JWST reveals the spectrum of a giant starspot on the M-dwarf TOI-3884

Star system: TOI 3884 | Distance from the Sun: 141 light years

TOI-3884 b is a rare super-Neptune planet that transits a fully convective M-dwarf star with a persistent giant polar starspot. Because the planet passes over this spot during every transit, astronomers can directly study the spot’s properties using the James Webb Space Telescope (JWST). Seven transits were observed with JWST, covering a wide range of wavelengths from 0.5 to 5.3 micrometers.

Each transit showed a recurring signature as the planet crossed the starspot, but the details varied, allowing the team to measure the star’s rotation period and orientation. They obtained the first direct spectrum of an M-dwarf starspot, finding it to be about 1,850 Kelvin cooler than the rest of the star’s surface. While current models match the observed spot contrasts at longer wavelengths, they underestimate the contrast at shorter wavelengths. This suggests that existing models may not fully capture how starspots affect the light from M-dwarfs, especially in the optical range. The results highlight the need for empirical spot spectra to accurately interpret planetary atmospheres around these stars.

Key findings
- JWST observed seven transits of TOI-3884 b, each crossing a giant polar starspot
- The starspot is about 1,850 K cooler than the surrounding photosphere
- The star’s rotation period and orientation were precisely measured
- Models match spot contrasts at long wavelengths but underpredict them at short wavelengths
- Empirical spot spectra are crucial for interpreting exoplanet atmospheres around M-dwarfs

Scientific study

A Panchromatic JWST Spectrum of a Giant Starspot on the Fully Convective M-dwarf TOI-3884 (external link)