TOI-561 b reflects three-quarters of its starlight

Webb watched TOI-561 b for 37 hours, covering nearly four orbits, and found evidence of a substantial global atmosphere on a world once thought too extreme to hold one. Climate models suggest silicate clouds on its dayside act as sunscreen, keeping the planet cooler than a bare rock this close to its star.

Source: University of Birmingham
Image credit: University of Birmingham

TOI-561 b is a lava world, a rocky planet hugging its star so closely that its dayside is hot enough to melt rock. Worlds like this were expected to be bare: any atmosphere should have been stripped away by the star's heat and wind long ago, leaving dark rock facing the star. Webb's 37-hour stare says otherwise. Watching for nearly four full orbits let the telescope see the planet through its whole cycle — dayside, nightside, and the transitions between — and the pattern of light it returned fits a substantial atmosphere wrapped around the whole planet, not bare rock.

The proposed explanation is rock weather. On the dayside, surface rock vaporizes into the air. Higher up, where it is cooler, that vapor condenses into clouds made of silicate minerals — essentially rock clouds. The clouds reflect starlight before it reaches the surface, acting as sunscreen, and the planet ends up cooler than a bare rock in the same orbit would be. The result is a world that reflects roughly three-quarters of the starlight hitting it, bright where a bare lava world would be dark.

That brightness is itself part of the evidence. A bare rocky surface is dark. A cloudy one is bright. The high reflectivity, the heat pattern through the orbit, and the climate models all point the same way: an atmosphere that survives where it should not, kept alive by a cycle of rock evaporating and raining back down. It changes the expectation for the whole class of ultra-hot rocky worlds. Extreme heat no longer guarantees a bare rock. Some of them keep their skies.