Forty-eight and a half light years from Earth, a world called LHS 3844b circles its red dwarf star without ever turning away. One hemisphere bakes under permanent starlight, reaching temperatures between 1,000 and 2,000 Kelvin — hot enough to melt rock. The other faces the void of space in total, unending darkness, approaching absolute zero, the coldest temperature physically possible. For most planetary scientists, that portrait was enough to close the case: no life, no further questions.

Daisuke Noto, a postdoctoral researcher at the University of Pennsylvania's Penn GEFLOW Lab, was not satisfied with that verdict. In a study published in Nature Communications, Noto and colleagues from the Japan Agency for Marine-Earth Science and Technology and Hokkaido University shifted their attention away from the surface and looked inward — deep into the planet's mantle, the thick rocky layer that lies between the crust and the core. Using a physical laboratory model rather than computer simulation alone, the team replicated the conditions that would exist inside a tidally locked world, tracing how heat moves laterally through rock when one side is constantly heated and the other constantly cooled. What they found complicates the familiar story of hostile alien worlds.

Tidal locking, it turns out, can drive sustained mantle convection that distributes heat across a planet's interior, potentially creating moderate thermal environments in regions that surface temperature maps alone would write off as dead zones. The terminator — the ring-shaped boundary between permanent day and permanent night — emerged as a candidate for localised habitability. The finding matters beyond one planet. Tidally locked worlds are, by a wide margin, more common in the galaxy than Earth-like planets with regular day-night cycles; moons and short-orbit planets routinely fall into this configuration.