The Kissing Sisters

You can definitely write a gripping story set in a relatively ordinary planetary system. Put in a couple of rocky planets, some gas or ice giants with exotic moons, an asteroid belt for good measure. Throw in interesting cultures and intriguing characters and off you go. You can even write a mind-blowing story around our good old Sol—look at The Expanse!

But why stop there?

For over a decade, I have been fascinated by the notion of two worlds sharing the same orbit. Their mutual physical interference, the cultural interaction of the settlements, the cross-world travel challenges… So much potential.

Lagrange is boring

What would prevent several planetary bodies from sharing an orbit? Well, physics certainly wouldn’t. Newton doesn’t care. And our own system shows several examples. Jupiter pushes and pulls swarms of Trojan asteroids in the L4 and L5 Lagrange points of its own ellipse around the Sun. Hell, even Earth has such co-orbiting companions: 2010 TK7 and 2020 XL5.

But Trojans are boring. They always stay 60 degrees away on their path, never getting nearer or farther for a proper story-worthy setup. And their mass must be negligible for the arrangement to work, otherwise the whole thing falls apart into the chaos of a three-body interaction. Well, okay, there might be a story there, but I think someone has already looked into that…

Throw in the horseshoe

I wanted to tell a story of two proper-sized co-orbiting planets with cultures and peoples and cities and whatnot. The inspiration came from Saturn. Best known for its spectacular rings, less known for Janus and Epimetheus.

These two guys are small, irregularly shaped moons circling their home planet at about 150,000 km. Technically, their orbits differ very slightly—by about 50 km. A tiny difference, smaller than each of the two moons, but a difference nevertheless. Enough to make one of the moons (the closer one) orbit at a slightly higher angular velocity than the other.

Eventually, the closer moon catches up with the farther one from behind, but instead of slamming into its brother, they start tugging on each other with their tiny gravities. The closer one gets pulled a little farther away from Saturn, and vice versa. With the total angular momentum conserved, they effectively swap their orbits, and the fast racer starts lagging behind, only to become the one caught from behind four years later. Rinse and repeat.

1The inner moon runs faster and catches the outer one from behind.

2At their closest, gravity swaps their orbits. They never touch.

3The chaser is now the slower, outer moon, and falls behind. Four years on, it repeats.

This is called a horseshoe orbit, because from a vantage point planted on one of the moons, the other one does this funny dance: catches up, moves a bit away from Saturn, then seems to fly away in the opposite direction to reappear from the other side, move closer to the planet just before it would collide with you, and rush off again. A nice, closed loop, like the outline of a round, smooth letter C.

Why not planets

Humanity has catalogued thousands of exoplanets but has yet to find a pair that behave like Janus and Epimetheus. But the science is sound and it is definitely doable. Maybe there is a world where a cataclysmic event of early planetary formation split one body into two pieces and, instead of creating an Earth–Moon analog, left two co-orbiting worlds.

That is the arrangement I chose for Through. Two planets, 1 AU on average from their mother star, catching one another every 86.5 years. Once that idea formed in my head, I saw the worldbuilding unfold. A once-in-a-lifetime Sisters’ Kiss that reshapes the heavens, with big cultural and logistical implications. Two worlds so close and yet so different: Big Sister as close to an Earth twin as can be; Little Sister smaller, locked in resonance with Mother—a world whose nights last an entire year…