Planet WASP-121 b always shows the same face to its star, like a rock in a campfire. The dayside is so hot that ordinary hydrogen, the very stuff most of the universe is made of, can't take it: its molecules tear apart into individual atoms, and the temperature flips upside down: the higher you go, the hotter it gets. This is an ultra-hot Jupiter 850 light-years away. Astronomers aimed the infrared spectrograph of the James Webb Space Telescope at it and tracked how the planet's glow changes as it moves along its orbit.

Such observations are called phase curves. As the planet orbits its star, it shows us now the bright disk of its day, now the dark pit of its night, and at each position its light tells a different story. Comparing with 3D models threw up a surprise: the planet radiates about 12% less than calculations predicted. That's not a trifle: it looks like something is stopping it from absorbing and re-emitting the star's heat, and that "something" has long puzzled scientists. But the atmosphere's motion clearly shows a powerful brake, as if an invisible hand is holding back the winds, keeping them from speeding up. And the nightside, where no direct rays fall, looks surprisingly smooth in the spectrum: no sharp dips like the ones gases usually leave. The best explanation is that clouds gather there in the dark, covering everything like a thick curtain.

But one detail doesn't fit the picture. For light with wavelengths shorter than 1.4 microns, the more we shift toward the blue edge, the farther the planet's hottest spot runs away from us. This doesn't look like reflection from dayside clouds, nor can it be blamed on atmospheric transparency. It's as if the planet has another trick up its sleeve, one we've yet to figure out. So WASP-121 b reminds us that even a rock in a campfire can hold secrets invisible at first glance.