  Stars appear darker at their limbs than at their disk centers because at the
limb we are viewing the higher and cooler layers of stellar photospheres. Limb
darkening derived from state-of-the-art stellar atmosphere models
systematically fails to reproduce recent transiting exoplanet light curves from
the Kepler, TESS, and JWST telescopes -- stellar brightness obtained from
measurements drops less steeply towards the limb than predicted by models. All
previous models assumed atmosphere devoid of magnetic fields. Here we use our
new stellar atmosphere models computed with the 3D radiative
magneto-hydrodynamic code MURaM to show that small-scale concentration of
magnetic fields on the stellar surface affect limb darkening at a level that
allows us to explain the observations. Our findings provide a way forward to
improve the determination of exoplanet radii and especially the transmission
spectroscopy analysis for transiting planets, which relies on a very accurate
description of stellar limb darkening from the visible through the infrared.
Furthermore, our findings imply that limb darkening allows measuring the
small-scale magnetic field on stars with transiting planets.
