Stellar Freckles in X-rays express
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.
Original abstract
as written by the authors arXiv:2606.16782
Transiting exoplanets provide a unique laboratory for studying stellar surface heterogeneities via starspot or facular occultations. When observed at multiple wavelengths, this configuration enables spectroscopic characterization of spot thermal contrasts, distributions, and morphology. In this work, we leverage JWST NIRISS/SOSS transit observations of the 20--30 Myr planets V1298 Tau bcd to study the surface properties of their solar analog host star V1298~Tau. We identify 14 starspot crossing events across two visits. We derive $$0.8-2.8\mu$$m starspot contrast spectra and demonstrate the contrasts can only be explained when accounting for the umbral and penumbral components of the starspots, robust to which stellar model grid is assumed. The spot temperatures are broadly consistent between visits, suggesting that V1298~Tau ($$T_\mathrm{phot}=4880\pm20$$ K) has starspots with $$T_{\mathrm{umbra}}$$ = 3265--3436 K umbrae and $$T_{\mathrm{penumbra}}$$ = 4388--4659 K penumbrae, and are $$\sim$$30% umbrae by area. The differences between these spot components and the stellar photosphere are consistent with sunspots. Additionally, the relation between the spot contrast and the ratio of umbral to penumbral area is similar to that of the Sun. Combining these JWST observations with long baseline multi-band photometry from the Las Cumbres Observatory, we also estimated the global unocculted spot distribution, revealing at least 5 additional large unocculted active regions. All together, these measurements suggest that while the total spot coverage evolves in time, the relative temperatures of surface heterogeneities on Sun-like stars may be consistent throughout their lifetimes. Furthermore, these results demonstrate that JWST exoplanet transit observations can be useful for starspot substructure characterization.
The temperature in a sunspot's umbra is about 3700°C. That's 1800 degrees cooler than the surrounding photosphere, but still hot enough to melt tungsten.
In Arthur C. Clarke's novel Rendezvous with Rama, the characters observe a transit of Mercury across the Sun's disk — a scene reminiscent of the transit that helped scientists study spots on another star.