From planetesimals to dwarf planets by pebble accretion

¹ Department of Astronomy, Tsinghua University, Beijing 100084, China
² National Astronomical Observatory of Japan, 2-21-1 Osawa, Mitaka, Tokyo 181-8588, Japan

^★ Corresponding author; chrisormel@tsinghua.edu.cn

Received: 12 December 2024
Accepted: 5 February 2025

Abstract

The size distribution of trans-Neptunian objects (TNOs) in the Kuiper Belt provides crucial insights into the formation and evolution of the outer Solar System. Recent observational surveys, including the Outer Solar System Origins Survey (OSSOS++), have revealed that dynamically cold and hot TNO populations exhibit similar size distributions for dimmer objects (H_r > 5), which are consistent with planetesimal formation by streaming instability (SI). However, the hot population contains a significantly larger number of massive bodies, including several dwarf planets. In this study, we investigate the role of pebble accretion in shaping the size distribution of hot TNOs, after their formation in the primordial disk (PB) between 20 and 30 au and before these bodies were dynamically implanted into their current orbits by a migrating Neptune. We find that pebble accretion grows the most massive bodies only, consistent with the flattening of the distribution brightwards of H_r = 5. All results point to a correlation (degeneracy) between the pebble aerodynamic size and the intensity of the gas motions. Nevertheless, accretion from an inward-drifting stream of pebbles is unlikely, as it puts extreme demands on the mass budget of pebbles. In particular, the masses of the cold classicals are too low to trigger pebble accretion. Accretion in an environment where pebbles are entrained, as believed to be the case in rings seen with the Atacama Large Millimeter Array (ALMA), is preferable. Combining the constraints obtained from this study with ALMA imagery morphology fitting reveals a typical pebble aerodynamic size of τ_s ~ 10⁻², a turbulent diffusivity parameter α_D ~ 10⁻³, and a total accreted pebble mass of ~10 m_⊕ in the primordial belt. Those TNOs formed through significant pebble accretion with masses exceeding ~10⁻⁴ m_⊕ are likely to satisfy the International Astronomical Union’s “round shape” criterion for dwarf planets.