Planet-disk interaction and orbital evolution (1203.1184v2)

Abstract: As planets form and grow within gaseous protoplanetary disks, the mutual gravitational interaction between the disk and planet leads to the exchange of angular momentum, and migration of the planet. We review current understanding of disk-planet interactions, focussing in particular on physical processes that determine the speed and direction of migration. We describe the evolution of low mass planets embedded in protoplanetary disks, and examine the influence of Lindblad and corotation torques as a function of the disk properties. The role of the disk in causing the evolution of eccentricities and inclinations is also discussed. We describe the rapid migration of intermediate mass planets that may occur as a runaway process, and examine the transition to gap formation and slower migration driven by the viscous evolution of the disk for massive planets. The roles and influence of disk self-gravity and magnetohydrodynamic turbulence are discussed in detail, as a function of the planet mass, as is the evolution of multiple planet systems. Finally, we address the question of how well global models of planetary formation that include migration are able to match observations of extrasolar planets.

• The paper reviews mechanisms of planet-disk interaction, including Lindblad and corotation torques, various migration types, and the roles of numerical simulations and disk characteristics in orbital evolution.
• Numerical simulations highlight that including realistic thermal physics can significantly mitigate rapid Type I migration.
• Planet-disk migration plays a crucial role in shaping the observed architectures of exoplanet systems, including resonant configurations.

An Analysis of Disk-Planet Interactions and Orbital Evolution

The paper of planet formation within protoplanetary disks has been a focal point in astrophysical research for its implications on planetary migration and the ultimate architecture of planetary systems. The paper "Planet-disk interaction and orbital evolution" by Kley and Nelson provides a comprehensive review of the mechanisms governing planet-disk interactions, focusing on how these interactions influence planet migration, specifically addressing the roles of Lindblad and corotation torques, the effects of planet mass on migration, and the implications of disk turbulence and self-gravity.

Disk-Planet Interactions

The essence of planet-disk interaction lies in the gravitational forces exchanged between an emerging planet and its natal disk, leading to angular momentum transfer and subsequent migration. The speed and direction of planetary migration are profoundly impacted by torques, primarily Lindblad and corotation, exerted by the disk. Lindblad torques are associated with spiral density waves launched in the disk, whereas corotation torques are linked to material co-orbiting with the planet. The migration process is typically characterized by three types: Type I for low-mass planets, Type II for gas giants capable of gap formation, and Type III or runaway migration for brown-dwarf-mass planets in massive disks.

Numerical Simulations and Modeling

The paper explores the intricacies of both linear analyses and numerical simulations used to assess disk-planet interactions. Numerical simulations have increasingly augmented our understanding, allowing researchers to explore non-linear effects and configurations beyond the assumptions of linear theory. Simulations reveal that rapid Type I inward migration, which poses significant challenges for planet formation models, can be notably mitigated by the entropy-related corotation torque in radiative disks. This insight stresses the importance of incorporating realistic thermal physics into models to better predict migration outcomes.

Impacts of Disk Characteristics

Critical to the evolution of a planet's orbit is the disk's physical state, encompassing its viscosity, thermal structure, and turbulence. Turbulent disks, influenced by magnetohydrodynamic (MHD) instabilities, introduce stochastic forces that modify migration pathways and timescales, adding complexity to predictions. Disks that maintain a layered structure with 'dead zones,' regions of suppressed turbulence due to low ionization levels, yield different migration outcomes than fully turbulent environments. These factors must be accounted for in global models attempting to reconcile theoretical predictions with observed exoplanet populations.

Implications for Planetary Systems

The paper discusses how disk-driven migration shapes observed planetary architectures. Notably, resonance lock between migrating planets explains the prevalence of configurations such as the 2:1 resonances evident in systems like GJ 876. This highlights scenarios where differential migration leads to resonant capture, which significantly influences eccentricity and inclination evolution, impacting long-term dynamical stability.

Conclusion and Future Directions

Kley and Nelson's review underscores the significance of planet-disk interaction models that integrate migration with observational data from exoplanet surveys. It propounds the need for enhanced models that blend migration theories with N-body dynamics and includes the effects of disk heterogeneity and MHD processes. As observational capabilities expand, providing richer datasets, these models will be pivotal in unraveling formation histories consistent with the diverse exoplanetary systems observed.

From a theoretical standpoint, understanding planet-disk interactions in increasingly sophisticated environments will advance our comprehension of planet formation scenarios, offering potential insights into conditions conducive to stable, life-supporting systems. Hence, the paper not only reviews current understanding but also paves the way for future exploration and development in the field of planet-disk dynamics.