Beyond solar metallicity. How enhanced solid content in disks reshapes low‑mass planet torques

The migration of low‑mass planets (M_ łeq10 M_⊕) is tightly controlled by the torques exerted by both the gas and solids in their natal disks. While canonical models assume a solar solid‑to‑gas mass ratio (ε p and neglect the back-reaction of the solid component of the disk, recent work suggests that enhanced metallicity can radically alter these torques. We quantify how elevated metallicities (ε=0.03 and ε=0.1) modify the gas and solid torques acting on an Earth‑mass planet, test widely used linear scaling prescriptions, and identify the regimes where solid back‑reaction becomes decisive. We performed global, two‑dimensional hydrodynamic simulations that (i) treat solid material as a pressureless fluid fully coupled to the gas through drag and (ii) include the reciprocal back‑reaction force. The low-mass planet was maintained on a fixed circular orbit, and thus we computed static torques. The Stokes number was varied from 0.01 to 10, and three surface‑density slopes (p=0.5, 1.0, 1.5) and three accretion efficiencies (η=0%, 10%, 100%) were explored. Predicted torques, obtained by rescaling canonical ε=0.01 results, were compared with direct simulations. and and Solid torques scale nearly linearly with ε, but gas torques deviate by 50-100% and can even reverse sign for mathrm St łeq1 in ε=0.1 disks. These discrepancies arise from strong, feedback‑driven, asymmetric gas perturbations in the co‑orbital region, which are amplified by rapid planetary accretion. Accurate total torques are recovered only for mathrm St independent of ε or η; for mathrm St łeq2 the linear prescription systematically overestimates the magnitude, sometimes predicting the wrong sign. Solid back-reaction in high‑metallicity environments can dominate the migration torque budget of low‑mass planets. Simple metallicity rescalings are therefore unreliable for mathrm St łeq2, 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.