Beyond solar metallicity: How enhanced solid content in disks re-shape low-mass planet torques

The migration of low-mass planets is tightly controlled by the torques exerted by both gas and solids in their natal disks. While canonical models assume a solar solid-to-gas mass ratio (epsilon=0.01) and neglect the back-reaction of solid component of the disk, recent work suggests that enhanced metallicity can radically alter these torques. We quantify how elevated metallicities (epsilon=0.03 and epsilon=0.1) modify the gas and solid torques, test widely used linear scaling prescriptions, and identify the regimes where solid back-reaction becomes decisive. We performed global, 2D hydrodynamic simulations that treat solid material as a pressureless fluid fully coupled to the gas through drag and include the reciprocal back-reaction force. The planet was maintained on a fixed circular orbit, thus we computed static torques. The Stokes number was varied from 0.01 to 10, three surface-density slopes (p=0.5, 1.0, and 1.5) and three accretion efficiencies (eta=0, 10, and 100%) were explored. Torques, obtained by rescaling canonical epsilon=0.01 results, were compared with direct simulations. Solid torques scale linearly with epsilon, but gas torques deviate by 50-100% and can even reverse sign for St