The intrinsic magnetic fields of exoplanets influence the structure of their atmospheres and plasmaspheres and, consequently, the observational manifestations of transit absorptions. In this talk, we present a novel method designed to constrain the presence or absence of relatively weak MFs. We will show how the quantum effect of atomic alignment, which alters the absorption probabilities of individual transitions within multiplets from their equilibrium 2J + 1 values, provides a sensitive probe for fields as weak as 0.001 G. We applied this method to some available observations of exoplanet transits and demonstrated that we indeed have the ability to estimate the intrinsic magnetic field of some exoplanets right now