Looking at the Moon at night, you may have noticed that the same side always faces Earth. Every time you look at the Moon, you will see the same features, the same canyons and craters, regardless of the phase of the Moon. The other side is hidden from view. He always faces.

This happens because the moon is tidally locked to Earth. In a kind of celestial synchronicity, it takes exactly as long for the moon to spin on its axis as it does to complete a full orbit around our planet. There are other examples of this phenomenon in our solar system. Io is locked to Jupiter, and Enceladus is locked to Saturn.

Now imagine that instead of a moon locked to its planet, a planet is tidally locked to its star. This would mean that one side would always face the star – it would be bathed in constant daylight. The other side would darken into eternal night. Temperatures on either side of the planet could be extreme. To give you an idea of ​​the effect, we can look at Mercury. This planet is not tidally locked with the Sun, but it rotates very slowly – three of its days equals two of its years. The day on Mercury is scorching hot at 430°C, while the night is -180°C.

It would seem that such a planet would not be very conducive to life. Yet some science fiction writers have dreamed of what life would be like on these worlds. (Isaac Asimov has dubbed such planets ribbon worlds.) A planet like this could be habitable, in theory, along a thin band between day and night—a twilight region where temperatures are just . Recently, a team led by University of California-Irvine researcher Ana Lobo modeled tidally locked planets to find scenarios that might support life.

Ribbon world planets in our galaxy

Our galaxy may actually be littered with tidally locked planets. They can be especially common around M-type stars, which are sometimes synonymous with red dwarfs. The most common type of star in the Milky Way, M-types make up about 70% of the stars in our cosmic neighborhood. For liquid water to exist on such a planet, it would have to be close to its host star. And the closer a planet is to its star, the more likely it is to be tidally locked.

We have discovered a few potentially locked planets in our galactic neighborhood. For example, TRAPPIST-1 is a red dwarf star orbiting at least seven planets with years ranging from 1.5 to 19 Earth days. At such a close distance, it is likely that these planets are closely related to their star. Proxima Centauri B, the closest exoplanet to us, is a super-Earth type planet, which means it is more massive than our own planet, but much smaller than a planet like Neptune. Its year is only 11 days long and it is likely tidally locked with its star.

Planets like this have the advantage of being easy to detect. As they orbit, their gravity creates a small but detectable oscillation in the motion of their star. Since these planets orbit very close to their stars and their stars are small, this oscillation is more pronounced than it would be around a more massive star with more distant planets.

Day and night on exoplanets

In order to see what conditions would look like on a tide-locked planet like this, Lobo and his collaborators used software that models climate conditions on Earth. By slowing down the rotation of the planet in the software, they were able to model what the climate would look like on the day and night side of these planets. Perhaps more importantly, they were able to model the so-called termination zone – that twilight band between night and day.

The conditions on these planets depend on the level and type of water present, and it’s a complicated relationship. Water affects the planet’s albedo – the nature of the starlight the planet reflects back into space. Lighter planets have high albedo and reflect more radiation back into space, causing cooling, while dark-colored planets absorb more radiation, have low albedo, and heat up. (It’s the same dynamic that makes you feel warmer when you’re wearing a dark colored shirt on a hot day.) Ice, for example in the form of glaciers, will reflect more radiation back into space. Clouds too. But that’s not all. Water quantity also affects how much water is retained in glaciers on the night side or how much turns into water vapor on the day side. This complex balance would help determine the habitability of the planets.

Lobo discovered that if a planet is covered in ocean, a fair amount of water can evaporate on the day side. This water vapor could trap more and more stellar radiation, warming the planet. Such planets would likely have a runaway greenhouse effect, raising temperatures on their surfaces. The researchers found that such planets wouldn’t be able to sustain the kind of temperate zone that life loves so much, even on the night side.

Things would be different, however, if there was only water mixed with dry earth. In such a case, there would be less water vapor, which would lead to a greater contrast in temperatures between the day and night sides of these planets. The termination area could house a larger surface section where temperatures are ideal for liquid water, and possibly life as we know it. These planets would also be more likely to have a stable climate over long periods of time and not continually lose water to vapor on the dayside or glaciers on the nightside.

Life on a planet like this would surely have a unique experience. Bathed in eternal twilight, he would know neither the deepest night nor the clearest day. Perhaps he would never see the stars, confined as he would be to a narrow strip of his planet. But it can exist. This research is helping us define the types of planets that could harbor life as we know it, advancing our quest to find life on another world.