Several years ago, I became obsessed with tidally locked planets. The notion of a world permanently caught between two extremes—with one half always illuminated, the other always in the dark—took hold of my imagination. I realized that planets like these were the surest bet in the search for Earth-like places that our descendants could settle on. Worlds of eternal darkness and never-ending sunlight could be the future of the human race—if we’re serious about living in other solar systems.
Astronomers believe that most of the planets in our galaxy that have Earth-like temperatures are likely to be tidally locked. Because their orbital period is the same as their period of rotation, these planets will always present the same face to their sun—just as we always see the same side of the moon, as it orbits Earth.
And the reason for this glut of tidally locked worlds is pretty simple. Up to three-quarters of suns in our galaxy are red dwarfs, or “M-dwarfs,” smaller and cooler than our sun. Any planet orbiting one of these M-dwarfs would need to be much closer to its star to support human life—as close as Mercury is to our sun. And at that distance, the star’s gravity would pull it into a tidally locked orbit.
For example, astronomers recently discovered seven Earth-size planets in the habitable zone of the TRAPPIST-1 system, all of which are likely to be tidally locked.
My obsession with these planets led to my new novel, The City in the Middle of the Night. To picture all their strange geological features and weird knock-on effects, I talked to Lindy Elkins-Tanton, the director of the School of Earth and Space Exploration at Arizona State University, as well as other scientists studying them, and I read as much of the latest research as I could. More than anything else, I became captivated by trying to imagine what it would be like for people living on a planet where the sky never changes.
For now, talking about these planets means indulging in speculation—which is the perfect situation for a science-fiction writer. But we are learning enough about the dynamics of tidally locked worlds to start to understand how they would work, and what kind of civilization we could build there.
The first question: Where would humans settle on a tidally locked planet? When I started working on my book, the clearest answer appeared to be the terminator, the strip of twilight between the dayside and the nightside. “That might be the Goldilocks zone,” neither too hot nor too cold, but stuck “between eternal dusk and eternal dawn,” says Daniel Angerhausen, an astrophysicist at the Center for Space and Habitability at Bern University.
In the terminator zone, Angerhausen suggests, humans might be able to generate geothermal energy, using cold water from the nightside and hot water from the dayside in “some kind of thermal reactor.”
To have access to liquid water on a tidally locked world, you need a system to cool down the dayside and heat up the nightside, says Ludmila Carone of the Max Planck Institute for Astronomy. Otherwise, all the liquid might become tied up in ice on the nightside, or worse yet, the atmosphere itself could get frozen in the dark.
“The habitability of these planets hinges very strongly on how well you can transport heat,” Carone says. Her computer models show that a tidally locked planet might have two strong wind jets, one in each hemisphere, that might act a bit like the jet stream here on Earth. But if the planet is too close to the sun, it might have only one wind jet, directly over the part closest to the sun. In that scenario, heat could be trapped on the dayside.
Even a relatively modest temperature differential (say, 50 degrees Fahrenheit) between the two sides could make these planets harder to live on. A comfortably mild climate on the dayside might still leave the nightside cold enough to freeze water, according to Laura Kreidberg, a junior fellow at Harvard University who studies the atmospheres of exoplanets. “Could all the planet’s water freeze out on the nightside? We don’t yet know,” she says. Ocean currents could help transport heat, too, but those effects depend on how much water the planet has to begin with and where the continents are.