Hunting Axion Dark Matter with Protoplanetary Disk Polarimetry

We find that the polarimetric observations of protoplanetary disks are useful to search for ultralight axion dark matter. Axion dark matter predicts the rotation of the linear polarization plane of propagating light, and protoplanetary disks are ideal targets to observe it. We show that a recent observation puts the tightest constraint on the axion-photon coupling constant for an axion mass m≲10^{-21} eV.