TOI-3884 is a fully convective M dwarf with a giant polar starspot that the planet TOI-3884 b transits directly. Seven JWST observations spanning 0.5 to 5.3 micrometers capture the spot-crossing signatures, producing the first empirical panchromatic infrared spectrum of an M-dwarf starspot.
The spot is approximately 185 K cooler than the surrounding photosphere. But the measured spectrum doesn't match what atmosphere models predict. At shorter wavelengths, the models systematically underpredict the spot-photosphere contrast — the real spot is darker than the models say it should be.
This matters beyond stellar physics because starspots contaminate exoplanet transmission spectra. When a transiting exoplanet crosses a spot, the transit depth changes, and the wavelength dependence of that change imprints false signals that can mimic or mask atmospheric features. Current correction methods rely on the same atmosphere models that this measurement shows are wrong.
The JWST observations also nail down the stellar rotation period (11.102 ± 0.003 days) and constrain the star's spin-orbit orientation. The spot persists across all seven transits, covering a substantial fraction of the visible hemisphere — not a sunspot-scale blemish but a major structural feature of the stellar surface.
The first measurement establishes the benchmark. Every prior starspot correction for M-dwarf exoplanet spectra was calibrated against models, not data. Now there's data, and the models are off. The contamination from stellar heterogeneity is more complex than current treatments assume.