A planet glides across a star's disk, and for a moment becomes a flashlight. It blocks a tiny patch of surface, revealing details that otherwise escape our view. That's exactly how astronomers peered into the dark spots on V1298 Tauri, a young twin of our Sun. They were aided by the James Webb Space Telescope JWST and three giant planets in orbit.

Starspots are not just dots. Like on the Sun, they have an umbra and a penumbra. The scientists caught 14 moments when a planet crossed such a spot and recorded a spectrum — splitting the light into colors. By tracking how brightness varied across wavelengths from 0.8 to 2.8 micrometers, they calculated the temperatures of the umbra and penumbra. The umbra turned out to be 1,500 degrees cooler than the star's photosphere, and the penumbra a few hundred degrees cooler. This exactly mirrors the pattern seen in sunspots.

Spots on V1298 Tauri cover about 30% of the star's surface, and the umbra-to-penumbra ratio matches that of the Sun. That's surprising, given the star is only 20–30 million years old, while the Sun is 4.5 billion. So the structure of spots, umbra and penumbra, does not change with age. What changes is their overall number: young stars have more active regions, and they are larger.

This method, transit spectroscopy of spots, opens a new window into stellar physics. Previously, we judged spots by a star's overall brightness; now we can discern their internal structure. This matters not only for understanding stellar evolution but also for the search for other worlds: stellar activity can mask or mimic the signatures of planets.