On a Planet With No Sunrises & No Sunsets, Scientists Hunt Life in the Twilight

EARTH, Laniakea Supercluster—Picture a world where the sun is bolted to one spot in the sky.

On one hemisphere it is always high noon, a searing 1,000 to 2,000 Kelvin. On the other, permanent midnight, cold enough to brush absolute zero. There is no dawn, no dusk, no golden hour—just a single line of eternal twilight running pole to pole.

It sounds like a death sentence. A team of physicists thinks it might be an address.

A collaboration published in a 2025 study in the journal Nature Communications between the University of Pennsylvania's GEFLOW Lab, Japan's Agency for Marine-Earth Science and Technology (JAMSTEC), and Hokkaido University, modeled what happens deep inside "tidally locked" exoplanets, the kind that keep one face turned toward their star forever.

While that may seem puzzling to perpetually revolving Earthlings, tidal locking is not particularly exotic across the universe. For example, it is why we only ever see one side of the Moon.

When a body orbits close to a much larger one, gravity slows its spin until a "day" and a "year" become the same thing. Around small, dim red dwarf stars (the most common stars in the galaxy), the planets most likely to hold liquid water are also the ones most likely to be locked this way.

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Researchers, led by postdoctoral scientist Daisuke Noto, focused on LHS 3844b, a rocky tidally locked planet roughly 1.3 times Earth's radius sitting an estimated 48.6 light-years away. But they couldn’t exactly requisition it.

"Building an actual exoplanet in the lab wasn't in the budget," Noto said in a Penn announcement regarding their findings. Instead, they built a tabletop stand-in: a tank of glycerol seeded with thermochromic liquid crystals—crystals that change color with temperature—heated unevenly to mimic the lopsided furnace of a locked world. Watching the fluid move let them map how heat would circulate through such a planet's rocky mantle.

What they found was not entropy but a strange, orderly engine.

"It's not chaotic like Earth's mantle. It's slow and steady. Predictable," Noto said of the circulation pattern. "Kind of boring—but in a good way."

That’s important because the model produced a gradual slope of heat from the scorching dayside to the frozen nightside, hinting that the planet's mid-to-high latitudes could hold moderate temperatures (and, potentially, liquid water) warmed from below by the planet's own geology rather than its star. It also means that similar conditions could potentially go beyond just that single exoplanet.

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Most of the potentially habitable worlds astronomers can currently study orbit red dwarfs, and most are locked. If internal heat can soften the brutal math of a fixed sun, the list of places worth pointing a telescope at gets a great deal longer.

"Life might find a way," Noto said.

Still, it’s worth being clear that this is only a laboratory analog and a mathematical framework, not a photograph of an alien coastline. The experiment models heat flow beneath the surface, not weather, oceans, or biology above it. Whether real liquid water actually pools along that twilight seam remains unproven.

"That's something we couldn't test in this experiment, but it's an exciting direction for future work," Noto acknowledged.

For now, the twilight line stays theoretical—a thin band of maybe, wrapped around a world that never turns away.

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