Riona Yamada
Theory Astrophysics Lab in Nagoya University, Japan

Abstract
The early growth of planets from planetesimals is governed by
collisions. Previous models have treated colliding bodies as solid rocks
with brittle fracture and friction-dominated mechanical
properties. However, observational evidence from comets, asteroids, and
protoplanetary disks increasingly points to a porous nature of early
planetesimals, which were likely fragile aggregates of dust.

We present a new Smoothed Particle Hydrodynamics (SPH) model that
incorporates the actual mechanical properties of dust aggregates. Using
data from high-resolution N-body simulations, our method captures how
these porous structures compress and pull apart, without the cost of
modeling every individual grain. We then perform systematic simulations
of collisions between 30-km planetesimals.

Our results reveal a drastically different picture from rock-based models. Porous dust aggregates dissipate impact energy through compression, making them remarkably resilient. This allows planetesimals to merge even at impact velocities more than ten times their mutual escape velocity. We find that the cohesion of dust dramatically lowers the threshold for catastrophic disruption, favoring accretion over fragmentation.

This study demonstrates that the fragile, porous nature of primitive
planetesimals was likely a key factor enabling their rapid growth into
planetary embryos.

2026 January 15, 11:00

IA/U.Porto
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