New migration patterns in high planet–star mass ratio systems in discs with low viscosity

Context. The migration of giant planets remains a complex and rich topic. While significant progress has been made in understanding migration in discs with high viscosity, the migration of planets with high planet-star mass ratios in low-viscosity environments is still not fully understood. Aims. Our aim is to study the migration of planets with high planet-to-star mass ratios embedded in low-viscosity protoplanetary discs, characterised by the viscous parameter α = 10−4, and to derive analytical prescriptions that could be used to study planet formation across a range of stellar masses, spanning from Sun-like stars to M dwarfs. Methods. We performed hydrodynamical simulations using the FARGO3D code, exploring the migration of planets with high planet-star mass ratios (10−3 ≤ q ≤ 2 × 10−2) under different disc conditions, including variations in gas surface density, scale height, and density slope. Results. Our simulations reveal a change in the migration direction at a mass ratio of q ≈ 0.002, with planets exhibiting outward migration for q > 0.002. Additionally, for planets undergoing outward migration, we find that the migration speed depends on the unperturbed local gas density. In most cases, outward migration is driven by a positive torque related to the fact that the planet’s eccentricity remains below e < 0.2. However, under certain disc parameterisations, planets with q > 0.01 can develop higher eccentricities in the range 0.2 < e < 0.45, which can lead to stalled migration. Conclusions. Our findings suggest that outward migration is a viable mechanism for massive planets in low-viscosity discs, which has implications for the formation and distribution of super-Jupiter planets around Sun-like stars and planets more massive than Neptune around very low mass stars. Given the challenges in detecting such planets, improving our theoretical understanding of their migration is essential for interpreting exoplanet demographics and guiding future observational efforts.