Sedna and the Oort Cloud around a migrating Sun

Recent numerical simulations have demonstrated that the Sun’s dynamical history within the Milky Way may be much more complex than that suggested by its current low peculiar velocity (Sellwood, J.A., Binney, J.J. [2002]. Mon. Not. R. Astron. Soc. 336, 785–796; Roškar, R., Debattista, V.P., Quinn, T.R., Stinson, G.S., Wadsley, J. [2008]. Astrophys. J. 684, L79–L82). In particular, the Sun may have radially migrated through the galactic disk by up to 5–6 kpc (Roškar, R., Debattista, V.P., Quinn, T.R., Stinson, G.S., Wadsley, J. [2008]. Astrophys. J. 684, L79–L82). This has important ramifications for the structure of the Oort Cloud, as it means that the Solar System may have experienced tidal and stellar perturbations that were significantly different from its current local galactic environment. To characterize the effects of solar migration within the Milky Way, we use direct numerical simulations to model the formation of an Oort Cloud around stars that end up on solar-type orbits in a galactic-scale simulation of a Milky Way-like disk formation. Surprisingly, our simulations indicate that Sedna’s orbit may belong to the classical Oort Cloud. Contrary to previous understanding, we show that field star encounters play a pivotal role in setting the Oort Cloud’s extreme inner edge, and due to their stochastic nature this inner edge sometimes extends to Sedna’s orbit. The Sun’s galactic migration heightens the chance of powerful stellar passages, and Sedna production occurs around ∼20–30% of the solar-like stars we study. Considering the entire Oort Cloud, we find its median distance depends on the minimum galactocentric distance attained during the Sun’s orbital history. The inner edge also shows a similar dependence but with increased scatter due to the effects of powerful stellar encounters. Both of these Oort Cloud parameters can vary by an order of magnitude and are usually overestimated by an Oort Cloud formation model that assumes a fixed galactic environment. In addition, the amount of material trapped in outer Oort Cloud orbits (a > 20,000 AU) can be extremely low and may present difficulties for traditional models of Oort Cloud formation and long-period comet production.

► We model Oort Cloud formation while the Sun migrates within the Milky Way.
► The radial distribution of the Oort Cloud depends on the Sun’s galactic orbital history.
► Sedna may be a member of the conventional Oort Cloud.
► The strongest field star passages set the inner edge of the Oort Cloud.
► Previous Oort Cloud models underestimate its central concentration and the erosion of the outer cloud.