Radial distribution of distant trans-Neptunian objects points to Sun’s formation in a stellar cluster

Abstract

The Scattered Disk Objects (SDOs) are a population of trans-Neptunian bodies with semimajor axes 50<a≲1000 au and perihelion distances q≳30 au. The detached SDOs with orbits beyond the reach of Neptune (roughly q>35 au) are of special interest here as an important constraint on the early evolution of the outer Solar System. The semimajor axis profile of detached SDOs at 50–500 au, as characterized from the Dark Energy Survey (DES), is radially extended, but previous dynamical models of Neptune’s early migration produce a relatively compact profile. This problem is most likely related to Sun’s birth environment in a stellar cluster. We perform new dynamical simulations that account for cluster effects and show that the orbital distribution of SDOs can be explained if a particularly close stellar encounter occurred early on (e.g., M dwarf with the mass ≃0.2M⊙ approaching the Sun at ≃200 au). For such an encounter to happen with a reasonably high probability the Sun must have formed in a stellar cluster with ηT≳104 Myr pc−3, where η is the stellar number density and T is the Sun’s residence time in the cluster.