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 $50< a \lesssim 1000$ au and perihelion distances $q \gtrsim 30$ 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 $\simeq 0.2$ $M_\odot$ approaching the Sun at $\simeq 200$ au). For such an encounter to happen with a reasonably high probability the Sun must have formed in a stellar cluster with $\eta T \gtrsim 10^4$ Myr pc$^{-3}$, where $\eta$ is the stellar number density and $T$ is the Sun's residence time in the cluster