Romain Allart

Planet WASP-121b orbits so near its star that the fierce heat boils its atmosphere away, streaming like a comet's tails. With the James Webb Space Telescope, astronomers continuously watched this process for the first time.

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.

When a planet slips behind the edge of its star, the star backlights the planet's atmosphere, just as a setting sun illuminates clouds on the horizon. This brief moment, lasting only minutes, carries the imprint of the gases through which the starlight has passed.