Presentation Information
[PPS04-P34]Spin Regulation of Giant Planets by the Balance between Circumplanetary Disk and Vertical Accretion Flows
*Shogo Moro1, Hiroyuki Kurokawa1, Kanon Nakazawa1 (1.The University of Tokyo)
Keywords:
giant planets,planet formation,planetary rotation,circumplanetary disk,accretion flow,angular momentum transport
Giant planets are expected to acquire a large amount of angular momentum comparable to their breakup limits through accretion via circumplanetary disks during the late stages of their formation. However, neither solar system giant planets nor observed exoplanets exhibit rotation rates close to these limits (Bryan et al. 2020). This discrepancy has often been explained by angular momentum extraction through magnetic torques (e.g., Batygin 2018). For this mechanism to operate efficiently, however, several conditions must be satisfied: the presence of sufficiently conductive material and vigorous convection inside the planet, as well as ionized gas in the circumplanetary disk that is strongly coupled to the planetary magnetic field. There is currently no direct evidence that these conditions were met immediately after the formation of giant planets. In particular, such conditions may be difficult to realize for low-mass giant planets, since the high internal pressures required for the formation of metallic hydrogen may not be achieved. Consequently, the physical mechanism that suppresses rapid rotation in giant planets remains unresolved.
In this study, we focus on a scenario in which gas supply to a forming planet consists of both low-angular-momentum vertical accretion flows from high latitudes and high-angular-momentum accretion through a circumplanetary disk (e.g., Tanigawa et al. 2008), and we investigate the possibility that the balance between these two pathways regulates planetary rotation. Using an analytical model based on Adams et al. (2025), we derive the mass and angular-momentum fluxes within the accretion structure, including the circumplanetary disk. This framework allows us to clarify how the relative contributions of low-angular-momentum vertical inflow and high-angular-momentum disk-mediated accretion influence the spin evolution of the planet.
In this presentation, we show that, in addition to the relative contributions of vertical and disk accretion flows, the treatment of the region where the gas accreting from the disk couples to the planet and the planetary mass–radius relation are crucial factors governing spin evolution. We further identify the conditions under which these elements can effectively suppress planetary spin-up and discuss accretion structures during formation that are consistent with the observed final rotation states of giant planets.
In this study, we focus on a scenario in which gas supply to a forming planet consists of both low-angular-momentum vertical accretion flows from high latitudes and high-angular-momentum accretion through a circumplanetary disk (e.g., Tanigawa et al. 2008), and we investigate the possibility that the balance between these two pathways regulates planetary rotation. Using an analytical model based on Adams et al. (2025), we derive the mass and angular-momentum fluxes within the accretion structure, including the circumplanetary disk. This framework allows us to clarify how the relative contributions of low-angular-momentum vertical inflow and high-angular-momentum disk-mediated accretion influence the spin evolution of the planet.
In this presentation, we show that, in addition to the relative contributions of vertical and disk accretion flows, the treatment of the region where the gas accreting from the disk couples to the planet and the planetary mass–radius relation are crucial factors governing spin evolution. We further identify the conditions under which these elements can effectively suppress planetary spin-up and discuss accretion structures during formation that are consistent with the observed final rotation states of giant planets.