Tidally locked planets always show the same face to their star. On these worlds, one side remains in perpetual light while the other side is engulfed in constant darkness.
This unique characteristic creates extreme conditions: a scorched face and a frigid face. Between these extremes, the terminator line, where day meets night, could be conducive to life, thanks to an eternally twilight climate.
Artistic concept of Kepler-186f, an Earth-sized planet in the habitable zone.
Credit: NASA Ames/SETI Institute/JPL-Caltech
These planets are increasingly interesting to astronomers because they are abundant in the universe. The phenomenon, similar to that observed with our Moon, which always shows the same face to Earth, is explained by tidal forces. These bodies, often in close orbits around smaller stars, such as red dwarfs, pose a challenge for the viability of life.
To remain habitable, these worlds must effectively transfer heat from the lit side to the dark side. This mechanism depends on several factors, like atmospheric composition, distance from the star, and the presence of water. Water-rich planets may benefit from ocean currents that distribute heat uniformly, potentially increasing the areas suitable for life.
However, even water-poor planets could offer viable living conditions, especially around the terminator line, where temperatures are milder. On these worlds, life would face a strange environment: a perpetually low star on the horizon and powerful atmospheric and oceanic currents, without the turbulence due to planetary rotation.
Studies continue, particularly with advanced instruments like the James Webb Space Telescope, to understand if these "ribbon worlds" can sustain habitability over the long term and if, indeed, they might host the first forms of extraterrestrial life we could discover.