One possible scenario for a tidally locked planet is what’s known as the “eyeball Earth” model, in which a planet starts out entirely covered with ice—which then melts on the side facing the sun. To an observer from space, this could look like an eyeball, explains Angerhausen. Or, with an ocean that transports enough heat, you could end up with a lobster-shaped ocean surrounded by ice.
In the most extreme scenarios, the heat on the light side becomes so extreme that water can’t exist. But with enough of a temperature difference, it can re-form on the nightside.
That’s what happens on a tidally locked planet called WASP-103b, a “hot Jupiter”–type world. According to Vivien Parmentier at Aix Marseille University, an author (along with Kreidberg) on a recent study of WASP-103b, water molecules are destroyed on the dayside of the planet, only to drift back to the nightside and recombine into water molecules that form clouds ... and then the process repeats.
Beyond the problems with finding liquid water, a tidally locked world around a red dwarf could have other issues, says Carone. Red dwarfs are “notoriously temperamental” and tend to go through long phases in which they flare up and eject material into space.
Read: Astronomers have found planets in the habitable zone of a nearby star
These flare-ups could heat the atmosphere of a planet in the habitable zone, while the star could also eject material that strips away the atmosphere. This happened to Earth early on, when our original atmosphere was torn away from us. Afterward, Earth “sweated out” another atmosphere from trapped carbon dioxide. But on a tidally locked world, a violent-enough solar disruption could get rid of a second atmosphere, too.
Even with an atmosphere, the dayside of the planet could be exposed to deadly radiation, says Parmentier. The light from a red dwarf wouldn’t provide enough of the UV wavelengths needed to make ozone—so this planet, unlike Earth, might not have an ozone layer. (In my novel, direct sunlight isn’t just too hot; it actually causes nasty burns, so people have to stay in the shade.)
Any humans living on the planet would also need to eat and breathe, and the physicists Joseph Gale and Amri Wandel of Hebrew University have been studying whether plant life could survive the flares and radiation exposure. At first, plants might evolve in the ocean to take advantage of the protective layer of water. But eventually, if the star became less violent, the planet could develop an atmosphere thick enough to allow plants to grow on land. Gale and Wandel have also calculated that there would probably be enough light in the visible spectrum to allow normal photosynthesis.
With an atmosphere that could sustain life, though, there would also be air currents strong enough to cool the planet’s dayside. The temperature might end up being about the same as in Earth’s tropical regions. An atmosphere could also help create a layer of cloud that would serve as a permanent sun shade. As scientists such as Carone have been making computer models of tidally locked worlds, they increasingly believe humans could live outside the terminator region.