The Trisolaran system of the Three-Body Problem could never evolve life
I have been reading the Three-Body Problem trilogy by Chinese science fiction author Cixin Liu which is soon to become a Netflix series. I want to talk about some of the physics in the first book. If you don’t want any spoilers for that book, do not read any further.
In the book, a fictional exchange takes place between a species dubbed the Trisolarans and Earth. One of the primary drivers for the book’s plot is that the Trisolarans live in a solar system with three suns. Orbital dynamics of three masses that are close in mass and distance from one another is chaotic. That means that these stars would not behave in any predictable way.
Isaac Newton famously struggled with the three body problem when trying to solve the equations for the Earth-Sun-Moon system. The difference in masses and distances, however, mean that system is not chaotic but stable. The Earth and Moon have very little gravitational effect on the Sun, so it is “mostly” a two body system. Newton developed perturbation theory to help correct some of the prediction to account for the influence of the Sun on the Earth-Moon system and the Moon on the Earth-Sun system.
Cixin Liu places the Trisolarans in the Alpha Centauri system, which is not only our closest star but also a tri-sun system. This is where fact gives way to fiction in the book. The Alpha Centauri system is not chaotic.
This is why:
The image above is not to scale. Alpha Centauri A and B are large and close together at about 11.2 to 35.6 Astronomical Units (AU) where one AU is the distance from Earth to the Sun. That means they orbit one another at about the distance of anywhere from Saturn to Pluto’s distance from the Sun depending on where they are in their elliptical orbit. Proxima Centauri, meanwhile, is 13,000 AU from the other two bodies. That is 0.2 lightyears away.
To put that into perspective, here are various distances in our own solar system:
This is a logarithmic scale so the actual distance represented by an inch on the image increases by 10x as you move from left to right. That’s the only way to fit all this on here. If the Sun is where Alpha Centauri A is, then Alpha Centauri B is somewhere between Saturn and Pluto/Neptune. Proxima Centauri would be in the Oort Cloud where comets live, outside the Heliopause, the end of the Solar Wind.
If I had a model of the Alpha Centauri system where Alpha Centauri A and B were one meter apart, Proxima would be about a kilometer away. On an American Football field, that means that if ACA and ACB are 10 centimeters apart at one endzone, Proxima would be at the other endzone.
We call this kind of trinary system a hierarchical system, and it is always stable because the outer star orbits the two inner stars as if they were one body. Hence, if there were intelligent life in the Alpha Centauri system, they would not experience the chaotic and stable eras that the book describes.
Still, perhaps Alpha Centauri is a bad choice, and we simply need to look farther away to find a star system like Trisolaris.
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