Definition of "accreted"

According to the reigning hypothesis, about 4.5 billion years ago, shortly after Earth had accreted down into a sphere from its little slub of circumsolar material, another newborn planet [Theia], still shaky on its feet, slammed obliquely into Earth with terrifying force.

2014 September 7, Natalie Angier, “The Moon comes around again ”, in The New York Times‎