The Long Dark - building a habitable planet

Almost a year ago, I talked about the unusual world setting of The Long Dark. This is a world with a roughly 90 degree axial tilt - so it’s effectively spinning on its side compared to the plane of its orbit.

The most obvious characteristic of such a world is its extreme seasons. Other than a narrow strip around the equator, everywhere will experience midnight sun and some day-long darkness. The closer you get to the poles, the longer those periods of perpetual day and night become.

The strangest points on the surface are the poles themselves. At midsummer, the sun will be stationary, directly overhead. Expect it to get very hot! As the days proceed, the sun will start to move in small circles, gradually getting wider and wider and closer to the horizon. When you reach the equinox, the sun will hug the horizon, then dip below, and you then have half a year of complete darkness. This is “the Long Dark” of the book’s title.

This extreme light/dark cycle was really the foundation for the story, but there are other important features I wanted to bring together, which meant some research to build a credible and consistent world.

When I first came up with the idea, I pictured the world circling a white dwarf. I don’t know why, but that was my mental image. As I developed the setting, though, I realized I wanted this world to have a very long year. I wanted my colonists to have several Earth years of “summer” in one hemisphere, before they had to migrate across the equator and set up camp in the other hemisphere.

This meant it had to be orbiting far out compared to Earth’s orbit, yet still warm enough at that distance to support liquid water and life. A white dwarf was not going to give me the conditions I needed.

I had an idea for how to resolve this, but no idea whether it was workable. I was delighted when some reading from a number of sources suggested I was on the right track.

In a few billion years, our own sun is expected to go through a red giant phase. It will expand to swallow the orbits of the inner planets, maybe even Earth’s. This led me to two very important realizations:

First, in this phase the “Goldilocks zone” will push outwards to cover the orbits of Jupiter and Saturn. The “Goldilocks zone” is what astronomers refer to as the habitable zone, where liquid water should be able to exist on a planet’s surface. Right now, anywhere beyond Mars is too cold, but when the sun expands, the frozen outer reaches of the solar system will get a lot more toasty.

The second note is that if this is the evolution of our sun, then a red giant like this must be roughly the same mass. This means planets’ orbital periods will be comparable to those of our own solar system for a given distance out. And somewhere around the orbit of Saturn gave me the length of year that I was aiming for!

Right there, I had a long orbit that lay in the habitable zone. My white dwarf became a red giant.

I’m sure there are other scenarios that would give me the right combination of conditions, but this one simply resonated with me. It also provided other useful features that I’ll talk about in future posts.