TOI-3884 is a fully convective M-dwarf with a giant starspot that covers a significant fraction of its visible hemisphere. When the transiting planet crosses the spot during transit, the transit depth changes — the planet occults spotted surface instead of unspotted surface, and the flux deficit differs. This chromatic transit depth variation reveals the spot's spectrum.
JWST provides the panchromatic spectrum: from 0.6 to 12 microns, covering the spot's signature across optical, near-infrared, and mid-infrared wavelengths simultaneously. The spot is cooler than the surrounding photosphere by approximately 400 K, and the temperature difference manifests as wavelength-dependent transit depth variations — deeper transits at wavelengths where the spot-to-photosphere contrast is largest.
The panchromatic coverage matters because the spot's spectrum breaks the degeneracy between spot temperature and spot size. A small hot spot and a large cool spot can produce the same transit depth variation at a single wavelength. Across the full JWST wavelength range, the two scenarios predict different spectral shapes, and the data distinguishes them.
The contamination problem: if the spot's contribution isn't correctly modeled, the planet's atmospheric spectrum is systematically biased. Molecular features attributed to the planet's atmosphere could actually originate in the star's spotted photosphere. JWST's sensitivity is high enough that the spot contamination exceeds the planet's atmospheric signal at some wavelengths. Characterizing the spot is not optional cleanup — it's prerequisite to characterizing the planet. The star must be understood before its planet can be.