The M8V star TRAPPIST-1 hosts seven roughly Earth-sized planets and is a promising target for exoplanet characterization . Kepler /K2 Campaign 12 observations of TRAPPIST-1 in the optical show an apparent rotational modulation with a 3.3 day period , though that rotational signal is not readily detected in the Spitzer light curve at 4.5 \mu m. If the rotational modulation is due to starspots , persistent dark spots can be excluded from the lack of photometric variability in the Spitzer light curve .
We construct a photometric model for rotational modulation due to photospheric bright spots on TRAPPIST-1 which is consistent with both the Kepler and Spitzer light curves .
The maximum-likelihood model with three spots has typical spot sizes of R _ { \mathrm { spot } } / R _ { \star } \approx 0.004 at temperature T _ { \mathrm { spot } } \gtrsim 5300 \pm 200 K. We also find that large flares are observed more often when the brightest spot is facing the observer , suggesting a correlation between the position of the bright spots and flare events .
In addition , these flares may occur preferentially when the spots are increasing in brightness , which suggests that the 3.3 d periodicity may not be a rotational signal , but rather a characteristic timescale of active regions .