A tidally locked planet around an M dwarf has one face permanently lit, one permanently dark. The conventional worry is atmospheric collapse: CO2 condenses on the nightside, the greenhouse weakens, the planet cools. This is supposed to kill habitability.
Taniguchi et al. ran climate models and found the opposite. Atmospheric collapse can preserve surface liquid water on the dayside — not despite the loss of greenhouse warming, but because of it.
The mechanism has two competing effects. A thinner atmosphere provides less greenhouse warming, which cools the surface. But a thinner atmosphere also transports less heat from the sunlit dayside to the frozen nightside. The dayside retains more of its stellar energy. When the transport loss outweighs the greenhouse loss, the dayside stays warm enough for liquid water even as the planet's average temperature drops.
This is not a minor correction. The standard picture says atmospheric collapse shrinks the habitable zone inward. The corrected picture says it can maintain habitable conditions at orbital distances where a thick, well-mixed atmosphere would fail — because the thick atmosphere would efficiently redistribute the heat away from the only surface that receives it.
The through-claim: the atmosphere's failure to function as a heat engine is what preserves the conditions for life. A well-circulating atmosphere is the enemy of a one-sided planet. It takes the energy from the only place that has it and delivers it to the place that wastes it — the permanently frozen nightside where additional warmth melts no ice and hosts no chemistry, only re-radiates to space.
Failure modes have structure. An atmosphere that collapses is not simply a worse version of an atmosphere that circulates. It is a different system with different properties, and some of those properties are advantages that circulation would erase. The collapse doesn't save the planet by doing something. It saves the planet by stopping.