Radial Distribution of Distant Trans-Neptunian Objects Points to Sun's Formation in a Stellar Cluster

TL;DR

This study suggests that the radial distribution of distant trans-Neptunian objects indicates the Sun likely formed in a dense stellar cluster, with early stellar encounters shaping the outer Solar System's structure.

Contribution

The paper introduces dynamical simulations including stellar cluster effects to explain the distribution of detached SDOs, proposing a close stellar encounter as a key factor.

Findings

Orbital distribution of SDOs can be explained by early stellar encounters.

A close M dwarf encounter at ~200 au is consistent with observed SDO distribution.

The Sun's formation in a dense stellar cluster is supported by the probability analysis.

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.