puzzle. We live in a system comprising a star and several planets. Surely this is a typical n-body problem and assuming we survived it, we should see a whirling mass of chaotically moving bodies, not the nice orderly system with the Sun at the centre and all the planets moving in orderly, concentric, almost circular paths around it.

Why the difference? With all these bodies gravitationally tugging at each other, we should experience orbital chaos.

The reason our solar system has been stable for at least since life appeared on Earth 3.5 billion years ago is we have just one really massive object at the centre of things—the Sun, and several much less massive planets.

That means the strongest gravitational pull holding things together comes from the Sun. All the little gravitational tugs the planets inflict on each other are far smaller. The Sun keeps its family mostly under control. However, there is one potential troublemaker—the planet Jupiter, the largest and most massive planet in the Solar System. Its gravitational pull is strong enough to upset things.

Years ago, at one of the annual conferences of the Canadian Astronomical Society, a scientist from the Canadian Institute of Theoretical Astrophysics presented a report on his study of the long-term stability of the Solar System.

His calculations showed long periods of stability interrupted by episodes of instability. Now, years later, research suggests the Solar System was different in its youth, with some planets not being where they are now. Then things settled down, enabling Earth to present a stable environment for life to develop and flourish.

However, we know that periods of instability can be triggered by tiny events, just like shifting a pebble can start a landslide, or a loud shout an avalanche. No doubt, at some time in the future, some trivial event could upset our nice, stable Solar System.

Keep an eye on the sky.

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Before dawn, Saturn is visible low in the southeast and Venus low in the east. In the evening, Mars lies in the west. The Moon will be full on June 11.

This article is written by or on behalf of an outsourced columnist and does not necessarily reflect the views of Castanet.