Simplify the following text for a Grade 6 native English-speaking person.
Original: Partly by analogy to what is known about our Moon , Earth is considered to have differentiated from an aggregate of planetesimals into its core , mantle and crust within about 100 million years of the formation of the planet , 4.6 billion years ago .
Simplified: The Earth 's mantle and crust formed about 100 million years after the formation of the planet , about 4.6 billion years ago .