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Building the ultimate solar system

How do you squeeze 60 Earth-like planets into a single solar system? Take one scientist and apply some heavy-duty planetary physics, reports Dr Karl.

Right now, the human race has living room only on our little planet, Earth — if you ignore the half-dozen people on the International Space Station. But let's imagine the ultimate solar system — with 60 Earth-like planets.

To build our ultimate solar system we have to assume that the future human race will be able to control massive energies. Right now this is science fiction, but looking thousands of years into the future, who knows?

Despite the challenges Sean Raymond, an astrophysicist from Bordeaux Observatory in France, set out to build our ultimate solar system as a computer game he called Fantasy Star System. Because he's a physicist, this solar system with 60 habitable Earth-like planets had to follow the laws of physics. So his arrangement of planets had to be both scientifically plausible in the short term, and gravitationally stable over the long term of billions of years. After all, he wanted life to be able to evolve.

If you ever read the fairytale of Goldilocks And The Three Bears, you will remember that one porridge was too hot, another was too cold, but one was just right.

So the astronomers (who all read their nursery rhymes as children) describe our planet Earth as being in the so-called 'Goldilocks zone'. It's not too hot like Venus, and it's not too cold like Mars — it's just right. Inside the Goldilocks zone, water can exist as liquid water, not just as freezing ice or scalding steam.

For his 'Ultimate Solar System 1', Sean Raymond chose a smaller star than ours — one called a red dwarf. Because it had a lower mass, it would live a lot longer than our star, giving life a longer time to evolve. Of course, it wouldn't be as bright, but you just shift your planets in a bit closer to the red dwarf.

He then worked out how to have 24 habitable Earth-like planets orbiting this single red dwarf star. The first trick is to know (from our knowledge of Lagrangian points, look it up), that you can have two similar mass bodies in the same orbit around a star — so long as they are 60 degrees apart. Yup, two planets in the same orbit can be perfectly stable.

The second trick is to realise that one Earth-sized planet can have an almost Earth-sized body going around it. In fact, these locked planets would circle their common centre of gravity. This gives you two pairs, or four planets per orbit. The third trick is to know that the Goldilocks zone is wide enough to carry six of these orbits and would be stable over billions of years.

Suddenly you have created 'Ultimate Solar System 1', with 24 potentially habitable planets. There are six orbits, with four Earth-like planets in each orbit.

But then Sean Raymond let his astrophysicist imagination go crazy.

In our solar system, the giant planet, Jupiter, has some quite large moons orbiting it. How many Earth-sized planets could orbit a Jupiter, he asked himself.

The answer turned out to five, in orbits that would be stable for billions of years. And then, using our new best friend forever, Lagrangian points, he realised that he could stuff in a few more Earth-sized planets.

One would be leading 60 degrees ahead of the Jupiter-with-five-orbiting-Earths, and the other would be trailing 60 degrees behind in the same orbit. And when he did the maths, it turned out that it would all be gravitationally stable, even when he changed the leading and trailing Earths into double-Earths. That gave a total of nine Earths in that orbit — a Jupiter-with-five-orbiting-Earths, two Earths circling each other at 60 degrees in front of the Jupiter and another two Earths circling each other at 60 degrees behind the Jupiter.

But could he stuff six orbits inside the Goldilocks zone? Unfortunately no — because Jupiters are big, and need lots of room. But he could shove four orbits into Goldilocks land. Four orbits multiplied by nine Earths per orbit works out to 36 Earth-like planets around a single red dwarf. That was Ultimate Solar System 2.

Which one should Sean Raymond go for? Twenty four planets or 36. The answer was obvious. Both.

It turns out that about 40 per cent of the stars in our galaxy are in binary systems — two stars circling each other.

All Sean Raymond had to do was place or locate his two ultimate solar systems away from each other by about 2.5 times the distance from Earth to Pluto — and they would be stable. Voila! Sixty potentially habitable planets in a single solar system.

But how would he actually build it? Well, I'll talk more about that, next time…

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