Welcome to the Building the Ultimate Solar System series about building planetary systems with as many life-bearing worlds as possible. This post presents a new “ninja move” to compactify planetary system and build systems with more planets in the habitable zone.
Tell the truth. When you saw the word retrograde in the title of this post, did you think I would be writing about “retro” planets with funny hairstyles or wearing disco clothes? That would be awesome, wouldn’t it? Maybe next time…
I stumbled upon a simple way to tighten up the Ultimate Solar System. And by stumbled upon, I mean I discovered that someone else had figured it out. And I took it.
It is a little technical, so let’s get to the basics. Two planets orbit the same star. If the orbits are far apart, the setup is stable because the planets don’t feel each other’s gravity too strongly. If the orbits are too close together, the planets give each other little gravitational kicks that add up. Over time, these kicks change the shapes of the planets’ orbits. Eventually the planets’ orbits cross and the two planets either collide or at least cause major re-arrangement of the system’s orbits. Not stable.
There is a stability limit. Two orbits closer than the stability limit are unstable. A central goal of Building the Ultimate Solar System is to create planetary systems that are just a little bit wider than the stability limit. This maximizes the number of planets that can fit into a given area. The area we care about is the habitable zone, where planets can have liquid water on their surfaces.
But there’s a twist. It turns out that the stability limit only applies to normal planetary systems. Systems in which all the planets orbit the star in the same direction.
In a great paper from 2009, Smith and Lissauer show that there is a different stability limit for systems in which half of the planets orbit the star in the opposite direction. In that case, the stability limit is closer, so planetary systems can be more compact.
It makes a big difference. You can fit about twice as many planets into a given stretch of orbital real estate. The requirement is simply that every other planet must orbit in the opposite direction. So, planets 1, 3, 5, and 7 orbit the star in a clockwise direction, and planets 2, 4, 6, and 8 orbit counter-clockwise.
Take Earth-mass planets orbiting a star like the Sun. On prograde orbits, 4 Earths fit within the habitable zone. For alternating prograde and retrograde orbits, 8 Earths fit.
The planets are all half an Earth-mass, and there are 6 orbits packed into the habitable zone. Each orbit has two sets of binary Earths, separated by 60 degrees on the same orbit. This setup is stable for billions of years, and puts 24 habitable worlds in the habitable zone of a single star. Not too shabby.
[We’re not going to mess with Ultimate Solar System 2 because the orbital spacing of that one is built on resonances (different story).]
Let’s tweak Ultimate Solar System 1 based on what we learned about retrograde orbits.
It’s simple. In between each set of orbits from Ultimate Solar System 1 we can insert another orbit as long as it goes around the star in the opposite direction. And there’s no reason we can’t put the same pair of binary Earths on each retrograde orbit. Our Ultimate Retrograde Solar System looks like this:
We basically just took two copies of Ultimate Solar System 1, flipped one of them onto retrograde orbits, and enmeshed the two together. Now there are 48 planets in the habitable zone instead of just 24! Boom!
The only downside of our retrograde setup is philosophical. Up to this point, everything in the Ultimate Solar System happens on its own naturally. Tightly-packed planetary systems exist. Gas giant planets really have large moons, as well as Trojan companions (well, asteroids although we think that Trojan planets must exist). And we know of complicated groups of many stars bound together. Of course, it’s unlikely for all of these things to happen at the same time in an optimal way, but not impossible.
With the Retrograde Ultimate Solar System we are now swimming in impossible waters. Two planets can end up orbiting the same star in opposite directions, but only if their orbits are widely separated. I don’t know of any way that nature could produce a system of tightly-packed planets with each set of planets orbiting in the exact opposite direction of its immediate neighbors.
This means that the Ultimate Retrograde Solar System would have to be engineered. Created on purpose by some very intelligent and powerful beings.
And if these beings were engineering a Solar System, they might take things even further. The next Ultimate Solar System post will show just how deep this rabbit hole goes…. (it’s way way deeper)