The Origin of Universality in the Inner Edges of Planetary Systems

The characteristic orbital period of the inner-most objects within the galactic census of planetary and satellite systems appears to be nearly universal, with $P$ on the order of a few days. This paper presents a theoretical framework that provides a simple explanation for this phenomenon. By considering the interplay between disk accretion, magnetic field generation by convective dynamos, and Kelvin-Helmholtz contraction, we derive an expression for the magnetospheric truncation radius in astrophysical disks, and find that the corresponding orbital frequency is independent of the mass of the host body. Our analysis demonstrates that this characteristic frequency corresponds to a period of $Psim3$ days, although intrinsic variations in system parameters are expected to introduce a factor of $sim2-3$ spread in this result. Standard theory of orbital migration further suggests that planets should stabilize at an orbital period that exceeds disk truncation by a small margin. Cumulatively, our findings predict that the periods of close-in bodies should span $Psim2-12$ days - a range that is consistent with observations.

Automated summary

This paper presents a theoretical framework that explains the characteristic orbital period of the inner-most objects within planetary and satellite systems. By considering the interplay between disk accretion, magnetic field generation, and contraction, the authors derive an expression for the magnetospheric truncation radius and find that the corresponding orbital frequency is independent of the host body's mass. The analysis predicts that the periods of close-in bodies should span a range of 2-12 days, which aligns with observations.