Astronomers caught the young planet AU Mic b in the act of hiding behind its star. The Spitzer telescope tracked its infrared glow: as the planet slipped behind, the system's total light dimmed slightly. The dip was just 171 parts per million, like a single drop of ink in ten buckets of water. But that was enough to measure the dayside temperature of this distant world.

The planet turns out to be scorching at 1031 Kelvin (about 758°C). For comparison, if it were heated only by its star's radiation, it would be around 606 Kelvin. That's the difference between a hot cup of tea and a sizzling skillet. The planet is clearly getting extra heat, but where from is a puzzle.

The scientists tested several explanations: maybe heat isn't distributed well across the planet, or it's contracting under its own gravity and warming up, or the star's pulsations are to blame, or even instrument errors. None fit. So we'll have to wait for new observations from the James Webb Space Telescope, which could peer into the planet's atmosphere and maybe find the source of the mysterious heat.