Reaccretion

Reaccretion is the process by which material that has been removed, dispersed, or ejected from a body or region is gravitationally gathered back onto a surface or into a system. It can operate on planetary scales, in circumstellar disks, and in galactic or extragalactic contexts. The term encompasses a range of scenarios in which gravity or other forces reassemble previously separated matter.

In planetary science, reaccretion commonly refers to ejecta from impact events or debris in a protoplanetary disk that falls back onto a forming planet or its satellites. After large impacts, debris can reassemble under gravity, delivering volatiles and modifying surface chemistry and differentiation. In some models of early Solar System evolution, part of the material expelled during planetary growth may later reaccrete, contributing to a late veneer of materials that affect isotopic signatures and volatile inventories.

In disk and planetary formation, reaccretion can describe the reassembly of fragments that fragment or drift apart, as mutual gravity and dynamical interactions pull them back into larger bodies. Timescales for reaccretion vary widely, from thousands to millions of years, depending on mass, orbital dynamics, and environmental conditions.

In astrophysics, reaccretion includes fallback accretion, where gas or stellar debris ejected by winds, radiation pressure, or explosive events falls back onto a central object such as a protostar, neutron star, or black hole. In galaxies, galactic fountains and wind recycling describe gas that is expelled and later reaccreted, influencing star formation, metallicity, and feedback cycles.

Modeling reaccretion is important for understanding planetary growth, atmosphere formation, and galactic evolution, and is constrained by observations of disks, halos, and isotopic evidence.