Sentence:  According to the reigning hypothesis, about 4.5 billion years ago, shortly after Earth had <WSD>accreted</WSD> down into a sphere from its little slub of circumsolar material, another newborn planet &#91;Theia&#93;, still shaky on its feet, slammed obliquely into Earth with terrifying force. 
Meaning of the word marked with <WSD> tags:  To grow together, combine; to fuse.