Growth of asteroids, planetary embryos and Kuiper belt objects by chondrule accretion (1503.07347v1)

Abstract: Chondrules are millimeter-sized spherules that dominate primitive meteorites (chondrites) originating from the asteroid belt. The incorporation of chondrules into asteroidal bodies must be an important step in planet formation, but the mechanism is not understood. We show that the main growth of asteroids can result from gas-drag-assisted accretion of chondrules. The largest planetesimals of a population with a characteristic radius of 100 km undergo run-away accretion of chondrules within ~3 Myr, forming planetary embryos up to Mars sizes along with smaller asteroids whose size distribution matches that of main belt asteroids. The aerodynamical accretion leads to size-sorting of chondrules consistent with chondrites. Accretion of mm-sized chondrules and ice particles drives the growth of planetesimals beyond the ice line as well, but the growth time increases above the disk life time outside of 25 AU. The contribution of direct planetesimal accretion to the growth of both asteroids and Kuiper belt objects is minor. In contrast, planetesimal accretion and chondrule accretion play more equal roles for the formation of Moon-sized embryos in the terrestrial planet formation region. These embryos are isolated from each other and accrete planetesimals only at a low rate. However, the continued accretion of chondrules destabilizes the oligarchic configuration and leads to the formation of Mars-sized embryos and terrestrial planets by a combination of direct chondrule accretion and giant impacts.

• The paper demonstrates that gas-drag-assisted chondrule accretion accelerates planetesimal growth, yielding mass distributions consistent with observed asteroid and embryo sizes.
• It employs advanced streaming instability simulations to reveal a runaway accretion regime where 100 km-scale bodies rapidly accumulate mass.
• The paper underscores chondrule-driven growth as a key process in both terrestrial planet and Kuiper Belt object formation, suggesting revisions to traditional planetary formation models.

Overview of "Growth of Asteroids, Planetary Embryos, and Kuiper Belt Objects by Chondrule Accretion"

Introduction

The paper explores the mechanism of asteroid formation primarily focusing on the accretion of chondrules—millimeter-sized spherules prevalent in primitive meteorites—within protoplanetary disks. A significant contribution of this research is its proposition that the incorporation of chondrules is a fundamental aspect of the accretion process that leads to the formation of asteroids, planetary embryos, and Kuiper Belt objects (KBOs). The authors explore the dynamics of gas-drag-assisted chondrule accretion, providing computational simulations that support the hypothesis of chondrule-dominated growth in planetary formation.

Key Findings

1. Gas-Drag-Assisted Accretion: The paper demonstrates that gas-drag enhances the accretion of chondrules onto the largest planetesimals within a few million years, forming mass distributions that align with observed asteroid sizes. Notably, planetesimals grow up to Mars-sized embryos, suggesting a parallel process in terrestrial planet formation regions.
2. Runaway Accretion: The research highlights a regime of runaway accretion for planetesimals around 100 km in radius within the asteroid belt with chondrules providing a significant mass addition. This could lead to isolated Mars-sized planetesimals capable of destabilizing oligarchic structures and facilitating further planetary growth through giant impacts.
3. Planetesimal Formation Simulations: Using high-resolution streaming instability simulations, this work indicates initial planetesimal formation triggered by cm-sized particles leading to gravitational instabilities. These initial bodies are suggested to be precursors to further chondrule-assisted growth.
4. Size Sorting of Chondrules: The accretion process inherently sorts chondrules by size, which aligns with the empirical observations of chondrule distributions within ordinary chondrites.
5. Role in Terrestrial Formation: The authors extend their model to speculate that chondrule accretion could be crucial in forming terrestrial planets, emphasizing a substantial growth phase driven by chondrules beyond an initial planetesimal period.
6. Kuiper Belt Objects: They also present scenarios for KBO growth, acknowledging the potential for chondrule or icy pebble accretion to shape the observed Kuiper Belt size distributions.

Implications and Future Directions

This research underscores the underappreciated role of chondrules in early solar system dynamics, offering a model where small particles are pivotal in accretionary processes. One implication lies in potentially revising existing models of planet formation to accommodate more significant roles of chondrule dynamics, especially in the asteroid belt and terrestrial region. However, given the sheer complexity of protoplanetary environments, future work requires fine-tuning models to incorporate particle interactions at various scales and exploring conditions enabling detailed empirical validation.

Understanding chondrule roles in planetary accretion zones offers keys to resolving discrepancies in planetary formation theories. The link between chondrule accretion, streaming instability, and resultant body size distribution can enhance predictive capabilities for both planetary and KBO formation models. Advances in simulation resolution, combined with comprehensive cosmo-chemical analyses, could cement chondrule accretion as a critical component in our understanding of solar system formation dynamics.

In summary, chondrule accretion affords a compelling lens into planetary embryo formation across different zones of a protoplanetary disk, providing a plausible mechanism that supports existing empirical data while inviting future observational and computational investigations.