abstract:Protoplanets are large planetary embryos that originate within protoplanetary discs and have undergone internal melting to produce differentiated interiors. They are believed to form out of kilometer-sized planetesimals that attract each other gravitationally and collide.
Currentthinking is that some 4 billion years ago, aMars-sized object smashed into the proto-Earth, knocking off a chunk ofplanet that eventuallyformed intothe moon.