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<=1 in epsilon=0.1 disks. These are due to strong, feedback-driven, asymmetric gas perturbations in the co-orbital region, amplified by rapid planetary accretion. Solid back-reaction in high-metallicity environments can dominate the migration torque budget of low-mass planets. Simple metallicity rescalings are therefore unreliable for St<=2, implying that precise migration tracks - particularly in metal-rich disks -- require simulations that fully couple solid and gas dynamics. These results highlight metallicity as a key parameter in shaping the early orbital architecture of planetary systems.