Tidally locked worlds are places of extremes. On one side it’s an endless day, the other a perpetual night. Yet scientists speculate that some may harbor conditions that could support life.
On some worlds, the sun never sets. Or rises. If a planet takes the same amount of time to spin once about its axis as it does to orbit once about its parent star, that star will appear to hang motionless in the sky. Such planets show only one face to their star, a situation known as tidal locking.
A tidally locked planet is a bifurcated world. On one side it’s always day; on the other, eternal night. The boundary between features a thin ring of twilit eternity, where the sun is forever setting (or rising, if you prefer). A number of worlds in our own solar system are tidally locked — including our moon — and any number of exoplanets that orbit their own stars in other solar systems may be tidally locked as well.